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.

Treated wastewater (effluent) has been used as a water source for aquifer recharge and sustaining perennial surface water flow. Artificial recharge basins allow effluent to seep into the ground relieving stressed aquifers. However, these basins frequently become clogged due to physical, chemical, and biological processes. Effluent is also used to replace baseflow for dry streambeds. However, little is known about the effect of effluent on stream-aquifer interactions. Effluent from the Nogales International Waste Water Treatment Plant sustains perennial flow in the Upper Santa Cruz River, Arizona. A series of monthly field campaigns were undertaken to understand the impact of effluent on the streambed at 16 different sites along a 30 km river reach. The field campaigns had two foci: physical transformations in the streambed and water source identification using chemical composition. Historic data sets including USGS stream gauging records, NIWTP outfall data, ADWR well transducer data and USGS well chemistry data were also analyzed to provide a larger context for the work. Results indicate that localized clogging forms in the Upper Santa Cruz River. The clogging layers perch the stream and shallow streambed causing a desaturation below the streambed. A clogging cycle is established in the context of a semi-arid hydrologic cycle: formation during dry and hot pre-monsoon months, and removal by a set of large flood flows (10+ m3/sec) during the monsoon season. However, if the intensity of flooding during the semi-arid hydrologic cycle is lessened, the dependent riparian area can experience a die off.