CHANGES IN HYDROLOGIC REGIME BY DAMS

We present the analysis of pre- and post-dam hydrologic changes that cover the spectrum of hydrologic regimes across the United States. We searched the entire National Inventory of Dams (NID) database for dams possessing longstanding USGS gages downstream of the dam, with approximately 30 years of daily and peak flow data both before and after dam construction. Our overall goals are to document the type, magnitude, and direction of hydrologic shifts due to impoundment, and we apply a hydrologic model, the Indicators of Hydrologic Alteration (IHA), to an array of dams ranging in watershed contributing drainage of four orders of magnitude including hydropower, flood control, water supply facilities, or some combination of all three types. IHA uses daily discharges and calculates 32 indices that describe the hydrologic regime for that station. The thirty-two indices generated by IHA consist of five major categories : (1) magnitude; (2) magnitude and duration of annual extreme conditions; (3) timing of annual extreme conditions; (4) frequency and duration of high and low pulses; and (5) rate and frequency of changes in conditions. The most significant changes across these 21 selected sites occur in minimum and maximum flows over different durations. For low flows, the 1-day through 90-day minimum flows increased significantly following impoundment and baseflows also increased. The 1-day through 7-day maximum flows decreased significantly across the sites. At monthly scales, April and May mean flows tend to decline while August and September mean flows increase. Other significant adjustments include changes in annual hydrograph conditions, primarily in the number of hydrograph reversals which have increased dramatically for all sites following impoundment. The number of high pulses has increased following impoundment but their average length declines. The mean hydrograph rise rate and fall rate have declined significantly. These results indicate that the major pulse of dam construction during the previous century has dramatically altered hydrologic regimes on a nation-wide scale, for both large and small rivers.