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KeywordsBoulders; Mountain streams; Steep gradients; Tropical.

Author and Co-author Contact Information

Fred N ScatenaAU15Earth and Environmental Science240 S 33rd StreetPhiladelphiaPA 19104-6316USA

A Gupta

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c0010 9.31 Streams of the Montane Humid TropicsFN ScatenaAU1 , Earth and Environmental Science, Philadelphia, PA, USAA Gupta

r 2012 Elsevier Inc. All rights reserved.

9.31.1 Introduction 19.31.1.1 Historic Perspective 29.31.1.2 Environmental Settings of TMSs 29.31.1.3 Tectonic Settings 39.31.1.4 Modern Climate 39.31.1.5 Paleoclimate 49.31.1.6 Vegetation of Tropical Montane Watersheds 59.31.2 Hydrology and Aquatic Ecology of TMSs 59.31.2.1 Runoff Generation in TMS 59.31.2.2 Floods and Storm Flows 69.31.2.3 Aquatic Ecology of Tropical Rivers 69.31.3 Water Quality and Denudation 79.31.3.1 Water Quality 79.31.3.2 Denudation 79.31.4 Channel Morphology of TMSs 89.31.4.1 Drainage Networks of TMSs 89.31.4.2 Longitudinal Profiles and Hydraulic Geometry 89.31.4.3 Channel Features 99.31.4.4 Floodplains and Riparian Zones 99.31.4.5 Role of Instream Wood 109.31.5 Response to Anthropogenic Disturbances 119.31.5.1 Land-Use Change 119.31.5.2 Dams and Water Diversions 119.31.5.3 Climate Change 119.31.6 Conclusions 12References 13

Abstract

TropicalAU2 montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastalregions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not

been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive

2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfallsthat alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these

streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic

landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging

view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a highfrequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic

processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to

transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in

other fluvial systems.

s0010 9.31.1 Introduction

p0010 Tropical landscapes have played an important role in the sci-

entific development of geomorphology and in evaluating the

relative roles of climate, structure, process, and time in land-

scape development. Understanding the fluvial geomorphology

of tropical streams in general and tropical montane streams

(TMSs) in particular is essential, as they produce a dis-

proportionately large amount of the sediment, carbon, and

material that reaches coastal regions (Milliman and Syvitski,

1992; Lyons et al., 2002; Meade, 2007; Goldsmith et al.,

2008). TMSs are also important water sources and drain some

of the planet’s most diverse ecosystems and areas that are

Scatena, F.N., Gupta, A., 2012. Streams of the montane humid tropics. In:

Shroder, J., Jr., Wohl, E. (Eds.), Treatise on Geomorphology. Academic Press,

San Diego, CA, vol. 9, pp. xx–xx. [Please replace ‘xx’ by correct page number

when available.]

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considered especially sensitive to environmental and climate

change (Emanuel et al., 1985; Pepin and Lundquist, 2008;

Colwell et al., 2008). Despite the importance of TMSs, their

fundamental properties have received only limited systematic

description (Wohl, 2006). This chapter reviews the current

state of knowledge of the fluvial geomorphology of streams in

montane tropical environments, starting with a historical

perspective and their environmental settings. This is followed

by a review of their morphology and their known responses to

disturbances. The chapter concludes with discussion of

knowledge gaps and research needs.

s0015 9.31.1.1 Historic Perspective

p0015 Studies of early tropical geomorphology were dominated by

the observations of short-term visitors from temperate lati-

tudes. This early work was inherently descriptive and typically

focused on attention-grabbing features such as inselbergs (see

Wirthmann, 2000; Thomas, 2006). In general, tropical land-

scapes were considered unique assemblages of landforms de-

veloped from long periods of intense chemical weathering in

climatically and tectonically stable areas. Thus, many tropical

landscapes were considered to be the end products of a

Davisian-type of landscape evolution. The highly productive

and diverse forests that covered these landscapes were recog-

nized as the unique end products of millions of years of

relatively undisturbed evolution. The rivers that drain these

landscapes have also been considered unique since Alexander

von Humboldt described the bedrock rapids of the Orinoco

lowlands (Wirthmann, 2000). Subsequently, many writers

have opined that the combination of intense chemical wea-

thering, forceful rainfalls, and assumed climatic and tectonic

stability has caused tropical rivers to incise and produce

unique assemblages of bedrock-lined rapids and low-gradient

reaches that flow over weathered bedrock covered by a thin

layer of boulders and alluvium (see Wirthmann (2000) for an

excellent review).

p0020Until the mid-1950s most researchers in tropical geo-

morphology were based in Europe and the research focused

on defining climatic–landform assemblages in cratonic set-

tings (Budel, 1982; Kesel, 1985). In these studies, tropical

mountain valleys were commonly described as narrow and

V-shaped. Lowland rivers were thought to exhibit little lateral

erosion and were expected to be dominated by incision (Kesel,

1985). In the past 50 years, tropical geomorphology has

shifted away from its historic fixation on steady change and

the hills and plains of the Gondwana continent and toward an

explicit recognition of the dynamic and diverse nature of the

tropics and the acknowledgment that landscapes are sculpted

by a range of formative events and the restorative processes

between these events (Wolman and Gerson, 1978; Scatena,

1995; Brunsden, 1996). Although inselbergs and planation

surfaces are still in vogue (Coltorto et al., 2007), a much larger

emphasis is currently focused on (1) quantifying the role of

tropical rivers in global biogeochemical budgets (Milliman

and Syvitski, 1992; Douglas and Guyot, 2004; Carey et al.,

2005; Meade, 2007; Goldsmith et al., 2008) and (2) evaluating

the relative roles of climate and tectonics in weathering and

landscape evolution (White et al., 1995 AU3, 1998; Riebe et al.,

2001; Hsieh and Knuepfer, 2001; Whipple, 2004; Latrubesse,

2006). Rivers of the humid tropical mountains play a central

role in these debates.

s00209.31.1.2 Environmental Settings of TMSs

p0025This chapter is focused on streams in mountainous regions of

the humid tropics that currently receive 2000–3000 mm or

more of precipitation each year. Figure 1 contains a general-

ized map of their occurrence that was developed from our

knowledge of their distribution and by identifying ecoregions

of tropical and subtropical humid montane forests (Olson

et al., 2001). Nevertheless, identifying TMS can be just as

challenging and as conceptually useful as defining large rivers

(Potter, 1978; Miall, 2006). As described in detail later, the

TMSs considered here are located in forested montane areas

30° North

30° South

0 2500 50001250Kilometers Zones of tropical montane streams

Isothermality >=50%

Major tracks of cyclones

N

S

EW

f0010 Figure 1 The distribution

AU16

of environments with tropical montane streams, paths of major cyclones, and areas where isothermality is Z50%.Isothermality is defined as mean diurnal air temperature range/monthly air temperature range and reflects tropical and subtropical climates. Seetext for details.

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below the alpine tree line and in areas that have cooler tem-

peratures and higher rainfall than adjacent lowland regions.

They also tend to drain spatially diverse and complex geology

before they enter the lowlands and coastal plain. For example,

in Central America, TMS streams flowing over Pleistocene

glacial deposits can be a few kilometers away from streams

draining highly weathered oxisols and carbonate platforms.

s0025 9.31.1.3 Tectonic Settings

p0030 In general, TMS streams drain continental and insular

mountains in active subduction zones, collision belts, rift

zones, and volcanic arcs. Transform faults have also been

important in the development of TMSs in the Caribbean,

Taiwan, and Indonesia. They also have diverse tectonic his-

tories and ages that have influenced their climate and geo-

morphic development in complex ways (Thomas, 2006).

India has traversed the equator through much of the Cenozoic

to form the Himalayan collision belt and the Western Ghat

escarpment that are now drained by TMS. This collision also

resulted in a series of strike-slip faults that traverse Southeast

Asia and influenced the location of many low mountain and

hill streams that are currently drained by TMS. Tropical South

America has TMS associated with Pacific and Car-

ibbean–South American plate boundary interactions that date

from the Cretaceous. Africa has slowly moved northward with

TMS draining rift zones and uplifted sedimentary rocks, while

Australia has moved from a near-polar position to its current

subtropical location and has TMS draining Paleozoic bedrock.

This diversity in geologic and tectonic history contradicts the

historic notion of the old tropical Earth and indicates that

many tropical landscapes and TMS have not developed under

fixed climatically or latitudinally defined conditions.

s0030 9.31.1.4 Modern Climate

p0035 Superimposed upon the geologic and tectonic variability of

TMS is a diverse set of climatic conditions. The tropics can be

defined as the area of surplus radiative energy that is bounded

by anticyclonic circulations near the 301 north and southlatitudes (Reynolds, 1985; McGregor and Nieuwolt, 1998;

Callaghan and Bonell, 2004). The climate of TMSs lacks very

cold seasons and they have a consistent diurnal range of air

temperature throughout the year (Hijmans et al., 2005). They

also have spatially complex patterns of precipitation that result

from the interaction of low-level circulation patterns, cyclonic

circulation, easterly waves, and the seasonal march of the

Intertropical Convergence Zone (ITCZ). Locally, precipitation

patterns are also influenced by land–sea breezes, orographic

uplifts, and the trade wind inversions. In many tropical

mountains, these processes interact such that the zone of

maximum annual rainfall occurs between 1000 and 1500 masl

(McGregor and Nieuwolt, 1998) and within the catchments of

TMS. The climates of these watersheds are typically within the

Af and Am groups in the Köppen–Geiger climate classification

system. In the Holdridge Life Zone system, these areas are

within the lower montane to upper montane altitudinal belts

of the tropical and subtropical moist, wet, and rain-forest life

zones (Holdridge, 1967).

p0040The major climatic feature that distinguishes the humid

tropics from the dry tropics is that average annual rainfall is

greater than potential evapotranspiration and there is enough

precipitation to support evergreen or semi-deciduous forests.

Many, if not most, TMS streams drain areas that receive an

annual precipitation greater than 2000 or 3000 mm yr�1. Be-

cause of the considerable seasonal variations in precipitation

and runoff, it is also common in the geomorphic literature to

explicitly acknowledge the presence of the seasonal and

aseasonal humid tropics (Gupta, 1975, 1988, 1995). The

seasonal humid tropics can be broadly defined as areas that

have a marked seasonal concentration of rainfall and runoff.

These areas are typically influenced by the ITCZ or the mon-

soonal rains and it is not uncommon that 80% of their annual

stream flow occurs in 4 or 5 months of the year. Whereas most

areas in the aseasonal humid tropics have a mean annual

rainfall between 2000 and 4000 mm yr�1, the mean annual

rainfall of the seasonal tropics is more variable and ranges

between 1000 and 6000 mm. In these areas, the interannual

variability of runoff is large (Mahe et al., 2004) and stream

channel geometry can change dramatically between wet and

dry seasons (Gupta, 1995).

p0045Interannual- to millennial-scale variability in rainfall,

flooding, drought and hurricane intensity, sediment transport

and deposition, water quality, and the structure of aquatic

populations of TMS have all been related, albeit complexly in

many cases, to changes in sea-surface temperatures, the El

Niño–Southern Oscillation (ENSO), monsoons, and other

global-scale circulation systems (see Douglas et al., 1999;

Rodbell et al., 1999; Giannini et al., 2001; Aalto et al., 2003;

Donnelly and Woodruff, 2007). In general, the interannual

variability of rainfall that influences TMS increases with de-

creasing rainfall, decreasing latitude, and the influence of the

ITCZ and ENSO (Dewar and Wallis, 1999). There is a general

impression, and in some places a misconception, that the

humid tropics are characterized by a domain of steady but

low-intensity rains, while the seasonal tropics have a higher

frequency of intense rainfalls. Although inverse relationships

between rainfall intensity and total rainfall have been shown

in some tropical areas, these relationships are not universal

(Yu, 1995).

p0050Frequent landscape-altering rainfalls are characteristic of

the watersheds of TMS and can occur in both dry and wet

seasons. Multiday rainfalls over 2000 mm are not uncommon

(Table 1 AU4), rainfalls greater than 500 mm d�1 typically have

recurrence intervals of 20 years or less, and daily totals greater

than 75 mm d�1 occur in most years (Gupta, 1988; Scatena

et al., 2004; Chu et al., 2009). Rainfall intensities and event

totals are commonly an order of magnitude higher in the

humid tropics compared to humid temperate regions and

rainfalls with intensities of 25 mm h�1 or more can account

for more than 30% of annual rainfall, whereas they typically

account for less than 5% in the temperate areas (Bonell,

2004). Maximum rainfall intensities also tend to occur in

tropical highlands below 1500 masl (McGregor and Nieuwolt,

1998), an elevation that is drained by TMS.

p0055Hurricanes and cyclonic depressions traverse many TMSs

but are rare in Africa, South America, and most of Southeast

Asia (Figure 1). In regions where they are common, an average

of 5–25 cyclonic storms can occur each year (Scatena et al.,

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2004). Where hurricanes pass directly over a TM watershed,

defoliation, landslides, and flooding are widespread and can

alter local and regional-scale hydrologic and nutrient cycles

(Scatena and Lugo, 1995; McDowell et al., 1996; Schaffer

et al., 2000; Gupta, 2000; Lyons et al., 2002; Carey et al., 2005;

Goldsmith et al., 2008). Although event rainfalls can exceed

2000 mm, average rainfalls within 222 km of the eye of a

hurricane are on the order of 100 mm d�1 (Anthes, 1982;

McGregor and Nieuwolt, 1998). Locally their impact is

strongly influenced by their storm tracks and physiography

and topography (Gupta, 1988). Likewise, not all the intense

rainfalls or peak discharges of TMS are associated with hurri-

canes or cyclonic depressions. Multiday rainfall events asso-

ciated with annual monsoons or the seasonal passage of the

ITCZ may actually generate more geomorphic work and have a

larger overall influence on fluvial landscapes than cyclonic

systems that have recurrence intervals on the orders of decades

in any location.

p0060 One characteristic that appears to be relatively common in

TMS is the basin-wide nature of the intense storms. Moreover,

monsoons, hurricanes, and ITZC-related storms tend to cover

such large regions that even large drainage basins receive

geomorphic significant rainfalls at the same time. This results

in large parts of the basin contributing to and experiencing

channel-modifying discharges at the same time. This is in

contrast to the spatially restricted contributions of snowmelt

or convective storms that often cause important but relatively

localized geomorphic impacts in temperate montane streams

(see Chapter 9.28 Specific Fluvial Environments: Steep

Headwater Channels (00253)).

p0065 Prolonged droughts and fires are an important, but com-

monly underestimated, disturbance in humid tropical forests

and in TMS (Walsh and Newbery, 1999; Grau, 2001; Malmer

et al., 2004; Sherman et al., 2008). Fires are less common than

droughts and the highest fire frequencies occur below the

cloud forest zone and where annual precipitation is seasonal

and/or less than 1000 mm yr�1. In most humid tropical for-

ests, cumulative rainfall deficits between 5% and 10% of mean

annual precipitation are common on annual and decadal

timescales (Scatena et al., 2004). During droughts with re-

currence intervals approaching a decade, riffles in headwater

TMS can dry up, pools can be isolated and reduced in volume,

and there can be localized crowding of benthic invertebrates

(Covich et al., 1998, 2003, 2006).

s00359.31.1.5 Paleoclimate

p0070Geomorphic legacies of past climates are widely recognized in

the fluvial environments of the mid-latitudes. By contrast,

because the tropics were historically considered as being cli-

matically stable, an explicit consideration of the geomorphic

legacies of past climates has not been a tradition in tropical

studies. It is now known that many tropical landscapes have

undergone considerable climatic changes during the Quater-

nary. Prior to about 28 000 14C year BP, the TMSs of Africa,

South America, and Australia had experienced humid forested

conditions for approximately 104 years (Thomas, 2003).

During the Last Glacial Maximum (LGM), between 21 000 and

18 000 14C yr BP, large parts of the tropics were cooler, rainfall

was reduced by 30–60%, and there was a reduction in the

extent of humid tropical forests (Servant et al., 1993; Mahe

et al., 2004; Kale et al., 2003;Goodbred, 2003; Thomas, 2003

and references therein). As the glaciers retreated, rainfall and

the extent of humid tropical forests increased and major

changes in fluvial activity apparently took place in several

tropical basins.

t0010 Table 1 Select examples of extreme rainfall events that have influences on tropical montane streams

Total rainfall (mm per event) Average mm d�1 during event Location Dates Source

Hurricanes5678 568 La Reunion 18–27 Jan 1980 Landsea, 19993240 1080 La Reunion 24–27 Jan 1980 Landsea, 19992467 1233 La Reunion 8–10 Apr 1958 Landsea, 19992287 327 Jamaica 4–11 Nov 1909 Gupta, 19882025 405 Cuba 3–8 Oct 1963 Gupta, 19881825 1825 La Reunion 7–8 Jan 1966 Landsea, 19991524 762 Jamaica 5–7 Oct 1963 Gupta, 19881248 1248 Taiwan 10–11 Sept 1963 Gupta, 19881168 1168 Philippines 14–15 July 1911 Gupta, 1988

Monsoon3388 484 India 9–16 June 1876 Gupta, 19883213 536 India 24–30 June 1932 Gupta, 19881036 1036 India 14 June 1876 Gupta, 1988

Frontal systems and the ITCZ2789 930 Jamaica 22–25 Jan 1960 Gupta, 19881109 1109 Jamaica 23 Jan 1960 Gupta, 1988911 304 Venezuela 14–16 Dec 1999 PAHO, 1999867 867 Cuba 1 June 1996 Planos, 2003

Modified from Scatena, F.N., Planos-Gutierrez, E., Schellekens, J., 2004. Impacts of natural disturbances on the hydrology of tropical forests. In: Bonell, M., Bruijnzeel, L.A. (Eds.),

Forest, Water and People in the Humid Tropics. International Hydrology Series. Cambridge University Press, Cambridge, ch. 19, pp. 489–513.

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p0075 The early Holocene in many TMSs was relatively wet. In

some areas, precipitation may have been elevated 20–35%

above recent means, and 40–80% greater than the LGM

minima (Goodbred, 2003; Thomas, 2003; Mahe et al., 2004

and references therein). Consequently, early Holocene chan-

ges in the fluvial environments were probably rapid and er-

ratic and there is some evidence that many tropical rivers

excavated deep rocky channels as a result of high discharges

that occurred during the Pleistocene–Holocene transition. The

maximum extension of modern tropical rainforests is thought

to have occurred in the early Holocene (i.e., 9500–8500 14C

year BP). This was followed by a mid-Holocene dry period that

created favorable conditions for forest contraction, forest fires,

and cut-and-fill episodes in alluvial reaches of lowland tropi-

cal streams. Humid tropical forests in Africa, Asia, Amazonia,

Central America, and the Caribbean have all experienced fires

and extended droughts during the past 10 000 years (Sanford

et al., 1985; Hodel et al., 1991; Guilderson et al., 1994).

Variations in the Holocene frequency of hurricanes and cata-

strophic rainfalls have also been linked to terrace formation

and channel incision in Taiwan (Hsieh and Knuepfer, 2001)

and coastal processes on Caribbean islands (Donnelly and

Woodruff, 2007).

p0080 Comparisons of the present hydrological and climatic

setting with the climate regime necessary to maintain full lake

levels at steady state can be used to gauge the degree to which

tropical drainages are adjusted to their present climatic regime

(Burrough and Thomas, 2009). This disequilibrium lake basin

index suggests that equatorial lakes are currently closer to their

full lake conditions at steady state than lakes in the subtropics.

A similar spatial pattern of channel disequilibrium may apply

to TMSs and their lowland counterparts. In any case, there is

ample evidence to suggest that many TMSs have fluvial fea-

tures related to Pleistocene climates and many may still be

adjusting to previous climatic regimes.

s0040 9.31.1.6 Vegetation of Tropical Montane Watersheds

p0085 Process-based classifications of tropical forests are farther ad-

vanced than classifications of tropical fluvial systems. Dis-

tinctions between forests are typically based on phenology

(i.e., evergreen or deciduous), climate (i.e., rain, wet, moist, or

dry forests), physiography (i.e., upland and lowland), and

hydrologic conditions (i.e., cloud forests, riparian, and wet-

land). The natural vegetation that typically covers the water-

sheds of the TMS discussed here are evergreen rain, wet, moist,

or cloud forests. On large equatorial mountains, the transition

from montane forests to subalpine forests or grasslands is

generally observed at elevations between 2800 and 3200 m

(Bruijnzeel, 2001). As such, this type of land cover is only

encountered in TMSs that drain the highest mountains, most

of which occur in Latin America, the Himalayas, and Papua

New Guinea.

p0090 Currently, most watersheds drained by TMS are covered by

mixtures of cut-over forests, pasture, coffee plantations, crop-

land, and small communities. Historically, the steep and

rugged terrain of TMSs provided them with some basic level of

protection. However, by the early 1990s tropical montane

forests were high on the list of the world’s most threatened

ecosystems and they were being deforested at a rate that was

considerably greater than that of lowland tropical forests

(1.1% yr�1 vs. 0.8% yr�1; Doumenge et al., 1995). The total

potential area of tropical montane forests has been estimated

by various methods to be between 3 and 5 million km2

(Bruijnzeel et al., 2010). Approximately 45–56% of these

forests remain and are drained by TMSs. Although protecting

these forests is still considered a critical conservation need, the

recognition of their importance as biodiversity centers and

water sources has resulted in some increased legal protection.

Abandonment and subsequent reforestation of watersheds

drained by TMSs is also occurring in some areas (Aide and

Grau, 2004). Although the impact of this reforestation on TMS

morphology is uncertain, hydrologic analysis suggests that

reforestation can decrease sediment yields, low-flow dis-

charges, annual runoff, and the proportion of rainfall that

contributes to stream flow (Bruijnzeel, 2004; Wu et al., 2007;

Bruijnzeel et al., 2010). The influence of reforestation on peak

storm flow discharge and stream power is less certain but may

be proportionately less than the influence of reforestation on

water quality and low stream flows.

s00459.31.2 Hydrology and Aquatic Ecology of TMSs

s00509.31.2.1 Runoff Generation in TMS

p0095It is commonly assumed that humid tropical landscapes have

quick hydrologic response times and high runoff coefficients

that result in flashy streams with high peak discharges that

ultimately incise channels. In practice, infiltration rates can

range from 0 to over 200 mm h�1 (Harden et al., 2003 AU5) and

runoff generation is complex and dependent on land cover,

antecedent conditions, bedrock lithology, and basin and ri-

parian morphology (Walsh, 1980; McDowell et al., 1992;

Dykes and Thornes, 2000; Elsenbeer, 2001; Schellekens et al.,

2004; Bonell, 2004; Niedzialek and Ogden, 2005; Saunders

et al., 2006 and references therein). Multivariate analysis has

been used to determine the relative influence of physical

characteristics and land cover on the hydrology and water

quality of several watersheds that contain TMS (Santos-Román

et al., 2003; Rivera-Ramirez et al., 2002; Soldner et al., 2004;

Harmon et al., 2009). The factors most commonly linked to

runoff quantity and water quality are bedrock geology, dom-

inant land cover (i.e., forest, agriculture, and urban), and

elevation, which is typically a cross-correlated surrogate of

precipitation and/or land use.

p0100The most characteristic feature of the response of TMS to

precipitation is the rapid and extremely flashy nature of

catchment runoff that is attributed to a variety of shallow

subsurface flow paths. US AU6DA Soil Conservation curve num-

bers (CNs) calculated for 28 storms in the predominantly

forested Rio Chagres Basin of Panama ranged from 64 to 98

and depended on storm intensity and antecedent conditions

(Calvo et al., 2005). These authors recommend a CN of 75 for

extreme storms in the wet season. The area-weighted CN for

TMS and their associated lowlands reaches of northeastern

Puerto Rico decreased from 74 to 60.7 as barren agricultural

lands and pastures were reforested (Wu et al., 2007). However,

stream flow response times and the ratio of runoff to rainfall

MORP 00256

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can vary within and between seasons and for some areas these

figures can be relatively high at the end of the dry season when

the soils are cracked and macro-pores are abundant (Nied-

zialek and Ogden, 2005).

s0055 9.31.2.2 Floods and Storm Flows

p0105 Flooding is common throughout the tropics and several dis-

tinct flood regimes have previously been distinguished and

include: (1) occasional short-term flood; (2) frequent or an-

nual short-term flooding; (3) annual long-term flooding; and

(4) annual submersion by floodwaters (Salo et al., 1986;

Scatena et al., 2004). Only the first two of these regimes are

common in TMSs and TMSs in both seasonal and non-

seasonal environments and are characterized by a regime

where short-term events capable of transporting bedload (see

Chapter 9.8 Bedload Kinematics and Fluxes (00233) and

Chapter 9.28 Specific Fluvial Environments: Steep Head-

water Channels (00253)) and removing periphyton (see

Chapter 9.12 Influence of Aquatic and Semi-Aquatic Or-

ganisms on Channel Forms and Processes (00237)) occur

several times each year. Peak discharges are several orders of

magnitude larger than base flow but average annual peak

flows are generally not channel-forming or -modifying events,

especially in areas with boulders and bedrock-lined channels

(Scatena et al., 2004; Pike et al., 2010; see Chapter

9.29 Bedrock Rivers (00254)). Major channel-modifying

events have been associated with peak discharges that range

from 20.9 to 65 m3 s�1 km�2 and have recurrence intervals on

the order of decades (Gupta, 1988; O’Connor and Costa,

2004; Garcin et al., 2005). The defoliation and uprooting as-

sociated with hurricanes can also produce considerable

amounts of nutrient-rich green litter and wood (see Chapter

9.11 Wood in Fluvial Systems (00236)) that can clog chan-

nels and even cause temporary reductions in suspended

sediment yields (Lodge et al., 1991; Gellis, 1993; see Chapter

9.9 Suspended Load (00234)).

p0110 Although the importance of flooding to TMS is widely

acknowledged, so is the difficulty in estimating the recurrence

intervals of moderate to extreme floods in ungauged TMS

(Pike and Scatena, 2010). In a well-gauged TMS network in

Puerto Rico, flood discharges that are close to the annual peak

are commonly experienced several times in a year. Compara-

tive analysis of these streams also showed that in these flashy

and relatively small streams, annual maximum flow series

analysis fails to capture the intra-annual flows that are re-

sponsible for structuring the vegetation in and adjacent to the

channels. A partial duration series based on 15-min discharges

is recommended for most analyses.

s0060 9.31.2.3 Aquatic Ecology of Tropical Rivers

p0115 Research on the aquatic ecology of tropical rivers has high-

lighted differences between tropical and temperate streams

(see Chapter 9.12 Influence of Aquatic and Semi-Aquatic

Organisms on Channel Forms and Processes (00237)), in-

cluding the latitudinal variations in diversity, radiation, tem-

perature, geostrophic effects, and the influences of continuous

litter inputs, warm water, the lack of ice, and common high

flows (Payne, 1986; Jackson and Sweeney, 1995; Talling and

Lemoalle, 1998; Dudgeon, 2008). Recent efforts have focused

on the spatial variability in ecological processes in relation to

waterfalls and other geomorphic conditions that influence

within-channel habitats and the migration and distribution of

species (Wantzen et al., 2006; Boyero et al., 2009).

p0120Although studies of tropical stream metabolism that ex-

tend for at least 1 year and/or extend along the longitudinal

profile of a basin are scarce, available information suggests

that rates of in-stream photosynthesis in forested TMS are

similar to those of similarly sized streams draining temperate-

deciduous forests (Ortiz-Zayas et al., 2005). However, con-

tinual herbivory and a high frequency of bedload-transporting

storms interact to suppress the abundance of periphyton and

submerged aquatic plants in TMS. Consequently, their rates of

respiration are much higher than in most temperate streams

and they can have ratios of photosynthesis to respiration of

less than 1 from their headwaters to their lower reaches.

Nevertheless, where tropical rainforest vegetation is present, it

can provide streams with sufficient amounts of labile organic

carbon to support high rates of respiration over long distances

and make tropical streams globally important sources of car-

bon inputs to oceans (Kao and Liu, 1996; Lyons et al., 2002;

Ortiz-Zayas et al., 2005).

p0125Short-term floods and droughts cause significant in-

vertebrate mortality and shifts in population-age distributions

in TMS streams in Malaysia, Hong-Kong, India, Ecuador, tro-

pical Australia, the Andean piedmont of Venezuela, and the

Caribbean (Flechter and Feifarek, 1994; Scatena et al., 2004).

However, native populations have numerous morphological

and behavioral adaptations to deal with common, substrate-

disturbing stream flows, including suction-cup-like append-

ages that can cling to bedrock surfaces and the ability to hide

under large boulders or occupy shallow-water channel-margin

habitats during floods and droughts. Some pan-tropical

shrimp and fish species can migrate past vertical waterfalls that

are tens of meters high by migrating in bedrock joints and in

areas with moss coverings and laminar sheet flow.

p0130Although frequent floods and steep gradients characterize

TMS, many, if not the majority, of the native macro-fauna

living in TMSs migrate between rivers and coastal zones over

the course of their lives (March et al., 1998, 2003; March and

Pringle, 2003; Crook et al., 2009). Consequently, waterfalls,

dams, and other geomorphic or anthropogenic migration

barriers can have important roles in determining the com-

munity composition and longitudinal variation of aquatic

species in TMS. Coastal conditions, the distribution of

waterfalls and migration barriers, altitude, drainage area, ri-

parian and watershed land cover, water quality, substrate size,

and pool volume have all been correlated to the abundance of

aquatic organisms and the structure of aquatic communities in

TMSs (Pyron et al., 1999; Fievet et al., 2001; Zimmerman and

Covich, 2003; Soldner et al., 2004; Blanco and Scatena, 2005,

2006).

p0135Because of the importance of floods, waterfalls, and other

geomorphic barriers to the distribution of aquatic organisms

in TMSs, the question has been raised whether the river con-

tinuum concept (RCC), which successfully explains longi-

tudinal patterns in species distributions and food webs in

temperate streams (Vannote et al., 1980), also applies to TMS.

MORP 00256

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In general, the RCC suggests that the longitudinal distri-

butions of aquatic species reflect downstream changes in

channel morphology, discharge, sediment load, riparian cover,

and incident radiation on the water surface. Although geo-

morphic barriers do cause discontinuities in the distribution

of organisms in TMS, the few existing controlled studies sug-

gest that the general patterns of food web and resource

availability that are predicted by the RCC do exist (March and

Pringle, 2003; Greathouse and Pringle, 2006). It has also been

hypothesized that the ecology of TMS might be functionally

closer to montane temperate streams than to their lowland

counterparts and that lowland tropical streams might differ

substantially from lowland temperate streams (Boyero et al.,

2009). Increased knowledge of the life histories and habitat

requirements of tropical species are desperately needed to

quantify and understand longitudinal patterns and differences

between TMS and temperate streams.

s0065 9.31.3 Water Quality and Denudation

s0070 9.31.3.1 Water Quality

p0140 TMS streams have warmer water, higher annual exports of

dissolved constituents and sediment, and less seasonal dif-

ferences in water temperature and water chemistry than their

temperate counterparts. Their water quality is not influenced

by freeze–thaw cycles, ice-induced bank erosion, or seasonal

pulses of plant litter (although hurricane deforestation can

create significant pulses). Several studies have related stream

water concentrations of certain elements to either shallow

near-surface flow or longer and deeper flow paths (Schellekens

et al., 2004; Bonell, 2004; Bhatt and McDowell, 2007; Saun-

ders et al., 2006). Surface soils drained by TMSs are typically

wet, commonly saturated, and have intensive microbial ac-

tivity that can reduce and remove nitrogen from soil and

groundwater before it enters the stream channel (Chestnut

et al., 1999; Chestnut and McDowell, 2000). Local riparian

nitrogen dynamics and their ability to remove nitrogen before

it enters the stream channel depend on local lithology and

geomorphology (McDowell et al., 1992) and some of the

planet’s highest basin-average rates of denitrification are found

in the humid tropic systems in South America and Africa

(Seitzinger et al., 2006).

p0145 Most carbon exports from TMSs are in the form of dis-

solved organic carbon (McDowell and Asbury, 1994; Chestnut

et al., 1999; Lyons et al., 2002). Land-use change and large-

scale hurricane-related defoliation can result in significant

increases in stream water cation and carbon exports. However,

post-hurricane exports are less than a few percentages of the

hurricane-derived plant litter inputs, which reflects the tight

nutrient retention these systems can have (Schaffer et al.,

2000). Nevertheless, because of their high carbon exports and

storm-initiated CO2 consumption from silicate weathering,

some TMSs subject to tropical cyclones may be important

global sinks of CO2 transport to ocean burial (Kao and Liu,

1996; Lyons et al., 2002; Goldsmith et al., 2008; Draut et al.,

2009).

s00759.31.3.2 Denudation

p0150The average rate of ground surface lowering of TMS can be

well over 100 m per million years, but typically ranges be-

tween 50 and 75 m per million years (White et al., 1998;

Hsieh and Knuepfer, 2001; Hartshorn et al., 2002; Thomas,

2003; Riebe et al., 2004 AU7; Whipple, 2004 and references

therein). The sediment in most TMS is ultimately derived from

weathered saprolite, which typically contains meter-diameter

core stone boulders in a matrix of clays, silts, and sands.

Saprolite thickness varies widely and ranges from a few meters

to more than 100 m and comparisons of long-term denuda-

tion rates with the rate of saprolite advance suggest that the

saprolites drained by some TMSs have reached their steady-

state thickness (see Buss et al., 2008 and references therein).

p0155Rates of chemical denudation in TMSs are some of the

largest in the world (Milliman and Syvitski, 1992; White and

Blum, 1995; Syvitski and Milliman, 2007) and TMSs under-

lain by granite or on volcanic islands where meteorologic

waters are impacted by high subsurface temperatures are

among the highest of TMSs (Brown et al., 1995; Riebe et al.,

2001; Rad et al., 2007). The ratio of physical denudation to

total denudation in the drainages of TMS is variable and there

are insufficient studies for a definitive analysis. Nevertheless,

available rates suggest that physical denudation for TMS

drainages can range between 40% and 75% of total denuda-

tion and averages around 60% (White et al., 1998; Riebe et al.,

2001; Buss et al., 2008; Harmon et al., 2009 and references

therein).

p0160Slope failures tend to contribute most of the river sediment

in TMS, irrespective of the scale of the basin. In some areas,

landslides producing rain storms occur on an average of once

every 1–2 years and rainfall intensity-duration curves indicate

that slope failures can occur with most types of tropical rain

events, including the annual migration of the ITCZ, hurri-

canes, convective storms, and cold fronts (Scatena et al.,

2004). Available comparisons suggest that the rainfall

thresholds needed to trigger slope failures may be higher in

the humid tropics than in temperate areas (Larsen and Simon,

1993; Gabet et al., 1994 AU8). However, because the frequency of

these rains is higher, landslides have a significant influence on

hillslopes and channel processes. The most common slope

failures are shallow translational failures that have depths less

than 10 m (Simon et al., 1990; Larsen and Torres Sánchez,

1992; Maharaj, 1993; Paolini et al., 2005). Whereas most

slope failures are moisture driven and occur in the wet season

or following large tropical storms, earthquake-generated

landslides can also be significant in many TMS drainages

(Garwood et al., 1979).

p0165It is also common in TMSs that large flows can transport

years to decades worth of average annual sediment and ma-

terial flux in a single event. Modeling studies on the humid

tropics of Australia indicate that 30% of the total rainfall

contributes approximately 87% of the total transported sedi-

ment but only 45% of the total runoff (Yu, 1995). In Puerto

Rican streams, the highest recorded daily sediment discharges

are 1–3.6 times the annual suspended-sediment discharge,

and runoff from major storms transport 1–32 times the me-

dian annual sediment load (Warne et al., 2005). In eastern

Jamaica, stream flows associated with Hurricane Gilbert

MORP 00256

Streams of the Montane Humid Tropics 7

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transported large quantities of coarse bed sediment and over

1700 times the daily suspended load of the dry season (Tho-

mas, 1991; Gupta, 2000). In Taiwan, individual typhoons can

have hyperpycnal sediment concentrations (440 g l�1) andcan transport between 72% and 95% of the annual particulate

organic carbon fluxes of the highest yielding world rivers

(Goldsmith et al., 2008). Studies in Taiwan also suggest that

valley lowering and channel incision are driven by relatively

frequent flows of low to moderate intensity, whereas large and

rare floods are more important in widening bedrock channels

(Hartshorn et al., 2002).

p0170 The highest sediment yields in the humid tropics also

originate from TMS in tectonically active regions (Douglas and

Guyot, 2004). Whereas the global-scale battle regarding who is

responsible for delivering the most fluvial sediment to the

ocean may be fought between geology, geography, and

humans (Syvitski and Milliman, 2007), TMSs underlain by

highly weatherable bedrock and in areas subject to hurricanes

and occupied by marginalized farmers appear to be the win-

ners. Moreover, soil erosion in anthropogenically disturbed

TM watersheds can be several orders of magnitude larger than

pre-disturbed rates (Anderson and Spencer, 1991; Douglas

et al., 1992; Hewawasam et al., 2003; Douglas and Guyot,

2004; Sidle et al., 2004; Warne et al., 2005). These studies

suggest that TMS draining undisturbed forested watersheds

typically have sediment yields around 100–500 t km�2 yr�1.

Large, mixed land-use watersheds can have sediment yields

between 1000 and 3000 t km�2 yr�1. Small, highly disturbed

areas associated with logging can have yields greater than

25 000 t km�2 yr�1 (Sidle et al., 2004).

s0080 9.31.4 Channel Morphology of TMSs

s0085 9.31.4.1 Drainage Networks of TMSs

p0175 The hillslopes that drain into TMS are typically characterized

by landslide scars and a dense network of intermittent swales

and channels that dissect the landscape into narrow inter-

fluves and deep valleys. Reported drainage densities of TMSs

range from 2.6 to over 20 km/km�2 (Scatena and Lugo, 1995;

Walsh, 1996; Terry, 1999). In general, drainage densities in

humid tropical areas are considered to be higher than in

humid temperate areas because of higher precipitation inten-

sities but lower than in semi-arid areas because of greater

vegetation cover (Chorley et al., 1984). Detailed analyses of

several tropical areas further suggest that the relationship be-

tween drainage density and annual rainfall in TMS networks is

nonlinear and influenced by extreme daily rainfall totals and

the permeability, mineralogy, and storage capacity of soils

(Walsh, 1996). Analysis of several TMS networks indicates that

drainage density increases relatively rapidly until approxi-

mately 2500–3000 mm yr�1, at which point it increases at a

reduced rate with further increases in annual rainfall (Walsh,

1996). These studies also indicate that these networks com-

monly failed to conform to Horton’s laws of stream numbers

and that while the high channel densities can develop in less

than 50 years, major changes in basin or network shape do

not occur before 14 000 years.

p0180The drainage networks of TMSs are commonly described as

rectangular and structurally controlled and as having straight

nonaccordant tributaries that join the main channels at high

angles (Ahmad et al., 1993; Hare and Gardner, 1995 AU9; Ng,

2006). On the Nicoya Peninsula of Costa Rica, drainage basin

asymmetry has been used to identify centers of uplift and

direction of tilt (Hare and Gardner, 1995). In the Greater

Antilles, the rectangular network morphology of TMS has been

related to strike-slip, plate-boundary tectonics (Ahmad et al.,

1993) and in Hong Kong the headwater progression of the

drainage network has been related to systematic variations on

landslide morphology and density (Ng, 2006). It is typically

unclear whether the slope breaks and steps at the junctions of

a tributary and the mainstem are actively retreating knick-

points, structural, or high-flow features. These nonaccordant

tributary junctions have been known to influence the up-

stream migration of aquatic species.

s00909.31.4.2 Longitudinal Profiles and Hydraulic Geometry

p0185Average stream gradients of TMS are typically well above the

0.002 m m�1 threshold that has typically been used to define

montane streams (Wohl and Merritt, 2005, 2008). Their lon-

gitudinal profiles are typically described as being segmented

by waterfalls and alternating steep and lower gradient seg-

ments with morphology correlated to bedrock morphology.

For example, along the upper Rio Chagres watershed of Pan-

ama, reaches flowing across granites, diorites, and tonalites

have lower gradients and wider channels than reaches flowing

across gabbros and diorites (Wohl, 2005). In the Luquillo

Mountains of Puerto Rico, lower stream gradients are associ-

ated with granodiorite, whereas steep gradients are associated

with more erosion-resistant contact metamorphic rocks (Pike

et al., 2010). In Fiji and Hawaii, waterfalls can also occur

where stream flows across more resistant lava flows (Terry,

1999).

p0190Hydraulic geometries of many mountain streams are

highly variable and complex (Wohl and Merritt, 2005; see

Chapter 9.18 Hydraulic Geometry (00243) and Chapter

9.28 Specific Fluvial Environments: Steep Headwater Chan-

nels (00253)). Nevertheless, the few available studies suggest

that TMSs may have better developed downstream hydraulic

relationships than their temperate counterparts, especially

when compared to montane streams in temperate areas that

have been recently glaciated (Pike et al., 2010). Unfortunately,

the interpretation of hydraulic geometry in mountain streams

in general, and TMS in particular, is complicated because of

the lack of identifiable bankfull or reference discharges to

compare channel geometry at constant flow frequencies. Be-

cause of the lack of well-defined floodplains or other bank-full

features, most studies of the hydraulic geometry of montane

streams have based hydraulic geometries on reference dis-

charges that are associated with recent floods (Pike et al.,

2010). Nevertheless, in TMS reaches of Puerto Rico that lack a

floodplain, a definable channel boundary that is characterized

by the incipient presence of soil, woody shrubs, and trees

corresponds to the same flow frequency as the bankfull dis-

charge of nearby alluvial channels. The reference discharge

based on these riparian features has an average exceedance

MORP 00256

8 Streams of the Montane Humid Tropics

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probability between 0.09% and 0.30%, and a recurrence

interval between 40 and 90 days.

s0095 9.31.4.3 Channel Features

p0195 Bedrock channels (see Chapter 9.29 Bedrock Rivers

(00254)), boulder bars and boulder-lined channels, step–-

pools (see Chapter 9.20 Step–Pool (00245)), pool–riffle se-

quences (see Chapter 9.21 Pool-Riffle (00246)), and all of

the morphologic features observed in other mountain streams

(Montgomery and Buffington, 1997; Thompson et al., 2006)

have all been observed in TMSs. Although it is generally

considered that the abundance of bedrock channels or bed-

rock–alluvial channels (sensu Whipple, 2004) is relatively high

in TMSs, a comprehensive data set does not exist to verify this

quantitatively. Nevertheless, reaches with a continuous or

deep cover of alluvial sediments are generally lacking, whereas

reaches with accumulations of large-diameter boulders as well

as boulder leeves, boulder bars, and boulder steps are com-

mon (Figures 2 and 3). The origin of the boulders varies, as

some large boulders are exhumed core stones whereas others

have been transported to channels during slope failures and

debris flows (Ahmad et al., 1993; Terry, 1999).

p0200 Globally, TMSs in the following areas are considered to

have the intense rainfalls, steep slopes, and the geologic sub-

strate that produce coarse-grained material to maintain the

morphology created during large floods (Gupta, 1988): (1)

river valleys of East Asia, especially Taiwan and the Philip-

pines; (2) upland areas of Vietnam, Sumatra, Java, and Burma;

(3) humid areas of the Indian subcontinent; (4) Madagascar

and neighboring parts of coastal East Africa; (5) North and

Northeast Australia; and (6) Caribbean basin and highlands of

Central America.

p0205 In some regions, the morphology of tropical stream

channels has also been related to a pronounced seasonality in

stream flow. Streams in the seasonal dry Kimberley Plateau of

tropical Australia have a unique channel system where narrow

bedrock-lined reaches alternate with wider alluvial reaches

that have sandy ridges and anabranching channels (Wende

and Nanson, 1998). In areas with large storms and large

seasonal fluctuations in discharge, alluvial reaches of TMSs can

have a nested morphology that consists of a large storm flow

channel and a smaller channel that carries interstorm dis-

charges (Gupta, 1995). The interstorm channels are box

shaped and have high banks and high width–depth ratios. The

high-magnitude floods can occupy the entire valley bottom

and are sufficiently frequent that the high-flow channels fea-

tures are maintained. This type of multiple low- and high-flow

channels appears to be most common and pronounced in

areas subject to monsoon rains.

s01009.31.4.4 Floodplains and Riparian Zones

p0210Continuous alluvial floodplains (see Chapter 9.22 Flood-

plains (00247)) or riparian zones (see Chapter 9.14 Re-

ciprocal Relations between Riparian Vegetation, Fluvial

Landforms, and Channel Processes (00239)) are rare in TMSs.

Instead, channel margins are most commonly boulder-lined,

steeply sloping hillslopes, or consist of smaller patches of al-

luvium associated with tributary junctions, slope breaks, or

former slope failures. Floodplains and associated terraces are

more common in middle and lower reaches of the drainages.

In Fiji, cesium profiles in floodplain sediments indicate ac-

cretion rates of 3.2 cm yr�1 over the past 45 years (Terry et al.,

2002). These high accretion rates are attributed to the high

frequency of tropical cyclones that pass the area (40 between

1970 and 2002).f0015 Figure 2 The Rio Mameyes River in the Luquillo Mountains of

Puerto Rico.

f0020Figure 3 Headwater tropical montane stream in the LuquilloMountains of Puerto Rico.

MORP 00256

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p0215 Although alluvial riparian zones along TMSs are dis-

continuous and typically occupy less than 10–15% of the

landscape, they can be significant sources of storm flow

(Schellekens et al., 2004) and have large influences on light,

temperature, and the carbon and nitrogen chemistry of TMS

water (Chestnut and McDowell, 2000; Chestnut et al., 1999;

Heartsill-Scalley and Aide, 2003; MacKenzie, 2008). In the

subtropical wet forests of the Luquillo Mountains in Puerto

Rico, steep topographic and hydraulic gradients between the

riparian zone and the stream were responsible for nearly

constant inputs of groundwater to these first- to third-order

streams (McDowell et al., 1992, 1996). However, the abun-

dance and hydrologic and biogeochemical influence of ripar-

ian zones were highly dependent on geology such that areas

underlain by granodiorite that weather into deep sandy soils

have stronger hydrologic and biogeochemical connection than

areas underlain by volcanic rocks that weather into clay. These

floodplains tend to be smaller and are characterized by surface

drainage and periodically dry surface soils.

p0220 The floodplains of larger lowland tropical streams offer

striking and well-documented examples of how the frequency

and duration of flooding and floodplain soil saturation are

linked to patterns of forest structure and biodiversity (Salo

et al., 1986; Kalliola et al., 1991; Mertes et al. 1995; Hamilton

et al., 2007 and others). Relationships between fluvial pro-

cesses and riparian vegetation have also been documented in a

few TMSs. Riparian zones along the first- to third-order TMS of

the Luquillo Mountains of Puerto Rico do not have distinct

riparian species or riparian communities but do have distinct

understory species (Heartsill-Scalley et al., 2009). Relation-

ships between the flood frequency and the structure of ripar-

ian vegetation have been documented in these streams (Pike

and Scatena, 2010). The width of their riparian zones defined

on the basis of canopy cover, understory vegetation, and soil

drainage averages 22 m for perennial channels and 10 m for

intermittent channels (Scatena, 1990). For comparison, tim-

ber harvesting guidelines for Australian tropical forests require

leaving a minimum strip of undisturbed forests of 10 m for

streams draining less that 60 ha and 20 m for channels

draining 100 ha or more. No buffer protection is required

where channels are less than 5m wide. In Peninsular Malaysia,

the amount of logging-derived sediment reaching the channel

declined after 40 m but the overall effectiveness of riparian

buffers depends on the hydrologic connectivity between hill-

slope and channel buffers (Gomi et al., 2006). In summary,

these studies indicate that TMSs can have distinct zones of

riparian vegetation that are on the order of 10–40 m wide. For

comparison, in the relatively flat terra firme landscape of the

Central Amazon, riparian zones defined by a distinct, high-

diversity riparian herb community can be 100 m wide

(Drucker et al., 2008).

s0105 9.31.4.5 Role of Instream Wood

p0225 Logjams and accumulations of coarse woody debris (CWD)

are known to play an important role in structuring the

morphology and habitat in forested temperate streams (see

Chapter 9.11 Wood in Fluvial Systems (00236)). The few

studies that have investigated CWD in tropical streams

indicate that although TMSs lack beavers and other large river

dwellers, debris packs created by CWD, palm fronds, and fine

litter do provide important habitat and food resources to the

detrital-based aquatic food webs of TMSs (Covich and Crowl,

1990). However, CWD appears to be less abundant in TMSs

than in some temperate counterparts. A detailed survey of 26

montane to lowland stream reaches in the Dominican Re-

public indicated that 62% had measurable CWD, but no reach

has more than 5% woody debris cover (Soldner et al., 2004).

In first-order TMS in a pasture–forest landscape mosaic in

Puerto Rico, the amount of CWD tended to increase with

forest cover and there were positive relationships between tree

cover and percentage of dissolved oxygen, and negative rela-

tionships between tree cover and percentage of substrata

covered by fine-grained sediments from eroded soil (Heartsill-

Scalley and Aide, 2003). A 4-month CWD addition experi-

ment in pools in headwater TMS streams also indicated that

the CWD additions were correlated to changes in aquatic

species composition but had no effect on the total number of

freshwater shrimp per pool area (Pyron et al., 1999).

p0230A study of the transport of numbered, 2-cm-diameter

hardwood dowels in a second-order, boulder-lined stream in

the Luquillo Mountains indicated the dowels are dispersed in

a negative exponential pattern and have high retention at the

reach scale, even during large, hurricane-related storm flows

(Covich and Crowl, 1990). This high retention is attributed to

CWD, palm fronds, and other plant material being entangled

in crevices between boulders. It has also been noted that

suspended sediment concentrations during hurricanes can be

lower than predicted from concentration–discharge relation-

ships derived from nonhurricane storms of similar magni-

tudes (Gellis, 1993). Apparently, defoliation by the hurricane-

force winds created temporary debris dams that trapped

sediment and reduced suspended sediment concentrations.

Nevertheless, because relatively high stream flow can persist

for several days, the total sediment transported during the

passage of a hurricane can be significant (Gupta, 2000; Warne

et al., 2005).

p0235Unlike some cold temperate streams where CWD dams can

last for decades or centuries, the CWD in TMS is removed on

the order of years. In the Malaysian State of Sabah, CWD dams

have an average life span of approximately 1 year, although

some can exist over 10 years (Spencer et al., 1990). In the

Upper Rio Chagres Basin of Panama, large wood dams pro-

duced by a widespread flooding and landsliding event lasted 2

years or less and the fluvial system appears to alternate be-

tween brief periods of moderate wood load and long periods

without (Wohl et al., 2009). Although a few of the CWD dams

that are produced by hurricane defoliation and uprooting in

the Luquillo Mountains of Puerto Rico last as long as 5 years

in some headwater reaches, majority of CWD dams in head-

water streams were broken and redistributed within less than 6

months and CWD dams have not been observed in third- and

fourth-order streams.

p0240The long-term average rate of CWD inputs into most TMSs

is unknown, but it should be similar to that observed in

temperate environments because tree mortality and the rate of

stand turnover are similar in tropical and temperate forests

(Lugo and Scatena, 1996). However, in hurricane-impacted

areas, and in areas undergoing deforestation, the average

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annual rate may be larger or at least more episodic. Available

decay rates of CWD tissue in warm tropical streams indicate

they can decay between 16% and 30% over 3 years (Beard

et al., 2005). These relatively rapid decay rates, combined with

the high frequency of storm flows and episodic inputs, ap-

parently interact to reduce the life span of CWD accumu-

lations and their ultimate influence on channel morphology.

s0110 9.31.5 Response to Anthropogenic Disturbances

p0245 Because TMSs drain hillslopes with abundant weathering

products and because they have many storms per year and

high sediment loads, it could be assumed that they can recover

from formative events or adjust to new environmental con-

ditions faster than their temperate or arid counterparts.

However, because many TMSs are also supply-limited with

respect to fine sediments, they may not have the material

needed to rapidly reform and adjust their channels in re-

sponse to environmental changes. Although time will tell

whether the presumed effectiveness in restorative processes of

TMSs actually is true or part of the dynamic but

stable mythology associated with TMSs, there is no doubt that

TMSs are undergoing significant changes because of human

activities.

s0115 9.31.5.1 Land-Use Change

p0250 The influence of land-use change (see Chapter 9.39 Impacts

of Land-Use and Land-Cover Change on River Systems

(00264)) on hydrologic process and runoff of TMS has been

documented in several locations (see many examples in Bonell

and Bruijnzeel (2004)). In general, the conversion of forested

watersheds to pastures and cropland increases erosion, runoff,

and sediment yields. Available studies indicate that PO4, K,

and Mg concentrations increased considerably with urban-

ization and the water-quality changes associated with agri-

culture and urbanization in the humid tropics are of similar

magnitudes and directions as temperate streams (Santos-

Román et al., 2003; Ramirez et al., 2009). However, the long-

term geomorphic response of tropical stream channels to

land-cover change and urbanization (see Chapter 9.41 Ur-

banization (00266)) is poorly quantified and no studies have

focused on TMSs (Douglas, 1978; Gupta, 1982, 1984, 2010;

Gupta and Ahmad, 1999; Ebisemiju, 1989a, 1989b; Chin,

2006; Jeje and Ikeazota, 2002; Ramirez et al., 2009). In humid

temperate environments, streams commonly aggrade during

the initial phases of urbanization in response to the increased

sediment loads that are generated during construction. As the

urban landscape becomes established, channels then tend to

enlarge in response to an increased frequency of high-flow

events that carry less sediment. A recent review of the limited

studies from urban tropical areas suggests that channel en-

largement from tropical urbanization tends to be smaller in

magnitude compared to temperate counterparts (Chin, 2006).

Slight downstream decreases in channel size have also been

observed in coastal plain streams in Puerto Rico and have

been related to the presence of sediment deposited in earlier

agricultural periods (Clark and Wilcock, 2007AU10 ). By contrast, in

streams draining established urban areas of Puerto Rico, the

amount of channel incision does not appear to be correlated

with urbanization and river connectivity seems to be more

important than urbanization in determining fish assemblage

composition (Ramirez et al., 2009). Unfortunately, existing

studies on the impacts of urbanization on tropical streams

have been short term and may be biased toward the initial

stages of construction and aggradation. Thus, the long-term

influence of urbanization or other land-cover changes on the

morphology of TMS is uncertain. However, given that these

systems have few alluvial reaches and already capable of

transporting more fine sediment than is supplied, they are not

expected to undergo the same pattern of aggradation and

widening as alluvial reaches in humid temperate areas.

s01209.31.5.2 Dams and Water Diversions

p0255Because of their high runoff and montane settings, TMSs are

often well suited for hydroelectric generation or gravity-driven

water diversions (Benstead et al., 1999; Pringle et al., 2000;

Brasher, 2003; March et al., 2003). In Central America,

hydropower from TMSs already generates approximately 50%

of the electricity and the number of dams and diversions is

expected to continue to increase in the future (Anderson et al.,

2006a, 2006b). From an ecological view, dams and diversions

can be similar to extended droughts and result in a reduction

in resident and migratory habitat and the crowding and ac-

celerated mortality of individuals (see Chapter 9.40 Flow

Regulation (00265)). The cumulative effects of these alter-

ations can be a decrease in riffle habitats and in the number of

fish species immediately downstream from the dams. The

cumulative impacts of multiple hydroelectric dams releasing

water at the same time each day are unclear but of concern to

many residents who live downstream of TMSs.

s01259.31.5.3 Climate Change

p0260As noted earlier, the climate of most TMSs has changed in the

past and will change in the future (see Chapter 9.42 Climate

Change (00267)). The general expectation is that in the next

century tropical mountains will undergo warming and drying

that will result in an upward shift in life zones (Colwell et al.,

2008). Local and upwind deforestation can also influence

precipitation patterns in the watersheds of TMSs (Bruijnzeel

et al., 2010). The relatively high atmospheric inputs of nutri-

ents and efficient internal nutrient cycles suggest that the

biogeochemical systems of tropical montane forests will rap-

idly adjust to future environmental changes (Bruijnzeel et al.,

2010). How and when the morphology of TMSs will respond

to climatic and environmental change is less certain. However,

if the future resembles our admittedly poor understanding of

the past that is discussed above, widespread drying should

result in cut-and-fill episodes in alluvial reaches in the foot-

hills of TMSs. Increases in precipitation and forest cover

should promote relatively fixed, stable channels with riparian

areas covered by mature rainforest. Changes to the morph-

ology of the steeper gradient, boulder- and bedrock-lined

channels of TMSs are expected to reflect changes in the fre-

quency of slope failures and debris flows. Changes in

MORP 00256

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biogeochemical weathering rates and in the fluxes of water

and sediment from TMSs are also expected and may be good

geoindicators of environmental change in these systems

(Osterkamp, 2002).

s0130 9.31.6 Conclusions

p0265 Although TMSs have a long history of fascinating and con-

fusing geomorphologists, it is only recently that a rudimentary

understanding of their fluvial geomorphology can be de-

veloped from process-based case studies. Whereas the paucity

and restricted geographic distribution of available studies still

limit our ability to develop rigorous predictions of their be-

havior, the emerging view that is summarized below can be

used to guide future research and management:

•p0270 Most TMSs drain orogenic terrains that have not beenglaciated but have undergone climatic changes throughout

the Pleistocene and Holocene. In many areas, early Holo-

cene precipitation was 20–35% above recent means and

40–80% greater than during the drier LGM. Drying and an

upward shift in life zones are expected for the future and an

ongoing challenge is to identify the geomorphic legacies of

these past climatic fluctuations and the response of TMSs

to future changes.

•p0275 TMSs typically receive 2000–3000 mm yr�1 or more ofprecipitation and have a high frequency of intense and, in

some cases, prolonged rainfalls that commonly impact the

entire watershed at the same time. Rainfall and discharge

can be seasonal and temporal changes in runoff co-

efficients are common.

•p0280 TMSs drain steep hillslopes with high drainage densitiesand shallow subsurface storm flow paths that rapidly de-

liver precipitation to stream channels. Their rectangular

channel networks closely reflect regional geologic structure,

whereas the slopes and widths of their segmented longi-

tudinal profiles reflect underlying bedrock. High channel

densities can develop in a few decades but drainage net-

works commonly fail to conform to Horton’s laws of

stream numbers and length.

•p0285 TMSs have high material fluxes from both physical andchemical weathering and are the headwaters of streams

that may contribute between 20% and 40% of the global

fluxes of dissolved load and sediment to the oceans. Their

chemical denudation is strongly influenced by deeply

weathered and thick saprolite and tight internal biogeo-

chemical cycles. Available data suggest that physical de-

nudation averages around 60% of total denudation and the

recurrence interval of landslide-generating events is on the

order of years.

p0290 The net result of these interactions are storm-dominated

fluvial systems that are characterized by a high frequency of

short-duration events that efficiently transport dissolved ma-

terial and fine sediment. Most TMS streams are considered to

be supply-limited with respect to fine-grained sediment and

transport-limited with respect to the large boulders that enter

the channel during debris flows or by in situ weathering. The

stream channels in these systems are characterized by:

• p0295Steep-gradient streams with numerous boulders, rapids, andwaterfalls that alternate with low-gradient reaches flowing over

weathered rock or a thin veneer of coarse alluvium. Knick-point

migration, differential weathering rates, and debris flo-

w–hillslope interactions are all responsible for the devel-

opment of waterfalls and rapids in TMSs. A future

challenge will be to determine the relative importance of

these processes in different tectonic and geologic

environments.

• p0300Better developed downstream hydraulic geometries than theirtemperate montane counterparts. This may be due to some

combination of the lack of recent glaciations and because

the deeply weathered saprolite is relatively deformable

given the high frequency of intense storms they experience.

• p0305Lack of permanent CWD that structures channel and reachmorphology. Although the long-term supply of CWD to

TMSs may be similar or even larger than the supply to

temperate montane counterparts, the combination of epi-

sodic inputs, rapid decomposition, and mechanical

breakdown by a high frequency of storms apparently re-

duces the residence time and overall geomorphic influence

of CWD.

• p0310Poorly developed and discontinuous floodplains. Distinct ri-parian zones can be identified on the basis of soils and

vegetation and typically extend 20–40 m from either side

of headwater channels. Because shallow subsurface flow

commonly passes through wet and nearly saturated ripar-

ian soils before entering TMS channels, the biogeochemical

transformations within the riparian zone can have a dis-

proportionate influence on stream-water chemistry.

Therefore, establishing riparian buffers zones can be an

effective best management practice in these systems.

• p0315Migratory aquatic species that are well adapted to floods and tomigrating steep bedrock channels. Their spatial distribution

within TMS networks is directly related to the distribution

of waterfalls or anthropogenic barriers. The high frequency

of bedload-transporting storms combined with continual

herbivory interacts to suppress the abundance of periph-

yton and aquatic plants. Consequently, rates of respiration

are much higher than most forested temperate streams.

p0320As the above review suggests, TMSs do not have diagnostic

landforms that can be solely attributed to their low-latitude

locations. However, there is general agreement that TMSs are

distinct fluvial systems. The distinctiveness of TMSs appears to

result from a combination of high rates of chemical and

physical weathering and a high frequency of significant geo-

morphic events rather than the absolute magnitudes of indi-

vidual events or processes.

p0325It is generally assumed that stream channels tend toward

quasi-equilibrium morphologies because their beds and pro-

files deform and adjust in response to changes in discharge

and sediment supply. In many TMSs, described relationships

between bedrock lithology, channel slope, and channel

morphology suggest that these channels do adjust and estab-

lish quasi-equilibrium morphologies. However, the abun-

dance of large and relatively immobile boulders and their lack

of fine-grained alluvial deposits suggest that the restorative

processes in these systems may be less responsive than those

where fine-grained sediment is actively involved in rebuilding

MORP 00256

12 Streams of the Montane Humid Tropics

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and sculpting channels between formative events. A future

challenge in understanding and managing these systems

under ever-increasing anthropogenic pressures is to dis-

tinguish the formative and restorative events that sculpt these

landscapes and maintain their aquatic resources and contri-

butions to regional biogeochemical cycles.

Uncited references

p0330 Blum (2007), Clark and Wilcock (2000), Gabet et al. (2004),

Harden and Scruggs (2003), Hare and Gardner (1985), Har-

mon (2005).

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