geomorphology of streams 1: stream classifications...environmental management 10:199–214. braided,...

Geomorphology of streams 1: stream classifications

Dr. John RichardsonFRST 386

courses.forestry.ubc.ca/frst386/

Terms to define parts of a stream network

Classifications of streams

How watersheds are formed

Processes that lead to large-scale features

Outline

Geomorphology (or “fluvial geomorphology”)

Morphology of stream networks – physical arrangements of the pieces, but especially the study of the processes creating those arrangements

Scales from whole watersheds to the exact location of a bacterium on a piece of rock

We’ll include the stream channel, and the surroundings that the water works it way through (watershed, floodplain), including the closest part, referred to as the riparian area.

Catchment

Watershed

Basin (or drainage basin)

From hydrology we would call this the contributing area, i.e., the area generating flow to the channel

In North America we often use “watershed”, but in other places, and more correctly, watershed means the divide between catchments. These three terms are often used interchangeably, but the preferred term is catchment.

Channel

Hillslope (or “upslope”)

Floodplain

Terraces (of different ages)

Parts of a catchment

Channel – characterised by mineral bed material, evidence of fluvial processes

Active channel – reworked on at least annual basisWetted channel – water in the channel at time of observationBankfull channel – margin where annual, or active, channel gives way to the true channel margin or bank

Floodplain – evidence of occasional overtopping of banks

Terraces – built up, former floodplains, now vegetated (usually) and above the floodplain level

Hillslope – the part of the contributing area beyond the floodplain (doesn’t have to have much slope)

Channel

Hillslope (or “upslope”)

Floodplain

Terraces (of different ages)

Parts of a catchment

Hierarchical classification of stream habitats showing approximate linear spatial scales

Stream system - catchments

Frissell CA, Liss WJ, Warren CE & Hurley MD. 1986. A hierarchical framework for stream habitat classification: viewing streams in a watershed context. Environmental Management 10:199–214.

Classifications

We use streams to mean all flowing waters, incl. brooks, becks, branch, creeks, streams, rivers, etc.

In Europe, in particular, there is a system of classification not often used in North America

Krenon or Kryalspring source

RhithronSmall streams

PotamonLarge streams

Stream order

Stream link number (cumulative number of 1st order streams)

Some measure of average annual discharge or unit discharge

flow

Stream orders Proposed originally by Strahler (1954)

Based on a map scale of 1:50,000

1

23

1

1

1

2

2

11

1

3

1

2

3

4

Link size = 12

Stream orders

Depends on map scale

Depends on landscape

Small streams very easily missed, due to canopy closure

Several estimates that ~80% of small, permanently flowing streams are missed (Meyer & Wallace 2000)

Corollary: small streams very heavily influenced by their surrounding forests

Vannote RL, Minshall GW, Cummins KW, Sedell JR & Cushing CE. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences 37: 130-137.

Riparian class

Average channel width (m)

Reserve zone width (m)

Management zone width (m)

Total RMA width (m)

S1 large rivers

≥ 100 0 100 100

S1 (except large rivers

> 20 50 20 70

S2 > 5, ≤ 20 30 20 50

S3 1.5 to 5 20 20 40

S4 < 1.5 0 30 30

S5 > 3 0 30 30

S6 ≤ 3 0 20 20

BC’s Riparian Management Area Guidelines (1995)

BC Ministry of Forests (1995) Riparian Management Area Guidebook

Shaded: Fish stream or community watershed

River Continuum Concept

Vannote RL, Minshall GW, Cummins KW, Sedell JR and Cushing CE. 1980. The River Continuum Concept. Canadian Journal of Fisheries and Aquatic Sciences 37: 130-137.

Gradual transitions in many properties of streams, not easily captured by strict classification

Properties of streams depend on what goes on upstream

A somewhat idealised concept, but useful for many generalisations about streams

Area or length0.1 1 10 100

Mag

nitu

de

Stored sediment

Stream flow

Size of bed material (grain size)

Channel gradient

As stream size increases

Stream flow (Q)

Amount of stored sediment

Channel gradient

Grain size

Tractive force Tg (taug) in N/m2 (also referred to as shear stress)

Τg = ρ · g · R · S

where,

ρ = density of water (1 kg / L)

g = gravitational acceleration (9.81 m/s)

R = hydraulic radius (m) – ratio of cross-sectional area to wetted perimeter

S = slope of the energy line

Τg = ρ · g · R · S

This can be approximated as Tg ≈ D · S

i.e., depth · slope

Tractive force Tg (taug)

Most forces act on stream channel at high flows –forces can be hard to imagine

This is the primary reason we get concerned about peak flows when people discuss forest hydrology

Τg = ρ · g · R · S

This can be approximated as Tg ≈ D · S

i.e., depth · slope

Tractive force

Large, rare events (e.g. 1 in 10 year flood) can impose large changes on a channel

Some features of stream channels may last centuries from rare events, with smaller changes superimposed upon those larger-scale features


Stream reaches and segments


Colluvial Alluvial Bedrock

colluvial braided regime pool/riffle plane-bed step-pool cascade bedrock

Transport Limited Supply Limited

Stream beds made of mineral substrates (organic materials decompose or are washed away)

A stream channel is defined as showing evidence of “fluvial processes” (fluvial = running water) such as bare sediments (e.g. no organic soils) – this is how channels were defined for purposes of the Forest Practices Code (BC 1995)

(from: Montgomery & Buffington 1993)

Alluvial channel – sediment transport and reworking

Note: energy of water at high flow rearranges the channel and its banks

Alluvial channel - Meandering reach

Bedrock dominated

Colluvial – stream power not sufficient to move dominant pieces of rock –these pieces are either in place (e.g. from glacial tills) or fall in from hillslope

Colluvial Alluvial Bedrock

colluvial braided regime pool/riffle plane-bed step-pool cascade bedrock

Transport Limited Supply Limited

These are reach types (may also be applied to stream segments)

Sources of materials to streams

A lot of material forming stream channels comes by way of mass wasting, i.e., slope failures on hillsides delivering rock, soil and wood

Mass wasting occurs naturally, but the rate is often greatly increased by land-use activities such as roads and forest harvesting

In the case of “colluvial” streams, the stream bed works down through existing materials

Channelised debris flows from upstream bring rocks and wood to downstream reaches

Debris flows also occur naturally, but the rate of them tends to increase dramatically in catchments with forest harvesting

Summary

Parts of a catchment (watershed) – channel, floodplain, terrace, hillslope

Classifications – order, for management purposes, etc.

River Continuum Concept as a general model of stream function

Tractive force – process that leads to formation of the morphology (geomorphology) of stream channels

Geomorphology of streams 2: stream channels, channel units, and hydraulic habitat

Dr. John Richardson

FRST 386

courses.forestry.ubc.ca/frst386/

Reaches, channel units, hydraulic habitat (microhabitats)

Substrate types, including large wood in streams

Describing fine-scale habitats

Outline


Reach types and channel unitsGeomorphology


Braided, alluvial reach

Squamish River, BC

Twentieth century disturbance maps for two study reaches along the Queets River in the Olympic Mountains. The 16 dates represented are the years for which accurate spatial data were available.

Naiman RJ et al. 2010. A process-based view of floodplain forest patterns in coastal river valleys of the Pacific Northwest. Ecosystems 13:1-31.

Riffle-pool reach

Step-pool reach

Cascade (bedrock dominated)

Disturbance regimes - channelised debris flows

Infrequent for a given channelHigh magnitude effect

Channelised stream reach

Micro-habitat or hydraulic habitats

“Micro-habitat” depends on scale of organism – fish may find micro-habitat in a channel unit, whereas an invertebrate may select a very specific part of a single rock


Two primary kinds of “substrate” (bottom material)

Mineral – rocks, inorganic particles

Wood – large wood (>10 cm diam.), small wood (<10 cm diam.)

(e.g. trees or branches falling into streams)

Large wood in streams

Buffington JM, Lisle TE, Woodsmith RD & Hilton S. 2002. Controls on the size and occurrence of pools in coarse-grained forest rivers. River Research and Applications 18:507–531.

Functions of wood

• Creates steps that cause plunge pools

• Backs up finer sediments, forming locally gentle slopes (may include spawning gravels)

• Contributes to channel stability (in channel or on the banks)

• At the edge may create back eddies and scour pools

• Cover for fish (hide from flow, hide from predators)

• A source of food for some organisms – fungus, bacteria, some invertebrates

• Traps leaves and smaller particles of organic matter

Richardson JS. 2008. Aquatic arthropods and forestry: large-scale land-use effects on aquatic systems in nearctic temperate regions. Canadian Entomologist 140:495-509.

Fluvial geomorphology and sediment transportEast Creek, BC

Red (12 mm)

Blue (45 mm)

photos: Dr. Marwan Hassan, UBC

Wetmore SH et al. 1990. Characterization of the hydraulic habitat of Brachycentrus occidentalis, a filter feeding caddisfly. J. N. Am. Benthol. Soc. 9:157-169.

Characterising hydraulic habitat – position based on the force and turbulence of flow

SummaryReaches are classified on the basis of their channel units (e.g. riffle – pool, step – pool, etc.)

Channel units created by tractive forces at peak flows – what remains is what resisted the forces

Large wood & boulders are the dominant structural elements in most small to mid-sized streams

Fine-scale habitat can be characterised based on structure or hydraulic characteristics

geomorphology of streams 1: stream classifications...environmental management 10:199–214. braided,...

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