East Fork Owyhee River Salmon and Steelhead Recovery Project

The Shoshone-Paiute Tribes’ completed a habitat assessment in the East Fork Owyhee River and its tributaries estimating the watersheds capacity to support natural production of anadromous Chinook salmon and steelhead. The assessment was multi-faceted with three components: 1) Literature review of water quality conditions and native trout populations in the Basin, 2) extensive juvenile rearing and spawning habitat surveys designed to document physical habitat conditions in mainstem and tributary stream segments during the summer of 2013 from Wildhorse Dam downstream to China Dam, and 3) quantified salmon and steelhead rearing capacity throughout the Basin using a habitat-based carrying capacity model known as the Unit Characteristic Method (UCM). Hydraulic modeling was also used to extrapolate baseline capacity estimates across a range of stream flow conditions. Ancillary study objectives included identifying natural and man-made barriers that may impede salmonid migration, evaluating health and genetic structure of resident salmonid populations, and documenting potential locations for a put-and-take salmon and steelhead fishery.

Summer habitat conditions in the East Fork Owyhee River were found to be a mix of large pools with an average depth of 0.8 meters and shallow riffles, typically not deeper than 0.3 meters. Substrates were comprised predominantly of cobbles and boulders at higher elevations in the watershed, with progressively larger quantities of gravel and fines as distance increased downstream of Wildhorse Dam. Habitat conditions in Skull, Slaughterhouse, and Van Duzer Creeks (the three tributaries surveyed which had flowing water in 2013) had similar mesohabitat composition. Tributaries were dominated by beaver ponds and shallow riffles, with abundant quantities of fine sediments and moderate quantities of gravel.

Carrying capacity estimates calculated using habitat data collected in 2013, were 17,590 juvenile steelhead and 11,442 juvenile Chinook in mainstem reach 1 directly downstream of Wildhorse Dam. Comparatively, juvenile capacity estimates for mainstem reaches 2 and 3 were roughly 65% less for steelhead and 50% less for Chinook. Mainstem spawning capacity estimates indicate more potential for redd deposition at lower elevations, where the stream gradient is reduced and the channel broadens. Mainstem reaches 2 and 3 were estimated to possess steelhead redd capacities greater than 4,000 and Chinook redd capacities greater than 600, while mainstem reach 1 redd capacities were approximately 50% less for both species.

Carrying capacity estimates derived from habitat data collected in 2013 were heavily influenced by drought-like stream flow conditions. In an average water year, summer base flows in the mainstem would be approximately 50 cfs, which would equate to a juvenile rearing capacity estimate roughly twice the baseline values calculated from 2013 survey data. When accounting for the full range of summer flow and temperature conditions (90%-10% exceedance flow, 18-22oC), it is estimated that the annual summer rearing capacity for the entire study area could range between 3,300 and 43,000 juvenile steelhead trout and from 3,600 to 41,000 Chinook salmon.

The analysis suggested there are three primary factors that limit Chinook salmon and steelhead trout production in the East Fork Owyhee River; these are high summer water temperature, low stream flow, and high volume of fine sediments. Other noteworthy limiting factors include impaired water quality and migration passage conditions at irrigation diversion structures. Restoration activities that address these issues are likely to produce the greatest benefit for anadromous salmonids.

Confirmation that the watershed retains the capacity to support natural production of anadromous salmonids and assist in identifying adult and juvenile trap locations for implementing a potential trap and haul program.