The hydrologic regime directly affects the physical and biological attributes of a river system, including sediment transport (Section 3.3), large woody debris (Section 3.6), water temperature (Section 2.2), and aquatic habitat (Section 4.1). Hydroelectric projects often substantially alter the hydrologic regime by
- storing water;
- altering the magnitude, timing, and duration of peak flow downstream of reservoirs; and
- creating diversions that reduce natural flow in bypass reaches or augment natural flow through inter-basin transfers.
Hydrologic studies are an integral component to a hydroelectric relicensing process. Unregulated and regulated flow records are required to evaluate sediment and large woody debris transport processes that influence changes in sediment storage, channel morphology, and bed surface texture. Hydrographs are important tools when evaluating current aquatic habitat condition, and changes from unimpaired to regulated flow regimes are necessary to quantify a project's past and potential future effects on aquatic habitat. Spill histories from project dams and measurements of flow fluctuations are common components of fisheries studies, including evaluation of aquatic habitat connectivity and fish stranding. Whitewater boating feasibility studies also commonly rely on flow records and spill histories generated from hydrologic studies.