human impacts on the stream–groundwater exchange...

Human Impacts on the Stream–GroundwaterExchange ZonePETER J. HANCOCK*Ecosystem ManagementUniversity of New EnglandArmidale, N.S.W. 2351, Australia

ABSTRACT / Active exchanges of water and dissolved mate-rial between the stream and groundwater in many poroussand- and gravel-bed rivers create a dynamic ecotone calledthe hyporheic zone. Because it lies between two heavily ex-ploited freshwater resources—rivers and groundwater—thehyporheic zone is vulnerable to impacts coming to it throughboth of these habitats. This review focuses on the direct andindirect effects of human activity on ecosystem functions ofthe hyporheic zone. River regulation, mining, agriculture, ur-ban, and industrial activities all have the potential to impairinterstitial bacterial and invertebrate biota and disrupt the hy-drological connections between the hyporheic zone and

stream, groundwater, riparian, and floodplain ecosystems.Until recently, our scientific ignorance of hyporheic processeshas perhaps excused the inclusion of this ecotone in rivermanagement policy. However, this no longer is the case as webecome increasingly aware of the central role that the hyporheiczone plays in the maintenance of water quality and as a habitatand refuge for fauna. To fully understand the impacts of humanactivity on the hyporheic zone, river managers need to work withscientists to conduct long-term studies over large stretches ofriver. River rehabilitation and protection strategies need to pre-vent the degradation of linkages between the hyporheic zoneand surrounding habitats while ensuring that it remains isolatedfrom toxicants. Strategies that prevent anthropogenic restrictionof exchanges may include the periodic release of environmentalflows to flush silt and reoxygenate sediments, maintenance ofriparian buffers, effective land use practices, and suitable ground-water and surface water extraction policies.

As global exploitation of both stream water andgroundwater increases, it is becoming more evidentthat managers need to develop an awareness of thelinkages between these two systems, the roles that theselinkages play in maintaining water quality, and howhuman activities may impair them. The impact of hu-man activity on surface stream habitats has been exten-sively studied throughout the world (Hynes 1960, John-son and others 1997, Townsend and others 1997,Parker and others 2000), and this scientific informationis now the basis for many stream restoration efforts(Stanford and others 1996, Wissmar and Beschta 1998).However, the surface stream forms only the visible partof a continuous freshwater ecosystem that includes thegroundwater, alluvial, and riparian systems (Gibert andothers 1990). Central to all of these is the area ofexchange called the hyporheic zone (Figure 1), whichoccurs in the sediments of many gravel- or sand-bedstreams. The hyporheic zone is a dynamic ecotonebetween streams and groundwater and, as well as host-ing a unique and diverse invertebrate fauna (Mar-

monier and others 1993), is frequently the site of in-tense biogeochemical activity (Morrice and others2000).

Exchanges between the hyporheic and surround-ing surface, groundwater, riparian, and alluvial flood-plain habitats occur over a wide range of spatial andtemporal scales (Boulton and others 1998). The vol-ume and rapidity of these exchanges varies greatly,governed by surface water discharge, bed structure,and ambient conditions. Exchanges are necessary, asinflowing water brings with it material to support theresident bacteria and invertebrates that play a role inthe “biological filter” of the hyporheic zone. How-ever, this reliance on exchange also makes the hypo-rheic zone vulnerable to contaminants that impacton subsurface processes. As the hyporheic zone andits linkages to other habitats are seldom consideredin planning decisions, the potential for disturbanceoriginating from human activity (Table 1) is substan-tial. This paper synthesizes scattered ecological liter-ature to illustrate how instream and catchment scalehuman activity impacts on the hyporheic zone. As thefocus is from a river management and restorationperspective, the paper concludes with several recom-mendations for the effective management of streamswhere the hyporheic zone is an important ecosystemcomponent.

KEY WORDS: Human impacts; Hyporheic zone; Mining impacts; Agri-cultural impacts; Urban impacts; Industrial pollution; Riverregulation; Sedimentation; Stream restoration; Streammanagement

*email: [email protected]

DOI: 10.1007/s00267-001-0064-5

Environmental Management Vol. 29, No. 6, pp. 763–781 © 2002 Springer-Verlag New York Inc.

The Hyporheic Zone

Exchanges between surface and groundwater in po-rous-bed streams create a transitional ecotone calledthe hyporheic zone (Orghidan 1959, Schwoerbel 1961,Gibert 1991, Sabater and Vila 1991). It is characterizedby the presence of water moving into, through, and outof the sediment interstices in a downstream direction(Figure 2). In many rivers the hyporheic zone containsa unique invertebrate fauna, termed the hyporheos(Williams and Hynes 1974), which is composed of sur-face and subsurface species. It also contains many formsof fungi and microbes that transfer, release, and stabi-lize some forms of transient nutrients. The size of thehyporheic zone depends on the extent and strength ofthe surface water–groundwater interactions, which area function of sediment porosity, channel morphology,strength of groundwater upwelling, and stream dis-charge (Dahm and others 1998). However, although ithas been more than 65 years since hyporheic researchbegan (Karaman 1935), a specific set of quantitativedefining criteria for this zone is difficult to find (Find-lay 1995). Its boundaries have been described usinggradients in temperature (White and others 1987), dis-continuity break-lines in alkalinity and oxygen (Wil-liams 1989), and the composition of the hyporheos(Williams 1989, Boulton and others 1992, Bretschko1992), but with limited success. Triska and others(1989) used chemical tracers to define the hyporheiczone as being the area where 98%–10% of water orig-inated from the surface stream. This definition goessome way toward quantifying and illustrating the vari-ability of hyporheic boundaries and is a suitable defini-tion for this review. As well as extending vertically belowa stream, the hyporheic zone reaches laterally into thesediments beside the stream and the floodplain (Figure2). This lateral extension is often referred to as theparafluvial zone (Holmes and others 1994, Claret andothers 1999).

Exchange between surface and subsurface water maybe the most important regulator of biological activity inthe hyporheic zone. Without flow to renew nutrientsand oxygen and to flush wastes, the sediments becomeunsuitable for the majority of life that inhabits them.The importance of hydrologic fluctuations in control-ling water and nutrient exchange between the hypo-rheic zone and surface, groundwater, riparian, andfloodplain habitats is recognized in the dynamic eco-tone model of the hyporheic zone (Gibert and others1990, Vervier and others 1992). Fluctuations in hydrau-lic and geomorphological variables mean that theboundaries and flow paths of water through the inter-stices of the hyporheic zone are constantly changing. Insome large floodplain rivers the hyporheic zone canextend laterally for several kilometers [e.g., FlatheadRiver in Montana, USA (Stanford and Ward 1988, Stan-ford and others 1994)], vertically for some meters [e.g.,Rhone River in France (Marmonier and others 1992)],and have a volume that is several times greater than thatof the surface stream (Valett and others 1990). Hypo-rheic zones in upland streams tend to be less extensive,but still make important contributions to some streamecosystem processes (Boulton and Foster 1998, Morriceand others 2000).

Importance of the Hyporheic Zone

In terms of biodiversity, the hyporheic zone may notharbor as great a number of invertebrate species asother ecotones (Gibert and others 1990). However, asan area of intermediate biodiversity between the streamand groundwater, it is relatively species-rich, especiallyin evolutionarily ancient taxa (Boulton 2001). Its roleas a refuge for surface invertebrates has been well doc-umented (Grimm and others 1991, Boulton and Stan-ley 1995, Brunke and Gonser 1997), as has the occur-rence of groundwater dwelling Crustacea (Marmonierand others 1992) and other groups that occur exclu-sively in the hyporheic zone (Vervier and others 1992).Relatively short food webs and a low diversity of foodsources make these organisms sensitive to environmen-tal change (Marmonier and others 1993) and have ledto some suggestions that hyporheic fauna could be usedas biological indicators (Malard and others 1994, Ple-net 1995).

For some species of salmonids, the hyporheic zone isused for spawning and egg incubation (Geist andDauble 1998, Dauble and Geist 2000, Woessner 2000).These processes require loose gravel and an adequateoxygen supply (Kondolf 2000), as well as the bufferedtemperatures (Shepherd and others 1986) that are pro-vided by the interstitial habitat of the hyporheic zone.

Figure 1. The hyporheic zone as a modulator for linkagesbetween the stream, groundwater, riparian, and alluvial aqui-fer systems (after Boulton 2000a).

764 P. J. Hancock

Once the eggs hatch, the larvae of some species maystay in the sediments for up to a month (Dent andothers 2000).

The porous sediments in banks adjacent to streamscan act as buffers to rising water levels and reduce,delay, or even prevent the occurrence of flooding(Gardner 1999). This process is known as bank storage(Brunke and Gonser 1997). The ability of the sedi-ments to absorb a flood through bank storage dependsupon the extent and porosity of the riverbed and itsalluvial floodplain, and the extent of sediment satura-tion already present.

The ecological importance of the hyporheic zone isstarting to be recognized by water managers in someparts of the world (Winter and others 1998, Boulton

2000a). A primary reason for this recognition is therealization of the significance of the links between twoof the main exploitable sources of fresh water in main-taining water quality by biological filtration, supportingbiodiversity and providing buffers for floods.

Physical, chemical, and biological conditions of thehyporheic zone allow it to have a filtering effect on thewater that travels through it (Vervier and others 1992).In the Rhone River the first meter of bed sedimentsfunctions as a filter between surface and interstitialwater, changing the chemistry of the pore-water, metaldistribution, and the hyporheic faunal composition(Gibert and others 1995). Three filtration mechanismscan occur concurrently or consecutively in most hypo-rheic zones. The most obvious is the physical mecha-

Table 1. Human activities with potential to directly or indirectly impact the hyporheic zone

Activity Potential impact

Dam construction Creation of upstream no-flow areas, downstream sediment starvation, barriers formigration of fauna and river channel

Release of cool water from dams, orwarm water from hydrothermaldams

Change in physicochemical conditions in the hyporheic zone, alteration of exchangesthrough instream macrophytes

Channelization and embankments Removal of river bends, which slow water and allow down-welling, isolation of riverfrom floodplain

River regulation and surface waterextraction

Alteration of natural hydrological exchanges between hyporheic zone andsurrounding habitats

Groundwater extraction on alluvialfloodplain

Rapid influx of surface water pollutants, decreased residence time for hyporheicwater

Gravel extraction or mining onfloodplain

Lowering of alluvial water table, disruption of parafluvial flowpaths

Instream gravel, rock, or mineralextraction

Direct removal of hyporheic habitat, disruption of flowpaths, resuspension of silt,destabilization of river channel

High silt loads in runoff fromagriculture, forestry, roads,mining, and urban areas

Clogging of sediment interstices, creating anaerobic conditions

Increased nutrients from fertilizers,effluent, burning, livestock, etc.

Enhanced algal growth in surface water, leading to colmation of the surfacesediments

Failure to fence stock from river Increased nutrients and particulate matter, resuspension of silt, gravel compaction,destruction of riparian plants, introduction of weeds

Permeable irrigation ditches andchannels

Increase risk of contaminants entering groundwater, loss of groundwater or influx ofsurface water

Instream and groundwater salinityfrom agriculture and mining

Changes in physicochemical hyporheic conditions, potentially affecting biofilm andmaking conditions unsuitable for invertebrates

Introduction of exotic plant species Change distribution of gravel bar, change in nutrient inputs and out takes fromstream

Clearing of riparian vegetation foragriculture, forestry, and urbanbeautification

Removal of riparian regulation of groundwater fluxes, increased sunlight on streamleading to more algal growth and higher hyporheic temperatures

Herbicide and fungicide pollution May poison interstitial bacteria, riparian vegetation, or the benthic algae or plantswhich assist with exchanges

Pesticide pollution Poisoning of hyporheic invertebrates and microbesHeavy metal and chemical pollution

from industrial, urban, andmining sources

Poisoning of hyporheic invertebrates and microbes

Increased acidity from acid rain andmine drainage

Alter hyporheic physicochemical conditions and reduce precipitation of dissolvedmetals, may make habitat unsuitable for fauna

Human Impacts on the Hyporheic Zone 765

nism (Figure 3a), where the sediment particles impedethe flow of silt and particulate matter as water entersand moves through the hyporheic zone (Vervier andothers 1992). A second, biological filtering mechanism(Figure 3b) works in a manner similar to the tricklefilters of sewage treatment plants (Ward and others1998). Nutrients dissolved in either ground- or surfacewaters are taken up or transformed by microbial bio-films coating the sediments (Vervier and others 1992,Boulton 1999). The efficiency of biological filtration iscorrelated to microbial activity (Marmonier and others1995). Subsurface invertebrates then feed upon thebiofilms, and the nutrients enter the food chain. Athird filtering mechanism (Figure 3c) is provided pri-marily by the chemical conditions prevalent within thehyporheic zone, which allow the precipitation of dis-solved minerals and metals (Wielinga and others 1994,Harvey and Fuller 1998). The precipitate is thentrapped by the physical filter, where it may be degraded

biologically (Gibert 1990). Therefore, by increasing sol-ute residence time and contact time with substrates inenvironments with spatial gradients in dissolved oxygenand pH, the hyporheic zone influences the biogeo-chemistry of stream ecosystems (Bencala 2000). In Sy-camore Creek, Arizona, USA, the influence of patchysubsurface processes was found to effect surface nutri-ent patterns for up to several kilometers downstream(Dent and others 2001).

Human Activities

Human activities impact the hyporheic zone in twogeneral ways (Boulton 2000a, Dent and others 2000).The first affects transfers between the component sys-tems by impairing water exchange, and the secondimpairs biological activity directly through the poison-ing of invertebrates and bacteria. Often the occurrenceof one will lead to the other. When exchanges between

Figure 2. Aerial and side view of the hyporheic and parafluvial zones showing connections with the stream, groundwater,riparian, and floodplain systems. Dashed arrows show subsurface flow.

Figure 3. Three filtration processes that occur concurrently in the hyporheic zone: (a) physical filtration by the sediment matrix,(b) biological filtration by the microbial biofilm, and (c) chemical filtration by reactions such as mineral and redox processes

766 P. J. Hancock

the surface and hyporheic zone are disrupted by col-mation (i.e., the blocking of interstitial spaces), condi-tions can become anoxic, inhibiting faunal and aerobicmicrobial activity (Brunke and Gonser 1997). Colma-tion is caused by physical (e.g., inputs of silts and finesediments, compaction of sediment), biological (e.g.,overgrowth of microbial biofilms and surface algae), orchemical processes (e.g., clogging by precipitants). Ingeneral, a high level of colmation leads to decreasedoxygen and nitrate concentrations and increases inammonification. Apart from direct hydrologic disrup-tions, the poisoning of the invertebrate fauna from aninwelling toxicant may lead to a blocking of the sedi-ments, as there would be no regulators to prevent theirclogging through microbial biofilm grazing (Gibertand others 1994), burrowing (Palmer and others 1997),and pelletization of fine particles (Danielopol 1989).

The following sections are separated into categoriesof human activity, with each section investigating anactivity’s potential impact on the hyporheic zone. Manyactivities share impacts. For example, mining, agricul-ture, and urbanization all may contribute silt to thehyporheic zone and lead to colmation.

Mining

Heavy metals associated with mining can pollutesurface streams, groundwater, and alluvial aquifers(Paulson 1997), which in turn may limit both the hy-drological exchange and biological activity of the hypo-rheic zone. Runoff that contains silt may lead to colma-tion, but it may also contain high levels of heavy metals(Ciszewski 1998, Soares and others 1999). More di-rectly, gravel mining (Kondolf 1997, Boulton 1999) andalluvial gold and gem mining disturbs and removessediments and resuspends silt, allowing it to block theinterstices of hyporheic zones downstream. In-streammineral extraction can also cause deepening of thechannel, leading to a loss of pool–riffle sequence, bankslump (Bravard and Petts 1996), and a lowering offloodplain water levels (Kondolf 1997). All of thesehave the potential to disrupt hyporheic exchanges bothlaterally and vertically.

The influx of acid or saline groundwater into themine excavation (Bochenska and others 2000), has thepotential to lower the groundwater table through in-creased drainage or evaporation and might also impacton the hyporheic zone if it enters a nearby stream.Increasing the acidity and salinity of down-welling sur-face waters is likely to lead to a general failure in one ormore of the hyporheic filtering mechanisms.

In some cases the hyporheic zone can be a barrierbetween surface and groundwater, preventing or slow-

ing down the release of pollution from one habitat tothe other. Wielinga and others (1994) found that thehyporheic zone of Silver Bow Creek, Montana, USA,acted as an intermediate boundary between groundwa-ter, with low pH and high metal (iron and manganese)concentrations, and surface water, with high pH andlow metal concentrations (Figure 4). The mixing ofhyporheic water and groundwater formed a 10-cm-thick layer where bacteria and pH dropped substan-tially and a dense band of precipitated metals formed.In this case, the specific geochemical conditions pro-vided by the hyporheic zone slowed the rate of releaseof mine-originated pollution into the surface stream.

In the Pinal Creek basin, Arizona, USA, sediment-borne microbial oxidation removed approximately20% of the dissolved manganese pollution resultingfrom copper mining (Harvey and Fuller 1998). Therate of the manganese uptake depends on a balancebetween chemical reaction rates, hyporheic residencetime, and the turnover of stream water through thehyporheic zone. Associated with this increase in man-ganese oxides were decreases in the concentrations ofother metals (cobalt, nickel, zinc), which were taken upduring the oxide formation (Fuller and Harvey 2000).An experiment conducted specifically to test the abilityof subsurface gravel (limestone and river gravel) inremoving manganese from acid mine drainage foundthat a pH of between 6.8 and 7.2, dissolved oxygen(DO) concentration ranging from 3 to 5 mg/liter, anda redox greater than 500 mV provided ideal conditionsfor precipitation (Sikora and others 2000).

The ability of the hyporheic zone to act as a physi-

Figure 4. Precipitation of metals in the hyporheic zone ofSilver Bow Creek, Montana, using Fe(II) as an example (afterWielinga and others 1994).


cochemical filter should not be taken as an invitationfor complacency, as the filter may break down if thechemical conditions change too excessively or if theyare not able to fluctuate. For example, if water drainedfrom a mine increased the acidity of the hyporheicwater, contaminants such as metals could remain dis-solved and toxic to the fauna. Without fluctuations inthe locations of conditions that favor the precipitationof metals, the sediments risk becoming clogged. If fluc-tuations in both hydrological and chemical conditionsdo occur, the distribution of precipitants may be dis-persed over a broader spatial scale, allowing more timefor microbial breakdown.

Changed hyporheic conditions can also lead to thepoisoning of invertebrates from contaminants. Arsenicand iron from a stream in Montana occurred in higherconcentrations in the hyporheic zone than in the sur-face and groundwater (Nagorski and Moore 1999).Furthermore, the dissolved arsenic existed in the hypo-rheic zone as As(III), which is more toxic to inverte-brates. It was thought that the physical and chemicalconditions of the hyporheic zone (pH between 6 and 7,DO levels of 0–3 mg/liter) allowed sediment-boundarsenic to occur in its dissolved form of As(III), whereit then reentered the stream. In this case, the hyporheiczone acts as a source of toxicant to the surface stream,which was derived from a less toxic sediment-boundform. Heavy metals from mining can occur in highconcentrations in river bottom sediments (Ciszewski1998, Soares and others 1999), where they affectbenthic community structure (Clements and others2000). Heavy metals such as zinc and copper (Plenetand Gibert 1994) are also toxic to groundwater inver-tebrates. Losses in the hyporheic invertebrate faunamay impair processes in the rest of the hyporheic zone,as they would not be present to stimulate bacterialactivity and maintain interstitial porosity (Ward andothers 1998, Boulton 2000b).

Urban and Industrial Impacts

Surface Water

Streams flowing through urban areas are usuallysubject to river training works and receive substantialinputs of effluent, stormwater, and industrial discharge(Laws 1993, Williams 1999). Removal of surface waterfor domestic or industrial use can lower the river level,the effects of which may be especially pronounced inarid or semiarid areas. Lower water levels often dimin-ish the volume of the hyporheic zone by decreasing thearea of sediment that is covered by water and by lower-ing the hydraulic gradient, reducing the penetration of

downwelling water into the sediments (Stanley and Val-ett 1991).

River training is not unique to urban environments,and the straightening and enlarging of river channelshave often been used to reduce the frequency of flood-ing (Erskine 1992). This increases flow velocity, causingerosion and removal of bed and bank material (Erskine1992) and a loss of the natural pool–riffle sequence(Clifford 1993) that creates the hydraulic conditionsconducive to hyporheic exchange. The loss of sinuosityin the river reduces the amount of water flowingthrough the parafluvial zone (Figure 5). Increased im-permeable areas, such as buildings, car parks, androads, raise the volume of runoff. Without hyporheicbank storage, the frequency of flash flooding increasesand enhances scouring of sediments. Runoff may alsocontain a number of contaminants that affect the hy-porheic zone. Stormwater from an urban area in Mari-copa County, Arizona, USA, carried sediments contain-ing cadmium, copper, lead, zinc, and arsenic as well asthe organic compounds chlordane, dieldrin, polychlo-rinated biphenyls (PCBs), and toxaphene (Parker andothers 2000). These sediments can clog the interstitialspaces, while the contaminants poison microbes andinvertebrates.

Embanking in some larger rivers (e.g., Danube,Rhone, and Rhine) has long been used to improvechannels for shipping and provide protection againsterosion and flooding (Bravard and Petts 1996). Thishas often involved separating the channel from itsfloodplain via the construction of longitudinal dikes,low submersible embankments, and other structures.The isolation of the river from the floodplain severslinks with the lateral parafluvial zone as down-wellingoxygen rich water is prevented from entering the sed-

Figure 5. Before channelization the sinuosity and gradient ofa river allows hyporheic exchange at riffles and parafluvialzones (a). Channelization reduces sinuosity and increasesstream velocity, removing hyporheic exchange (b).

768 P. J. Hancock

iments. Disruptions such as these, which alter the con-nectivity of interactions that structure ecotones, invari-ably lead to reductions in biodiversity (Ward and others1999).

Channel diversion removes flow going over andthrough the sediments. The effects of diversion areparticularly prominent in riffles. Without surface flow,there is no deposition or erosion in the riffle area, andin a bypassed channel of the Rhone River these pro-cesses were found to have a large role in determininghyporheic community structure by influencing intersti-tial water circulation (Creuze des Chatelliers and Rey-grobellet 1990). Although water may still flow throughthe sediment when surface flows are diverted, causingthe hyporheic zone to become parafluvial, the require-ment for regular flushing flows to prevent the sedi-ments becoming clogged are not met (Schalchi 1992).This can lead to the formation of an impermeablesurface layer, uncoupling the stream and hyporheiczone. Depending upon the extent of the diversion,flushing flows may only occur during flooding, or not atall.

Effluent can be a major source of nutrients down-stream of urban environments, resulting in eutrophica-tion, algal blooms, and colmation (Boulton 2000a).However, the hyporheic zone is often able to transformsome of these nutrients. In the South Platte River down-stream of Denver, Colorado, USA, where 95% of thebase flow consisted of effluent from a treatment plant,denitrification in the anaerobic zone of the sedimentswas a key process (Bradley and others 1995). When thiswas combined with the nitrification of ammonium inthe oxygen-rich area, the predominantly coarse-sandbed was found to be an effective method of removinganthropogenic nitrogen. This example further illus-trates the role of hyporheic zone filtration.

Waterborne pollution from industrial sources cancontribute metal contaminants that alter hyporheic in-vertebrate communities. Surveys of hyporheic inverte-brates from the Rhone River upstream (Plenet andGibert 1994) and downstream (Plenet and others 1996)of the city of Lyon, France [with moderate pollutionfrom chemical and metallurgic industries as well aswater-purification stations (Plenet 1995)] revealed thatthe invertebrate community was affected by metal con-tamination of the sediments, especially iron, chro-mium, and nickel.

Groundwater Extraction and Pollution

Pumping groundwater from wells close to streamscan draw stream water through the hyporheic zone andreduce its residence time, impacting on biological andhydrological activity and thus reducing the efficiency of

the biochemical and physical filters. In an alluvial aqui-fer of the Rhone River, Mauclaire and Gibert (1998)monitored variations in physical and chemical condi-tions (water table, temperature, pH, redox potential,conductivity and dissolved oxygen). They observed thatpumping from a 200 m3/h capacity pump for a mean ofthree hours every day reduced the amount of time ittook for surface water to influence the aquifer from twodays to two hours. This rapid influx of surface watermade the hyporheic zone more vulnerable to surfacewater pollution, and the reduced contact time betweensediments and water probably reduced the effectivenessof biological filtration. Alternatively, Bourg and Bertin(1993) suggested that by careful positioning of bores inalluvial aquifers, some pollutants could be filtered fromthe water. It was found that the alluvial sediments of theLot River, France, effectively filtered out zinc within thefirst 10–15 m of sediment. However, they also note thatin some sediments, biological activity can have detri-mental effects, such as the release of iron and manga-nese due to the dissolution of their respective oxides.

Pollution of groundwater by human effluent is ofconcern in aquifers where septic tanks are in use orwhere the sewer piping is old and prone to leaks. Whilethe hyporheic zone can intercept effluent-pollutedgroundwater and remove some of the nutrients beforeit enters the stream, its ability to effectively removecoliform bacteria is uncertain. Sinton (1986) investi-gated the effects of two septic tank effluent disposalmethods (deep soakage pits and injection bores) onthe microbial quality of an alluvial gravel aquifer inNew Zealand. A 5.5-m-deep soakage pit allowed approx-imately 80% of the effluent to percolate into an uncon-fined aquifer where fecal coliform bacteria were found9 m from the pit. An 18-m-deep injection bore leakedeffluent into a confined aquifer, where coliform bacte-ria dispersed up to 42 m from the bore. A separatestudy (Sinton and others 1997) found bores situated onan alluvial aquifer near Christchuch, New Zealand, tobe contaminated with bacteria originating from efflu-ent irrigation up to 445 m upslope. Coliform dispersalover these distances indicates that bacteria may still beable to reach the hyporheic zone via its linkages withthe alluvial floodplain, and perhaps then enter thestream.

Not much work has been done on the specific effectsof groundwater sewage on the hyporheos, but therehave been several studies of its impact on groundwaterinvertebrates that indicate high levels of sewage pollu-tion do impact on the structure of associated faunalcommunities. Epigean organisms dominated thegroundwater fauna of the Terrieu area in France dur-ing low water periods when there was a high sewage


infiltration rate (Malard and others 1994). Whengroundwater recharge from the sewage-polluted Ter-rieu Stream was high, these epigean organisms weredisplaced downstream and the fauna temporarily be-came dominated by hypogean taxa. Sinton (1984)found a depauperate fauna (98% were three crustaceanspecies) in a sewage-contaminated aquifer in New Zea-land. However, a lack of background knowledge hasnot made it clear if this difference was natural or due tothe pollution. About 10% of the crustaceans containedcoliform bacteria and high sewage contamination cor-responded with high crustacea mortality rates. Wilsonand Fenwick (1999) also suggest that effluent-pollutedgroundwaters may create a suitable habitat for the iso-pod Phreatoicus typicus. The high death rates found bySinton (1984) may be a result of some detrimentaleffect of the bacteria on groundwater invertebrates andthe organic matter, low oxygen, and enriched nutrientlevels often associated with effluent. Because of theoverlap between the groundwater fauna and the hypo-rheos, it is likely that they share similar responses toeffluent pollution. Groundwater invertebrates may usethe hyporheic zone as a refuge when conditions deeperdown become unsuitable.

River Regulation

Changes in Flow Regime

Rivers are regulated to provide water for variouscatchment land uses, for hydroelectricity generation, orfor flood control or mitigation. Fluctuations in waterlevel alter the strength of hydrological interactions andthe amount of bed exposed to below-stream hyporheicor parafluvial activity (Figure 6). In a Canadian stream,fluctuations in river discharge simulating a hydroelec-tricity regime altered the flow pathways and chemistryof hyporheic water at trout spawning and incubationsites (Curry and others 1994). Although detrimental totrout reproduction in this case, fluctuating water levelsmay be essential for the hyporheic zone to functioneffectively (Brunke and Gonser 1997, Boulton and oth-ers 1998). Natural fluctuations often differ in timing,magnitude, and duration from those induced anthro-pogenically. Stream organisms tend to be linked to thenatural disturbance and flow regimes with which theyevolved (Jones and others 2000) and, as a result, may bevulnerable to any changes in these.

As river regulation alters flow regime, it potentiallyhas far-reaching and permanent spatial and temporal

Figure 6. Expansion and con-traction of the parafluvial zone(PZ) and hyporheic zone (HZ)with changes in surface water(SW) stage: (a) high flow in-creases saturated the hyporheiczone (HZ); (b) base flow main-tains HZ below the stream andPZ below the floodplain (FP);and (c) low flows reduce PZ andHZ as groundwater infiltratesmore freely.

770 P. J. Hancock

effects on the hyporheic zone. Under high stream flows(Figure 6a), water enters and travels through the sedi-ments with relative ease. However, if it moves too rap-idly, there will be little time for biological and chemicalfiltration to take place (Grimaldi and Chaplot 2000). Asthe surface flow decreases (Figure 6b, c), so too doesthe volume of water moving through the sediment.Because the water is moving slowly, it deposits silt andfine particulate matter in the sediment interstices, caus-ing colmation (Milan and Petts 1998, Brunke 1999).For some processes, especially reduction, low flowscould be as important to the hyporheic zone as highflows because they provide conditions favorable toanaerobes in saturated areas. After a period of low flow,a greater hydraulic pressure, resulting from high flows,is needed to resuspend the silt and move it rapidlydownstream. The cyclical wetting and drying of sedi-ment in lateral and mid-channel bars may aid in theiroxygenation, since when they are dry, air is able topenetrate the interstices. When river flow levels are tobe regulated, the reliance of hyporheic and other flu-vial habitats on fluctuations in discharge needs to berecognized in order to sustain natural rates of filtration.

Dam Construction

The concept of the hyporheic zone acting as a cor-ridor for the migration of interstitial fauna incorpo-rates channel-aquifer interactions into river continuummodels (Stanford and Ward 1993). Connections be-tween interstitial communities are often only presentthrough alluvial aquifer systems. The Global InterstitialHighway Model (Ward and Palmer 1994) views thealluvial aquifer as the center of a spatially continuoushypogean habitat, which follows a river along its lengthand connects it through groundwater linkages to allu-vial aquifers in other catchments. The construction ofdam walls can sever parts of the interstitial highway bydrowning alluvial aquifers upstream of the wall andisolating those downstream to downstream migration ofhyporheic fauna, many of which do not have the aeri-ally dispersing stages that are present in surface-dwell-ing invertebrates. Dams also act as barriers for thedownstream transport of sediment and movement ofchannel change in up- and downstream directions(Kondolf 1997, Shields and others 2000). Releasingsediment from a reservoir to compensate for its accu-mulation within the profundal zone is one method thatis adopted by some managers to prevent the reservoirsilting up. Below the Guernsey Reservoir, on the NorthPlatte River in Wyoming, USA, sediment releases cor-related with fluctuations in benthic invertebrate com-munities (Gray and Ward 1982). Some groups de-creased initially and then recovered or even increased

in density after cessation of sediment release. The im-pact of such releases on the hyporheic zone woulddepend upon the size distribution of the sedimentsreleased, the rate of release (gradual steady releaseversus pulse releases), bed porosity and roughness, andthe energy of the water used to transport them. Com-pensating for the hyporheic zone when planning forsediment release is complicated by many factors. If toolittle sediment is released, or the water is too fast, thensediment starvation may occur. If the opposite occurs,existing hyporheic zones could be colmated and smoth-ered, or there would be too little water to properly sortthe sediments.

Reservoirs that reduce the frequency and durationsof high flows can lead to channel migration and bankerosion (Shields and others 2000). As the channelmoves away from its original course, it abandons theoriginal hyporheic zone. Because high flows have beenreduced, there is little down-welling water to flush outsilt to create new interstices for hyporheic colonisation.Lentic areas that occur immediately upstream of damwalls can inundate sediment and remove any movingwater that might allow exchange with the hyporheiczone. Alternatively, continuous erosion downstream ofthe wall could scour the bed sediments to such anextent as to expose bedrock (Kondolf 1997), therebyremoving the hyporheic zone.

With many dams there is often a temperature differ-ence between the stream above the reservoir and thewater being released. In the Dyje River in the CzechRepublic, the water temperature regimes were re-corded upstream and downstream of the Vranov Res-ervoir (Helesic and others 1998). Fluctuations of0–23°C above the reservoir fell to 2–14°C downstreamof the wall. A decrease in temperature may encouragesurface-dwelling fauna to take refuge in the hyporheiczone (Sterba and others 1992) potentially changing thecomposition of the fauna through competition or pre-dation. Cooler hyporheic temperatures will slow bacte-rial activity (Hill and others 2000), and the lower oxy-gen levels often associated with hypolimnic water wouldchange the predominant biogeochemical processes toreductive ones. Changes in temperature can alter thecomposition of instream macrophytes, affecting the ex-changes that occur between their surface leaf and sub-surface root regions (White and Hendricks 2000). Res-ervoirs may also increase the amount of water flowingthrough the hyporheic zone by preventing the riverfrom freezing in winter. Regulated flows from HungryHorse Dam on the Flathead River, Montana, USA, com-bined with warm water from the Kalispell Valley aquiferallow hyporheic activity to continue throughout the


winter despite air temperatures of �30°C (Stanfordand others 1994).

Agricultural and Forestry Impacts

In many regions agricultural and forestry activitiesbring about catchment-wide alterations to the land-scape, potentially affecting the hyporheic zone (Table2). These impacts result from excessive nutrients, live-stock trampling and grazing, the modification of natu-ral riparian buffers, increased silt load, pesticides andherbicides, salinization, and hydrological alterationscaused by landscape modification, and groundwaterand surface water extraction. As many of these impactsact in concert, the effects of individual adverse pro-cesses are hard to isolate.

Excessive Nutrients

Agriculture is a major source of nitrogen and phos-phorus in aquatic ecosystems. The sources of thesenutrients may be diffuse and originate from animalwaste, fertilizers, or the ash of forestry waste or stubbleburned after harvest. High nutrient levels can lead toalgal blooms, loss of oxygen, and other problems thataffect surface waters (Carpenter and others 1998). Theyalso impact on the hyporheic zone (Table 2).

Anoxic down-welling water impairs interstitial bio-logical oxidation and in doing so reduces the efficiencywith which the biochemical filters are able to deal withan excess in some nutrients. Bacterial nitrification re-quires oxygen, as do the invertebrates that graze thebacteria, preventing clogging and stimulating growth(Ward and others 1998, Boulton 2000b). However, pro-cesses such as ammonification may increase under an-oxic conditions, accelerating nitrogen removal in thisway. Phosphorus tends to accumulate in hypoxic areasof the hyporheic zone. If oxygen levels become too low,the sediment-bound phosphorus is freed and can be

released into the stream or absorbed by periphyticbiofilm on the surface of upwelling zones (Hendricksand White 2000). Algal blooms brought about by highnutrient levels can lead to a clogging of the surfacesediments at down-welling zones, preventing exchangebetween the stream and hyporheic zone (Schalchi1992, Boulton 2000a).

Filtration of nutrients in the hyporheic zone can berelatively successful in agricultural catchments if condi-tions are suitable. A study of two moderate-intensityagricultural catchments in France (La Roche and Mou-linet) found that substrate origin governed how rapidlythe hyporheic zone was able to process nitrate(Grimaldi and Chaplot 2000). Granite-derived sub-strate, which had a sandy or peaty texture, allowed theinflow of nitrate-rich stream water to regions of deni-trification in the hyporheic zone. As a result, streamnitrate concentrations decreased in a downstream di-rection. This was not the case when the substrate isderived from schist, and it is thought that the redoxconditions due to low hydraulic conductivity are a keycontributing factor for this (Grimaldi and Chaplot2000).

Livestock

Failure to fence stock out of streams can lead todirect inputs of particulate organic matter and nutri-ents from waste (Harding and others 1999). Largestock such as cattle and horses can also resuspend silt(Fritz and Dodds 1999), compact the gravel in lateralbars and in the bed, and graze or trample riparianvegetation. Stock act as vectors for weeds, which estab-lish themselves on gravel bars, out-compete native spe-cies, and affect processes such as evapotranspirationand bar stability.

Intensive livestock industries (e.g., feedlots, pigger-ies) have the potential to contribute large amounts ofnutrients and silt to a stream if not managed properly.

Table 2. Potential impacts of agricultural activity and forestry on the hyporheic zone and their causes

Impacts Agricultural causes Forestry causes

Nutrients Fertilization, erosion Ash from burning, erosionSilt Erosion, resuspension of instream

silt by stockErosion of exposed postharvest soil

Riparian linkages Clearing and grazing streamsidevegetation

Clearing

Toxic chemicals Seepage from stock dips; herbicidesand pesticides

Herbicides and pesticides

Instream and bar disturbances Unfenced stock; road crossings Road crossingsHydrological Increased runoff in some cases,

decreased runoff due to dams inothers; higher groundwatertables; salinization

Increased runoff after logging;higher groundwater tables

772 P. J. Hancock

Chemicals leaching from stock dips and drenches nearstreams may also poison hyporheic invertebrates andbacteria.

Land Clearing and Modification of Riparian Habitat

Riparian ecosystems are important filters for streamwater quality, removing excessive nutrients and siltfrom overland flows and regulating groundwater dis-charge and chemistry before it enters the stream(Vought and others 1994, Hill 2000). In the WalkerBranch, Tennessee, USA, the riparian zone is a sourceof ammonium and phosphorus to hyporheic waterwhen the dissolved oxygen content is low, but acts as asink for phosphorus when dissolved oxygen levels arehigh (Mulholland 1992), underscoring the links be-tween the hyporheic and riparian habitats. Removingriparian vegetation during land clearing can lead to anincreased likelihood of bank collapse, burying the lat-eral bars and parafluvial zones (Boulton and others1997) and allowing more silt to enter the stream fromup-slope agriculture (Mulholland 1992, Maridet andothers 1996). In addition, greater amounts of light andnutrients can enhance algal growth (Townsend andothers 1997). All of these processes can lead to colma-tion.

Land use was correlated with the composition ofhyporheic fauna in several New Zealand streams nearHamilton, North Island (Boulton and others 1997).Streams draining native forest contained a diverse in-vertebrate fauna of mostly temporary hyporheic taxa(i.e., those which spend only a part of their life cycle inthe hyporheic zone). In streams flowing through pas-ture and in exotic pine forests, the fauna was less abun-dant and lower in taxonomic richness. The shadingeffect of the trees in pine and native forests meant thathyporheic water at these sites was cooler than at thepasture sites, while burial of lateral bars reduced dis-solved oxygen levels in pasture sites.

Exotic plant species could also cause problems forthe hyporheic zone. In Australia, various species ofintroduced willow (Salix spp.), which have a matrix offine filamentous roots, probably impede the subsurfaceflowpaths of water traveling through the hyporheiczone. Due to their deciduous nature, the contributionof willow organic matter to the stream is greater inautumn than the contribution made by the native ev-ergreen eucalypts. This, when combined with lessshaded water, can lead to increased growth in epilithicalgae throughout autumn and winter (Read and Bar-muta 1999). As well as changing the timing of dissolvedorganic carbon (DOC) inputs into the hyporheic zoneand the surface stream, this can also change the type ofDOC, which is required as a source of carbon and

energy for hetertrophic organisms. A change in thetiming, type, or amount of DOC can have detrimentaleffects on hyporheic filtration and stream ecosystemmetabolism. In some areas where eucalypts are intro-duced (e.g., Spain and California) the various impactsmentioned above would be expected to differ predict-ably.

Silt and Sediment

Forestry and agriculture can be major sources of thesilt and fine sediments that clog interstices and causecolmation. Cattle farming and forestry in two Frenchcatchments increased erosion so that silt accumulatedin the sediments (Maridet and others 1996). Fine sed-iment was found to accumulate in the hyporheic areasof Bear Valley Creek in Idaho, USA, where it influencedthe invertebrate colonisation rates and assemblages(Richards and Bacon 1994). Roads associated with for-estry can act as conduits for silt and water. They alsocreate regions of instability which can act as sources forlandslides (Jones and others 2000). The clogging ofsediments with silt will reduce the numbers and activityof hyporheic invertebrates, in turn affecting the poros-ity of interstitial sediments through the absence of theirfeeding and burrowing (Brunke and Gonser 1997).Due to their sensitivity to elevated silt levels, hyporheiccommunities may be potential indicators of poorlymanaged land use practices (Mary and Marmonier2000, Govedich and others 1996).

Agricultural development in the Ozarks area of theUnited States has contributed gravel-sized sediments tothe Current River, Missouri (Jacobson and Gran 1999).Superficially, this could be good for the hyporheiczone, as it provides sediments and goes some way tooffset the losses of gravel from extraction. However,accumulations of instream sediment can lead to chan-nel instability and a decrease in channel storage, lead-ing to the resuspension of silt stored in the floodplainsediments (Jacobson and Gran 1999) and predictableimpacts on the hyporheic zone. Furthermore, thesmothering of surface stream habitats may have impor-tant implications for hyporheic ecology. Siltation ofsalmonid spawning beds impedes fish breeding success(Milan and Petts 1998). To combat this, managers seekmethods to clean fine particles from the gravel. Ofthree such methods investigated in four rivers in south-ern England, pump-washing was found to be moresuitable than both high-pressure jet washing and trac-tor rotovating (Shackle and others 1999), although theauthors suggest that an overall reduction in silt may bemore beneficial. The ensuing loosening of the gravelfrom all these methods could lead to a spatial shift oralterations in the chemistry of the upwelling hyporheic


area, which is generally typified by low oxygen condi-tions. Resuspended silt or sediment may cause colma-tion in downwelling zones downstream.

Agricultural Chemicals

Pesticides impact on invertebrate communities bothin surface (Liess and Schulz 1999) and groundwaters(Notenboom and van Gestel 1992) and can enter thehyporheic zone via either of these pathways. Levels ofthe pesticides atrazine, deethylatrazine, and deisopro-pylatrazine in stream water infiltrating an alluvial aqui-fer of Walnut Creek, Iowa, USA, reached levels between270 and 3060 (�g/day)/m2 where average vertical hy-draulic conductivity through the streambed rangedfrom 35 to 90 m/day (Squillace and others 1993).Pesticides also entered the groundwater through leach-ing, but were recorded at levels two to five times lessand moved at a slower rate than those in surface water.

A preliminary investigation comparing the estimatedgroundwater concentrations of several pesticides to tox-icity data of several species of groundwater crustaceanswas carried out in a Dutch aquifer (Notenboom andvan Gestel 1992). Aldicarb, 1,3-dichloropropene, andethoprophos were at concentrations high enough toexceed European Community risk boundaries, whilemodeling indicated that nine others were at sufficientlevels to pose a potential risk to groundwater ecosys-tems (Table 3).

Concentrations of the lamprey pesticide 3-triflu-oromethyl-4-nitrophenol (TFM) in the hyporheic zoneof Dam Creek, Ontario, Canada, were found to begreatest at a depth of 55 cm one day after treatment(Jeffrey and others 1986). At concentrations highenough to kill juvenile lampreys, TFM has been found

to be toxic to a number of surface-dwelling microinver-tebrate species (Smith 1967). However, of eight taxathat occurred in the hyporheic zone, only tubificidworms decreased in abundance after its application.Although the hyporheic zone is not impermeable toTFM, it may act as a refuge for surface invertebratesduring treatment and as a source for recolonizationafter surface concentrations return to normal. Thereappears to be no literature on the impacts of herbicidesor fungicides on the hyporheic zone. However, linkageswith riparian and instream plants and algae imply thatfurther work in this area may reveal some such impacts.

Salinization

The concentration of salt in running waters is afunction of surface flow regime, catchment conditions,and groundwater discharge patterns (Schofield andRuprecht 1989). In 13 streams in south Western Aus-tralia, hyporheic salt levels at depths of only 20–40 cmoften differed significantly from those in the surfacewater (Boulton and others 1999). In some of thestreams, hyporheic salinity was lower than surface con-centrations by 20%; in others it was 20% greater. Dif-ferences in salinity between hyporheic and surface wa-ter could affect the chemical processes of each habitatand create toxic conditions for invertebrates. In thestreams with saline subsurface water, hyporheic speciesrichness was much lower (A. Boulton and P. Mar-monier unpublished data).

Hydrological Effects

Agricultural and forestry practices alter the hydrol-ogy of a catchment. Large-scale land clearing decreasesthe evapotranspiration, increases stormwater runoffand erosion, and decreases groundwater infiltrationand base flow in streams (Winter and others 1998).Pumping for irrigation from groundwater lowers thewater table and may change the extent and efficiency ofthe hyporheic zone. Pumping from the stream, eitherdirectly onto crops or pasture or into storage dams orirrigation ditches, reduces the hydraulic pressure withwhich water is pushed into and through the hyporheiczone. The lowering of the river height may also reducethe areal extent of hyporheic exchange sites and of thelateral parafluvial zone (Figure 7).

Irrigating from groundwater with pumps situatednear rivers can induce rapid streamwater infiltration ofthe hyporheic zone (Mauclaire and Gibert 1998). Evenwhen pumping is not carried out near streams, inap-propriate groundwater irrigation can lead to problemssuch as salinity and lower water tables for the hyporheiczone (Boulton and others 1999). In recharge areas,increasing groundwater exploitation could lead to in-

Table 3. Pesticides in Dutch aquifer detected ormodelled at levels exceeding the risk boundaries forpesticides in groundwater set by the EuropeanCommunitya

Exceed risk boundaries(detected)

Expected to exceed risk boundary(modeled)

aldicarb cypermethrin1,3-dichloropropene 1,3-dichloropropeneethoprophos fenpropathrin

pendimethalinpirimicarbpirimiphos-methylpropoxurterbufosthiramtrichlorphon

aRisk boundaries: 0.1 mg/liter for separate compounds, and 0.5 mg/liter for mixtures (based on Notenboom and van Gestel 1994).

774 P. J. Hancock

creases in groundwater pollution along shallow flowpaths (Hobma 1997), which could flow into the hypo-rheic zone.

Irrigation ditches can increase the area of surfacewater–groundwater interactions, leading to the infiltra-tion of water into the groundwater system. In the Kara-Kum canal in the Turkmenistan region of Russia, it isestimated that 45%–55% of water is lost through seep-age from permeable irrigation ditches (Zaletaev 1997).In these cases, irrigation ditches probably acceleratethe entry of agricultural chemicals to the groundwaterand then the hyporheic zone. Conversely in arid areasirrigation ditches can increase the loss of groundwaterby evaporation (Zaletaev 1997).

Global Issues

Since stream morphology and hydrology are impor-tant determinants of hyporheic processes and becausethese are largely determined by climatalogical variables,the hyporheic zone is likely to be affected by climatechange (Meyer and others 1999). The predicted in-crease in temperatures and changes in precipitationpatterns linked with global warming are expected toresult in an increase in the influence of groundwatervolume in streams (Winter 2000). In some regions thiscould lead to permanent streams becoming temporaryand to current flow duration in temporary streams

becoming even briefer (Stanley and Valett 1991).Streams fed by groundwater may experience an in-crease in the influence of subsurface flow. This willresult in a diminished hyporheic zone, since exchangewith the surface stream will be less.

It appears that the vulnerability of a stream to cli-mate change depends upon whether its dominantsource of water is through direct precipitation or fromgroundwater recharge. Because of the buffering capac-ity of large groundwater flows, it is thought that thelatter will be more stable (Winter 2000). A greaterfrequency and magnitude of spates and drying in sur-face waters could render the more stable hyporheiczone increasingly important as an invertebrate refuge.Within the hyporheic zone itself, the effects of in-creased periods of drying in the surface stream will leadto lower levels of exchange of down-welling waters, withthe repercussions identified earlier.

Hyporheic pH and redox conditions can also bechanged by acid precipitation, resulting from atmo-spheric concentrations of chemicals like sulfate andnitrate. If the acidic water first leaches through to thegroundwater, there is a chance that it will becomeneutralized before entering the groundwater–streamecotone (Winter and others 1998). However, neutral-ization is less likely for water that enters the hyporheiczone across the stream interface, and over time couldresult in chronic changes in pH and redox conditions.

Figure 7. (a) Plan view and (b) longitudinalcross section showing the effects of low surfaceflows on hyporheic boundaries (dashed lines).


Management Suggestions

Our knowledge of the hyporheic zone has increasedsubstantially over the past two decades. However, we donot understand hyporheic processes as we do those insurface and riparian ecosystems. Effective managementrequires decisions to be based on the best scientificinformation, and close collaboration between manag-ers and hyporheic ecologists is a necessary step inachieving this. We must have river rehabilitation andprotection strategies that consider all the interactionsbetween stream, hyporheic, floodplain, and groundwa-ter components. Strategies that prevent anthropogenicrestriction of exchanges may include the periodic re-lease of environmental flows to flush silt and reoxygen-ate sediments, the planting and maintenance of ripar-ian buffers consisting of native species, effective landuse practices, and suitable groundwater and surfacewater extraction policies. In many cases these policiesare already in place, mostly for the benefit of the sur-face stream, and it is only a matter of extending theirboundaries to incorporate subsurface processes. How-ever some, such as gravel extraction policies in Austra-lia, do not acknowledge the potential effects on thehyporheic zone.

Rehabilitation of a degraded hyporheic zone maynot be too difficult a task in a stream that is undergoingrehabilitation for its surface component. For example,a stream that has a well planted riparian zone in acatchment where land use practices minimize silt, nu-trient, and chemical runoff to the stream may onlyrequire a surge of water to flush silt from the sedimentin order to establish a suitable hyporheic zone forsubsequent recolonization of invertebrates. Somestreams will require more work, including the carefulintroduction of gravel, the loosening of existing gravelby mechanical means, and the reintroduction of bends,large boulders, and logs to induce down-welling andsediment deposition. However, since it is often notpossible to ascertain what hyporheic conditions werelike before the river became degraded, it may be diffi-cult to determine a restoration aim. This may beachieved using a combination of upstream conditions,conditions in similar but unimpacted systems, and thelatest scientific information.

Although increasing numbers of scientists and man-agers are becoming aware of the hyporheic zone, theawareness of the general community does not seem tobe growing. Raising public consciousness should beconsidered as an important step in hyporheic zonerestoration and preservation. A way of achieving this isthrough the inclusion of river users (e.g., rural andurban stream care groups, mining companies, etc.) in

monitoring and rehabilitation programs that incorpo-rate the hyporheic zone as an essential part of a healthyriver (Boulton 2000a). The establishment of a commu-nity-based hyporheic monitoring protocol may includesampling for microbial activity using the cellulosebreakdown potential through loss in tensile strength ofthe standard sized strips of cotton (Boulton and Quinn2000). Sampling for fauna and nutrients will also giveuseful monitoring data. Karaman-Chappuis pits(Mathieu and others 1991) and the pump method(Boulton and others 1992) are two simple techniquesthat can be used for this. The field methods describedabove are relatively cheap and should well be within thebudget of most community stream care groups. How-ever, as the fauna are small and nutrient measurementsrequire expensive equipment and chemicals, the sam-ples may need to be sent away for analysis. The estab-lishment of government funded “hyporheic monitoringcenters” at the state level may be one way of minimizingthe cost here. It is hoped that once people are con-scious of the existence of the hyporheic zone and itsimportance in river ecology, there will be a wider un-derstanding of any management decisions required.

Our lack of knowledge on the inhibition of inverte-brate and microbial activity by toxic levels of chemicalssuch as petrochemicals and agricultural chemicalsneeds to be addressed. More research is required intospecific pollutants, the mechanism by which they reachthe hyporheic zone, and the impact they have on spe-cies.

Many of the examples given in this review comefrom studies that span either a short temporal or smallspatial scales. Impacts from human activity are not soconfined. They may be immediate or take years todecades to show any effects. Similarly they may affectthe hyporheic zone of a single gravel bar or that of anentire river catchment. To assess the true impact ofhuman activity, more long-term studies are requiredover large stretches of river (catchment or trans-catch-ment). Appropriate hyporheic monitoring programscould go some way toward increasing this knowledge.

Conclusion

The hyporheic zone is a central part of most sand-and gravel-bed rivers. Its location at the interface be-tween stream surface water and groundwater exposes itto a variety of anthropogenic pressures. Impacts on thehyporheic zone potentially jeopardize the quality ofwater in rivers and groundwater. The hyporheic zone isable to withstand natural levels of disturbance and isable to transform nutrients, stabilize metals, and re-cover from drying and floods. However, many human

776 P. J. Hancock

activities have the potential to disturb hydrological ex-change and biological activity beyond the thresholdwhere recovery is possible. Managers must recognizethe importance of links between the hyporheic zoneand the surrounding habitats and incorporate this intotheir restoration and management plans.

Acknowledgments

I would like to thank the NSW Department of Landand Water Conservation, the River Basin ManagementSociety, and the University of New England (UNE) forfunding. Much of the research for this document wasundertaken as part of a Masters of Resource Science atUNE. I would like to thank two referees for their re-marks. This manuscript benefited from comments byProfessor Pierre Marmonier (University of Rennes),Associate Professor Andrew Boulton (UNE), MelaniePearson (Murray Darling Freshwater Research Centre),and Jodie Reseigh (UNE).

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