Hyporheic+zones

Author: Ryan Webb

 * __Introduction __**

 The hyporheic zone is one of the more challenging zones of a river to investigate. A basic conceptual definition of the hyporheic zone may be stated as the "saturated interstitial areas beneath the stream bed and into the banks that contain some channel water" (White, 1993). There is no doubt that the hyporheic zone is the zone in which groundwater and surface water interactions occur within an open channel system. However, the boundaries of the hyporheic zone may differ depending on what discipline an investigator focuses in. The hyporheic zone may be bounded based upon various factors such as hydrologically, chemically, zoologically, and metabolically (Brunke & Gonser, 1997). Malard (2002) defines the hyporheic zone as "a three-dimensional mosaic of exchange patches over multiple spatial scales where groundwater and surface water mixing occurs." The Malard definition is how this paper will define the hyporheic zone. Figure one displays a few of the various hyporheic conceptual cross-sections. Often times such exchange patches may be summarized as mainly surface water gaining reaches near the headwaters of a natural stream system, with the mid-reach section of the system containing a number pool-riffle-pool sequences in which flow paths enter and exit the ground numerous times, and finally as a system becomes a large river with a developed floodplain most of the hyporheic exchange is that of groundwater recharge (White, 1993).



 Given a definition of the hyporheic zone, there are also numerous conditions that may alter the boundaries or exchange processes within the zone temporally. A generalized statement can be made concerning the hydraulic conductivity of the top layers of soil being the highest; although, as sedimentation occurs this may change due to silting or clogging of these top layers and therefore the various erosion processes present in a natural system may influence the hyporheicechange through altering the conductivity of the soils (Chen, 2012). In fact, hyporheic pathways have been shown to change with flow pulses from seasonal runoff and precipitation events that drive natural fluvial action and alter geomorphic features (Malard, 2002). In addition to soil conductivity, the hydraulics of a system may also impact the exchange processes within the hyporheic zone. For example, should a hydraulic jump be present flow paths of the hyporheic zone may change to represent downwelling below a pool in which the water is plunging and an upstream directed upwelling at the base of a step that would be inducing such a jump (Endreny, 2011).

 Within the hyporheic zone that has just been described, there are many functions provided such as spawning habitat, biogeochemical processes, as well as aquifer and riparian exchange that are vital for a sustainable, healthy stream. The objectives of this study are to: (1) define the hyporheic functions, (2) state how such functions have been degrade due to anthropogenic influences, and (3) define potential next steps needed for hyporheic restoration to be integrated to stream restoration efforts.

=**__Hyporheic Function __**=

One of the functions of the hyporheic zone is that of providing a spawning habitat but certain species. A study conducted by Malcolm et al. (2003) investigated just how the heterogeneity of exchanges between surface water and groundwater impacts the survival of salmonid eggs in a stream in northeast Scotland. The study provided evidence suggesting that the riparian h<range type="comment" id="110470">illslope and stream channel are hydrologically connected such that the hillslope may be indicative of whether the stream reach is predominantly upwelling or down-welling. Alkalinity is heightened as dissolved oxygen and nitrate levels are low along reaches of the stream in which upwelling water is provided from a groundwater source with higher residence times. Conversely, down-welling reaches displayed higher dissolved oxygen and low alkalinity. Thus<range type="comment" id="540204"> the hyporheic chemistry <range type="comment" id="919732">holds the potential to change rapidly from<range type="comment" id="628551"> influence of local groundwater sources over the course of hydrological events. This influence upon the interstitial chemistry of the streambed then impacts the survival of salmonid eggs. Malcolm et al. (2003) provides results that show how the survival of salmonid eggs can vary considerable with a correlation to this interstitial chemistry. Higher influence from groundwater sources of higher subsurface residence time reduces the survival rate.
 * <span style="font-family: 'Times New Roman','serif'; font-size: 18px;">Spawning Habitat **

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> <range type="comment" id="417344">It has been previously discussed that sediment permeability influences the hyporheic zone. This influence contributes to <range type="comment" id="569653">what type of hyporheic patches <range type="comment" id="128013">are present within a stream reach, and these patch types then contribute to the biogeochemical processes and invertebrate assemblages which occur. The frequency and distribution of hyporheic patches control the nutrient cycling and biodiversity within the hyporheic zone. Smaller patches produce higher rates of nutrient processes whereas larger patches produce lower rates of nutrient processes. A healthy hyporheic zone will have reaches with<range type="comment" id="70238"> both many small patches and large patches. Small patches provide an environment better suited for preferential organism accumulation whereas larger patches are better suited as protection during sever flood events due to higher stability. Representation of biological diversity that may be present in the hyporheic zone can be seen in figure 2 (Malard, 2002).
 * <span style="font-family: 'Times New Roman','serif'; font-size: 18px;">Biogeochemical Processes (biofilms & nutrient processing) & Habitat **



<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> Boulton et al. (2010) conducted a study that shows nutrient rich water that is upwelling from hyporheic exchange becomes a biological “hot spot” of primary productivity whereas the <range type="comment" id="408102">down-welling areas provide organic matter and dissolved oxygen for larger organisms such as invertebrates. Figure 3 displays these different interactions within the stream bed (Boulton et al., 2010).



<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Depending on the flow, biotic activity, and dissolved organic carbon, the hyporheic interface can act as either a source for dissolved organic matter or a sink. The grain size distribution of the sediment <range type="comment" id="653383">holds influence on the storage of organic matter, along with bed load movement. Organic matter is transformed by biota and mobilized following transient and abiotic storage. Through these processes, a biofilm forms on the hyporheic interface in which bacteria remains. In floodplain rivers, this process suggests a strong connection between groundwater and surface water flow in the ecosystem. Figure 4 displays a flow chart of the diffierent factors that influence the formation of these biofilms. (Brunke & Gonser, 1997)



<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> The final function of the hyporheic zone to be discussed in this paper is <range type="comment" id="959206">that of aquifer recharge and riparian exchange. Hyporheic exchange can influence the quantity and quality in water resources. Once more, the conductivity of the sediment will influence the exchange rates. Unconsolidated materials of higher hydraulic conductivity can be potential identifiers for higher recharge areas. Surface water and groundwater exchanges are most commonly associated with unconsolidated aquifer thickness variabilities, contrasts in permeability<range type="comment" id="739133"> form contact between lithologic units, increased hydraulic gradients as a result from formations within the channel, as well as the elevation of the riverbed relative to the base of the aquifer. Figure 5 shows the correlation between geologic units and streamflow gains and losses on the Methow River (Konrad, 2006).
 * <span style="font-family: 'Times New Roman','serif'; font-size: 18px;">Aquifer Recharge & Riparian Exchange **



<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">All of these mentioned factors impact the aquifer recharge and riparian exchange. In an unhealthy stream system, hyporheic exchange can deteriorate and water resources for human use will diminish.


 * __<span style="font-family: 'Times New Roman','serif'; font-size: 32px;">Hyporheic Degradation: __**

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> Once more, sediment hydraulic conductivity has a major impact upon hyporheic exchange. Exchange has been shown to be hindered by a reduction of pore volume from clogging of the top layers of soil beneath a stream (Brunke & Gonser, 1997). The clogging of a streambed can be the result of a number of different scenarios. A once meandering stream system that historically has been in a <range type="comment" id="996418">constant state of flux may be dammed for flood control <range type="comment" id="221497">which leads to consequences such as sediments consolidation without the flushing flows, clogging the top layers of soil (Elosegi et al., 2010). Other anthropogenic impacts can damage the nutrient exchange processes through organic contamination. <range type="comment" id="753532"> <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">For more on sediment transport click here.
 * <span style="font-family: 'Times New Roman','serif'; font-size: 18px;">Loss of exchange **

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> Groundwater flow is driven by gradients in the system ( <span style="color: #000000; font-family: 'Times New Roman','serif'; font-size: 16px;">Freeze & Cherry, 1979). <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> An undisturbed system will allow hydrological events to provide larger flood events that impose<range type="comment" id="12838"> larger gradients upon the hyporheic zone, thus increasing infiltration into groundwater systems for either supply for future upwelling or human consumption of aquifer systems. Numerous human activities have led to the hydromodifications along stream systems for many various reasons. Many of these modifications have caused flows to be lower than historically present within the river systems,<range type="comment" id="256154"> therefore degrading hyporheic exchange.
 * <span style="font-family: 'Times New Roman','serif'; font-size: 18px;">Hydromodifications (lower heads) **

For more information on environmental flows click here. For more about Dams and other structures click here.

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> The past tendancy in the US has been to channelize stream systems and make more homogenous landscapes to support societal growth. <range type="comment" id="317685">This includes, but is not limited to, deforestation in <span style="color: #000000; font-family: 'Times New Roman','serif'; font-size: 16px;">agricultural regions and afforestation in prairie regions which leads to decreased diversity at every level. As a stream becomes straightened, pressure gradients from the bends are lost and exchange becomes lowered. Bank stabilization efforts can also impact the conductivity of the bank <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">(Elosegi et al., 2010).
 * <span style="font-family: 'Times New Roman','serif'; font-size: 18px;">Loss of geomorphic Complexity **

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">For more information on channel geomorphology click here.


 * __<span style="font-family: 'Times New Roman','serif'; font-size: 32px;">Hyporheic Restoration Efforts __**

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> Currently, there is no established protocol for restoration assessment of the hyporheic zone. However, agreement can be found across disciplines and investigators that<range type="comment" id="564341"> permeability needs to be taken into account (Crispell & Endreny, 2009; Hester & Gooseff, 2010; Vaux, 1968). Many restoration actions already in practice may be utilized with additional thought towards hyporheic exchange. These efforts include added morphologic features such as pools, riffles, log dams, steps, and meander bends in order to create slope breaks or backwater to produce the appropriate gradients for healthy exchange. Some of these morphological features could potentially be created in such a way that they retain organic matter, large wood debris could be utilized for an additional source of carbon as well as riparian planting as another organic matter source. Sediment coarsening, which has many benefits for stream restoration, can be used to increase hydraulic conductivity. (Hester & Goseff, 2010)

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">For more information on current methods of channel design click here.

<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Crispell and Endreny (2009) implemented in-channel rock vane structures to attempt <range type="comment" id="324084">and induce <range type="comment" id="255130">a sequence of pool-riffle-pools that can be seen in healthy systems. The study was able to observe exchange patterns consistent with those seen in natural systems. Figure 6 displays photographs of the completed restoration structures.



<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Whatever method or methods that a restoration project implements, two temporal scales must be considered: one for direct impact immediately following restoration and a second for sustainable impact that evolves naturally over time (Hester & Goseff, 2010).

<span style="color: #000000; font-family: 'Times New Roman','serif'; font-size: 16px;"> At present <range type="comment" id="927529">time, the majority<range type="comment" id="535419"> of the research concerning the hyporheic zone is that of understanding the primary functions of the zone. These functions include habitat for a number of species, spawning habitat, nutrient circulation, biogeochemical processes, as well as aquifer recharge and riparian exchange. <range type="comment" id="336494">These functions have been degraded from human influences that have caused the sediments to clog, brought forth hydro-modifications that hinder exchange, and depleted geomorphic complexity that hinders exchange. <range type="comment" id="677897">There is no present protoco l for assessment of hyporheic restoration; however, efforts are beginning to occur taking hyporheic exchange into account. Research needs to continue in this field so that a better understanding can come into view and goals can be set for a healthy hyporheic zone. Once there is enough data to make educated <range type="comment" id="730615">decisions, such restoration practices can be utilized to reach a healthy system both above and below ground.
 * __<span style="color: #000000; font-family: 'Times New Roman','serif'; font-size: 32px;">Conclusion __**


 * __Reference List:__**

Brunke & Gonser (1997), The ecological significance of exchange processes between rivers and groundwater. Journal of Freshwater Biology 37, pg 1-33.

Ward et al. (2011), How can subsurface modifications to hydraulic conductivity be designed as stream restoration sturctures? Analysis of Vaux's conceptual models to enhance hyporheic exchange, Water Resour. Res., 47, W08512, doi:10.1029/2010WR010028.

Endreny et al. (2011) Hyporheic flow path response to hydraulic jumps at river steps: Flume and hydrodynamic models, Water Resour. Res., 47, W02517

Elosegi et al. (2010) Effects of hydromorphological integrity on biodiversity and functioning of river ecosystems. Hydrobiologia 657:199-215

Freeze & Cherry (1979) Groundwater. Prentice Hall Publishing co.

Hester & Gooseff (2010) Moving beyond the banks: hyporheic restoration is fundamental to restoring ecological services and functions of streams. Environ. Sci. Technol. 2010, 44, 1521-1525.

Xunong et al. (2011), Depth-dependent hydraulic conductivity distribution patterns of a streambed. Hydrological Processes2/11: 278-287.

Olivier & Jose (2011), The hyporheic refuge hypothesis reconsidered: a review of hydrological aspects, Marine and freshwater research 62:1281-1302

Crispell et al. (2009), Hyporheic exchange flow around constructed in-channel structures and implications for restoration design. Hydrological processes 23:1158-1168

Malcolm et al. (2003) Heterogeneity in ground water-surface water interactions in the hyporheic zone of a salmonid spawning stream, Hydrological processes 17:601-617

Konrad (2006), Location and timing of river-aquifer exchanges in six tributaries to the Columbia River in the Pacific Northwest of the United Sates, Journal of Hydrology 329:444-470