Changes in hydrologic regime by dams

Dams have major impacts on river hydrology, primarily through changes in the timing, magnitude, and frequency of low and high flows, ultimately producing a hydrologic regime differing significantly from the pre-impoundment natural flow regime. This paper presents the analysis of pre- and post-dam hydrologic changes from dams that cover the spectrum of hydrologic and climatic regimes across the United States. Our overall goals are to document the type, magnitude, and direction of hydrologic shifts because of impoundment. Using the entire database for the National Inventory of Dams (NID) for dams possessing longstanding U.S. Geological Survey (USGS) gages downstream, we identified 21 gage stations that met length-of-record criteria encompassing an array of types of dams and spanning four orders of magnitude in contributing watershed area. To assess hydrologic changes associated with dams, we applied a hydrologic model, the Indicators of Hydrologic Alteration (IHA), supplemented with orientation statistics for certain hydrograph parameters. Dams had significant impacts on the entire range of hydrologic characteristics measured by IHA. For many characteristics, the direction and significance of effects were highly consistent across the 21 sites. The most significant changes across these sites occurred in minimum and maximum flows over different durations. For low flows, the 1-day through 90-day minimum flows increased significantly following impoundment. The 1-day through 7-day maximum flows decreased significantly across the sites. At monthly scales, mean flows in April and May tend to decline while mean flows in August and September increase. Other significant adjustments included changes in annual hydrograph conditions, primarily in the number of hydrograph reversals that has generally increased for almost all sites following impoundment. The number of high pulses has increased following impoundment but the average length declines. The mean rate of hydrograph rise and fall has declined significantly. These results indicate that the major pulse of dam construction during the previous century has modified hydrologic regimes on a nationwide scale, for large and small rivers.