Hyporheic flow influences both biogeochemical cycling in streambeds as well as streambed ecology. Some biogeochemical processes may be temperature dependent; therefore, heat transport associated with hyporheic flow may be an important influence on such cycles. I separately and experimentally assessed the effects of hyporheic flow due to bed topography on thermal dynamics in the sediment using a custom, tilting flume with temperature controls. Diel temperature cycles of 6° C were imposed in the flume and propagation of temperature signals into the sediment was examined for different bed morphologies (plane bed, pool-riffle-pool, and rippled bed), channel flow rates, and sediment grain sizes. Temperature fields in the sediment were monitored using an array of embedded thermistors, and this data was used to identify zones of upwelling and downwelling within the hyporheic zone. Results suggest that bedforms do induce substantially deeper downwelling upstream and downstream of the bedforms, with upwelling near the crest. This in turn leads to substantial advective heat transport and distinct thermal patterns in the sediment. Variation in permeability and channel flow rates further affects the magnitude of this advective transport. These results corroborate existing theoretical models of coupled hyporheic exchange and heat transport under bedforms. Hyporheic flow therefore affects thermal patchiness in sediment, which may in turn exert a control on biogeochemical reaction rates, and form thermal refugia for fauna. / text

Dynamic exchange of water across the stream-riparian zone interface is important in increasing stream water transit time through basins and enhancing redox-sensitive biogeochemical reactions that influence downstream water quality and ecosystem health. Such exchange may be enhanced by beaver dams, which are common throughout low order streams in North America, Europe, and Argentina. Lateral exchanges of water and nitrogen (N) were observed along a beaver dammed, third-order stream draining a 1.3 km2 Canadian Rocky Mountain valley bottom capped in peat. Measurements of hydraulic heads and chloride concentrations from a network of 80 water table wells were used to identify areas of stream water and groundwater mixing in the riparian area, and their spatiotemporal dynamics in summer 2008. Beaver were found to be the greatest factor affecting lateral movement of channel water into the riparian area. Channel water flowed laterally into the riparian area upstream of the dams and back to the channel downstream of the dams. The hyporheic zone expanded by ¡Ü1.5 m in the un-dammed reaches, but upwards of 7.5 m or more when dams were present. High contributions of stream water were found far out in the riparian area where dams were not immediately present within the stream reach, suggesting that upstream dams directed stream water into the riparian area where it travelled down valley before returning to the stream. This suggests that multiple dams create hyporheic flow paths at multiple scales. Potential mass flux calculations show the riparian area immediately downstream of the beaver dam was a source of N and dissolved organic carbon (DOC) to the stream, and a sink along the rest of the reach. Cold spots of N and DOC availability were also found along the beaver-driven flow paths in the riparian area adjacent to the dam. This pattern likely developed due to flushing of nutrients along the beaver driven hyporheic flow vectors. This work enhances our understanding of stream-aquifer exchange and N dynamics in riparian areas, and the effects of beaver on these processes.

Dynamic exchange of water across the stream-riparian zone interface is important in increasing stream water transit time through basins and enhancing redox-sensitive biogeochemical reactions that influence downstream water quality and ecosystem health. Such exchange may be enhanced by beaver dams, which are common throughout low order streams in North America, Europe, and Argentina. Lateral exchanges of water and nitrogen (N) were observed along a beaver dammed, third-order stream draining a 1.3 km2 Canadian Rocky Mountain valley bottom capped in peat. Measurements of hydraulic heads and chloride concentrations from a network of 80 water table wells were used to identify areas of stream water and groundwater mixing in the riparian area, and their spatiotemporal dynamics in summer 2008. Beaver were found to be the greatest factor affecting lateral movement of channel water into the riparian area. Channel water flowed laterally into the riparian area upstream of the dams and back to the channel downstream of the dams. The hyporheic zone expanded by ¡Ü1.5 m in the un-dammed reaches, but upwards of 7.5 m or more when dams were present. High contributions of stream water were found far out in the riparian area where dams were not immediately present within the stream reach, suggesting that upstream dams directed stream water into the riparian area where it travelled down valley before returning to the stream. This suggests that multiple dams create hyporheic flow paths at multiple scales. Potential mass flux calculations show the riparian area immediately downstream of the beaver dam was a source of N and dissolved organic carbon (DOC) to the stream, and a sink along the rest of the reach. Cold spots of N and DOC availability were also found along the beaver-driven flow paths in the riparian area adjacent to the dam. This pattern likely developed due to flushing of nutrients along the beaver driven hyporheic flow vectors. This work enhances our understanding of stream-aquifer exchange and N dynamics in riparian areas, and the effects of beaver on these processes.

While restoring hyporheic flowpaths has been cited as a benefit to stream restoration structures, little documentation exists confirming that constructed restoration structures induce hyporheic exchange comparable to natural stream features. This study compares a stream restoration structure (cross-vane) to a natural feature (riffle) concurrently in the same stream reach using time-lapsed electrical resistivity (ER) tomography. Using this hydrogeophysical approach, I am able to quantify hyporheic extent and transport beneath the cross-vane structure and riffle. I interpret from the geophysical data that the cross-vane and natural riffle induced spatially and temporally unique hyporheic extent and transport, and the cross-vane created both spatially larger and temporally longer hyporheic flowpaths than the natural riffle. Tracer from the 4.67-hr injection was detected along flowpaths for 4.6-hrs at the cross-vane and 4.2-hrs at the riffle. The spatial extent of the hyporheic zone at the cross-vane was 12% larger than at the riffle. I compare ER results of this study to vertical fluxes calculated from temperature profiles and conclude significant differences in the interpretation of hyporheic transport from these different field techniques. Results of this study demonstrate a high degree of heterogeneity in transport metrics at both the cross-vane and riffle and significant differences between the hyporheic flowpath networks at the two different features. Our results suggest that restoration structures may be capable of creating sufficient exchange flux and residence times to achieve the same ecological functions as natural features, but engineering of the physical and biogeochemical environment may be necessary to realize those benefits.

Ecologically, the hyporheic zone (HZ) performs numerous roles within stream ecosystems (e.g. habitat, refugia from floods and droughts, nutrient cycling, pollution attenuation) with the dominant control on these various roles sediment composition. Recently, the body of literature on the role of sediment in the HZ has grown, though these studies rarely extend beyond the reach scale with little regional scale research undertaken in England and Wales. In this thesis, meio- and macrofaunal hyporheic assemblages at two depths (20cm and 50cm) across four geological regions (chalk – fine sediment, sandstone - fine to medium sediment and limestone – coarse sediment) two seasons (summer (n = 396) and winter (n = 192)) are described. The influence of recent glaciations (Devensian) on the distribution of stygobite fauna (summer (n = 192) and winter (n = 98)) is also considered. A reach scale experimental manipulation of sediment composition in the HZ of a highly dynamic, species rich and diverse study site (limestone) was also undertaken. I found the influence of glaciation on stygobite fauna still apparent in the two limestone areas with macrofaunal sized stygobite species rare or absent in both limestone areas. Meiofaunal sized stygobite fauna Antrobathynella stammeri (Crustacea: Syncarida) were recorded from the glaciated limestone site. These results suggest large stygobite fauna are rare or absent in the limestone areas of Northern England with meiofaunal stygobites possibly surviving in sub-glacial refugia. Stygobite fauna were abundant in southern England with alternative migratory routes north discussed (e.g. River Severn catchment and chalk aquifers). Geological regions had characteristic fauna with species richness and abundance of meio- and macrofauna high in the HZ of both limestone areas. Conversely, species richness and abundance was low in the chalk and sandstone HZ. The chalk HZ had a high abundance of macrofaunal sized Crustacea (Gammarus pulex) and low abundance of meiofaunal sized Crustacea (Copepoda). This suggested mechanical properties (burrowing) rather than morphology (body size and shape) was important in fine sediments, whereas in the limestone HZ morphology rather than mechanical properties was important. In the experimental study results were more intuitive with meiofauna abundant across all sediment treatments (fine, mixed, coarse and natural) with macrofauna abundant in sediment treatments containing a high proportion of coarse material. These results suggest Copepoda assume a greater role in ecological processes in fine sediment patches within a dynamic HZ, whereas when fine sediment dominates a system (e.g. chalk HZ) then the role of Copepoda is reduced. One implication could be the use of Copepoda as indicators of colmation and hydrologic exchange in the assessment of ecosystem health and give an indication of the refugial capacity of the HZ from projected high and low flow events in light of UK climate change forecasts.

Hydropower regulation of rivers exhibits a threat to the riverine ecosystems. Fragmentation of flow, landscape disturbances, and water retention are key features of regulated catchments, resulting in reduced floods and geochemical tr¬ansport, non-natural water level fluctuations, and thus disturbed exchange between the river and the aquifer. Storing of water in reservoirs reduces peak flow and turbidity, which increases particle settling and sometimes favours enhanced primary production and formation of a clogging layer. This in turn alters the interaction between surface water and groundwater, with potential secondary effects on the entire watershed. In Scandinavia, only eight large rivers (16%) remain entirely unregulated. The Lule River, the primary focus of this study, belongs to the most regulated rivers of Eurasia with a degree of regulation (i.e. the volume of water that can be stored in the reservoirs and used for regulation) of 72%, and is exposed to both seasonal and short-time regulation.Using hydrogeochemical analysis of two adjacent boreal rivers (pristine Kalix and regulated Lule River) discharging into the Gulf of Bothnia, the effects of regulation on river geochemistry were investigated. For the Lule River, the average maximum runoff was almost halved while the average minimum runoff was tripled as a result of the regulation. The winter transport fraction of total organic carbon, Fe, Si, suspended Mn and P in the Lule River was at least two to three times higher than in the pristine river. During summer, the suspended C/N ratio in the regulated river was 10-20, compared to <10 for the pristine river, suggesting a presence of predominantly decaying organic material due to longer residence times for the regulated river. This was supported by a virtually constant suspended P/Fe ratio throughout the year in the Lule River, indicating low abundance of phytoplankton. Hence, a pronounced impact on the ecosystem of the river, the hyporheic/riparian zone, and the Gulf of Bothnia is expected.In spite of vast anthropogenic pressure on riverine ecosystems, the knowledge regarding the hyporheic zone (the interface between rivers and aquifers where exchange between surface water and groundwater occurs) is limited for regulated rivers. Therefore, this study was extended to also cover the hyporheic exchange along the Lule River. Temporal changes in hyporheic fluxes across the river channel (rates and directions) were determined using seepage measurements and continuous observations of water stages, temperatures, and electrical conductivity for both the river and the groundwater. While the river water level changed frequently (typically twice a day with up to ± 0.5 m), the river remained gaining 90% of the time, and the largest number of observed changes in flow direction (observed at 5 m orthogonal distance from the river) was six times per week. Flow velocities ≤10-4 m d-1 ( zero flow) constituted 1.5% of the total observation time.Although no changes in water temperature were observed for the hyporheic zone, effects of river level variations were detected up to 5 m inland, where electrical conductivity occasionally decreased to surface water levels indicating infiltration of river water into the aquifer (negative fluxes). River discharge regulation may therefore have severe implications on biogeochemical processes and deteriorate the hydroecological functions of the hyporheic zone.

We investigated the transport of dissolved arsenic in groundwater, surface water and the hyporheic zone in a stream-aquifer system influenced by an abandoned arsenopyrite mine. Mine tailing piles consisting of a host of arsenic-bearing minerals including arsenopyrite and scorodite remain adjacent to the stream and represent a continuous source of arsenic. Arsenic loads from the stream, springs, and groundwater were quantified at the study reach on nine dates from January to August 2005 and a mass-balance approach was used to determine hyporheic retention. Arsenic loading from the groundwater was the dominate source of arsenic to the stream, while loads from springs represented a substantial proportion of the total arsenic load during spring. Arsenic loads in surface and groundwater were significantly elevated during summer. Elevated temperatures during summer may lead to increased arsenic loading by increasing dissolution rate of arsenic source minerals and/or increases in microbially-mediated dissolution processes. The hyporheic zone was shown to be retaining arsenic in the upstream-most sub-reach. Retention most likely occurs through the sorption of dissolved arsenic onto hyporheic sediments. In downstream sub-reaches, hyporheic sediments are derived from mine-tailing piles which have high arsenic content. The hyporheic zone in these sub-reaches was shown to be releasing dissolved arsenic. The historic influence of mining activity has resulted in multiple sources of arsenic to the stream which has increased arsenic contamination of the surface waters. / Master of Science

The hyporheic interstitial was recognized as an integral zone within the aquatic ecosystem bearing important functions for both adjacent compartments, surface and ground water, about 50 years ago. Since then, rather disciplinary works gained knowledge on the organismic community of this ecotone, its spatial extent, the role of distinct parameters such as hydrology and morphology, temporal characteristics, process dynamics, the role for stream or groundwater quality and restoration measures. However, a systematic study on the risks to the hyporheic functions was missing to date. This thesis combined existing methods in order to gather an integrated set of information allowing for the assessment of the ecotonal status. This approach was applied to investigate the functional behavior towards stressors like increasing nutrient and fine sediment input into a rather pristine environment. An interdisciplinary risk assessment and the establishment of adapted measures was called for as land-use scenarios for the studied catchment area indicated progressive onland erosion. Therefore firstly, an integrated monitoring scheme was drawn up and conducted at three sites along a river that underlay a stressor gradient such as mentioned before. Secondly, the data sets were analysed in order to evaluate the status of the hyporheic funtions at the riffles. Thirdly, a coupled surface-subsurface modelling approach was set up to further study the impact of the stressors on the ecotonal integrity. And fourthly, an interdisciplinary consideration combined with studies on the catchments sediment budget and the rivers ecological status was applied to identify measures for the restoration and protection of the aquatic ecosystem. The analysis of the data gathered with the help of the established monitoring scheme revealed that elevated nutrient or fine sediment input lead to biological or physical clogging, respectively, with consequences for the hyporheic zone functions. The surface - ground water connectivity was either lowered in summer months, when biofilm growth was highest, or permanently, as fine sediment particles infiltrated into the interstices of the riverbed sediment. Scouring did not seem to take place as high amounts of fine particles were found in the matrix after discharge events of snowmelt and summer precipitation. With respect to the biogeochemical regulation function, biofilm material appeared to provide an autochthonous carbon source boosting microbial substance turnover. The sediment underneath the physical clogged layer was cut off from carbon and oxygen rich surface water and thus was not reactive. However, the enhanced surface area provided by the fine sediment within the topmost sediment layer seemed to support microbial processing. The inclusion of the results of a study concerning the ecological status at the investigated reaches lead to the deduction that biological clogging at the present degree was not affecting habitat quality. Whereas the physical clogging had tremendeous and lasting effects on the macroinvertebrate community which carries to the conclusion that sediment management within the studied catchment is of uttermost importance. A scenario analysis reflecting distinct clogging degrees and types with a calibrated model of a studied riffle within a pristine reach proved the observed loss of hydrologic connectivity due to physical and biological clogging. Further, a treshold of oxygen consumption rates above which the reproduction of salmonid fish would be unsuccessful was identified for the settings of the middle reaches. In summer month with low discharge it seemed to be likely that this treshold might be reached. Following, a dynamic discharge may be decisive to protect the ecotonal integrity. The integration with the outcome of an investigation regarding the sediment sources within the catchment allowed for two suggestions. On the one hand, river bank restoration and protection within the middle reaches need to be prioritised, and on the other hand, the conservation of the natural vegetation at the steep slopes within the mountaineous areas need to be undertaken in order to secure the pristine aquatic environment of this area. Hyporheic zone research of the last decade was driven by testing hypotheses on the functional significance of distinct spatial and temporal configurations in the field and by new modelling approaches. However, data on the quantification of the ecological impact of clogging processes were lacking. The thesis contributed to the systemic understanding of the hyporheic zone being affected by physical and biological clogging and new field data within a degrading pristine environment were generated, accessible for further hyporheic research. The interdisciplinarity enabled comprehensive statements for the usage of an Integrated Water Resources Management plan.

I studied nine streams near Grand Teton National Park, Wyoming, covering a land use gradient (urban, agricultural, and forested) to assess influences of land use on denitrification rates and hyporheic exchange. I hypothesized denitrification in the hyporheic zone is governed by availability of chemical substrates and hydrologic transport. I tested this hypothesis by coupling measurements of denitrification potentials in hyporheic sediments with a 2-storage zone solute transport model. Denitrification potentials were lowest on average in hyporheic sediments from forested streams and highest from agricultural streams. Modeling results suggest, on average, agricultural sites are transport-limited by having the slowest exchange rate with hyporheic zone and longest transport before entering storage. Land use influences the capacity for hyporheic denitrification in two ways 1) agricultural and urban practices supply substrates that build the microbial potential for denitrification and 2) agricultural and urban activities alter channel form and substrates, limiting hyporheic exchange.

Recent research has shown that groundwater flow in hyporheic zone is critical in major hydrologic, ecological, and biogeochemical processes. Quantitative analyses from the literature show that there is a strong correlation between the diel cycles in pH, water temperature, and other parameters such as trace metal concentrations. There is, however, no controlled experimental data to illustrate how water temperature influences the trace metal concentrations and other parameters. The research study presented here illustrates the mechanism of heat is transported from stream water to groundwater in the hyporheic zone on different bed form. The work will serve as the foundation of future research in understanding the relationship of heat and trace metal concentrations in the sediments. / text