Instream Flows for Beaver Populations: Effects of a Midwinter Drawdown of the Snake River and Drought in Grand Teton National Park

Abstract

In 1976 and 1977, much of the western United States experienced severe drought conditions and by mid-winter of 1976-77, streams, lakes, and ponds in northwest Wyoming were noticeably affected. In apparent anticipation of water shortages for summer irrigation needs, the Bureau of Reclamation administering the Jackson Lake Dam in Grand Teton National Park reduced daily discharge to nearly 100 cfs in early February of 1977 and maintained abnormally low discharges until May of 1977. This compares with a mean February discharge of 606 cfs for the previous 20 years (U.S. Geological Survey, 1957-1977). The consequences of a mid-winter drawdown on fish and wildlife populations requiring relatively stable winter stream flows are generally unknown but it was immediately apparent that aquatic mammals in the park, especially beavers, were adversely affected (Mr. Peter Hayden, NPS Biologist, GTNP, Personal Communication, March 25, 1977). Meaningful assessment of this perturbation and the drought that continued through the summer of 1977 was possible since background data on beaver populations in the park were available (Collins, 1976a).