Hydropower schemes alter the hydrological and sediment regimes of rivers, hence modifying the aquatic ecosystems to varying degrees. Hydropower discharges are regulated, and although it is not possible to make sweeping generalisation, the downstream flow regimes may provide a higher percentage of average flow conditions and reduce the occurrence of high and low flow events.  For base load stations, discharges can be at a consistent flow for long periods; for peaking stations, flows can fluctuate rapidly on timescales of hours.  Where diversions have occurred into river basins, power stations can deliver prolonged periods of higher than natural flows, whilst dewatering the river system downstream of the diversion dam.  

Changes to downstream hydrology can impact on river hydraulics, instream and streamside habitat, and can affect local biodiversity.  Altered flow regimes from natural patterns can disadvantage native species to the advantage of introduced species.  Altered flow regimes create a number of potential issues depending on the specific context and degree of change - fish cues for migration, connectivity of habitat and quality of habitat refuges, habitat area available for macroinvertebrates and fish, changes to habitat quality through altered riparian zones, increased erosion or sedimentation, and delivery of organic materials and nutrients.   The retention of flood flows in the reservoir can affect the natural productivity and stability of riparian zones, floodplains and deltas.  In estuarine systems, altered flows can change the extent of salt-water intrusion due to changed freshwater inflow patterns to the estuary.