Abstract

By using the established hydraulic relationships among flood frequency, flood magnitude, and river-channel capacity, we develop a scale-independent assessment of the hydrogeomorphic impacts of 21 dams across the United States that have broad ranges in function and contributing drainage area. On the basis of generalized extreme value (GEV) analysis of pre- and post-dam hydrologic records, our analysis indicates that the 2 yr discharge has decreased ∼60% following impoundment, exceeding the magnitude of climatically triggered discharge reductions occurring during the Holocene. Reductions in the frequency of the pre-dam 2 yr discharge have been equally profound. The pre-dam 2 yr flood has occurred on average twice per site, whereas statistical analysis indicates that it should have occurred ∼20 times. Furthermore, floods greater than bankfull have been essentially eliminated by dams, completely disconnecting the riparian zone from riverine influence. Our analyses herein suggest that a critical threshold of disconnectivity exists and corresponds approximately to the pre-dam 5 yr flood. This similar recurrence probability exists independent of region, dam type, or catchment size.

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