Reclamation Information Sharing Environment (RISE)

Yellowtail Dam closed in 1966, changing the hydrologic, sediment, and temperature regimes for the Bighorn River in southeast Montana, allowing a cold-water trout fishery to thrive over the following decades. However, fishermen recently reported a loss in habitat. We conducted a series of geomorphic and hydraulic studies to assess changes in the Bighorn River, including geomorphic mapping and hydraulic modeling. The first studies in 2008-2010 followed a period of drought marked by lower releases from the dam. In 2019, we reassessed the river following a wetter decade with prolonged high flow releases. Our overall findings are that: (1) the Bighorn River lost significant geomorphic diversity between 1939 and 1980; (2) the rate of loss in geomorphic diversity slowed between 1980 and 2019; and, (3) high flows did remove vegetation and aggraded sediment at some channel entrances, but not all. Through time, many side channels require increasingly higher flows to connect to the main channel or are disconnected. The entrance to one channel was manually excavated in 2012 and remains open and flowing; this type of adaptive management approach offers promise. While the river is unlikely to achieve the spatial extent of geomorphic diversity observed prior to the closure of Yellowtail Dam, it may be possible to restore and maintain many of the side channels that existed into the 1980s.