Kachess Lake Bull Trout Population Group

Overview

The Kachess Reservoir Population Group consists of two bull trout populations that occupy tributaries to Kachess Reservoir. Detailed information on each of these can be found here:

Box Canyon Creek Bull Trout Population

Upper Kachess River Bull Trout Population

These two adfluvial populations make use of shared foraging, migration and overwintering (FMO) habitat in Kachess Reservoir, and historically may have used downstream habitat in the Yakima River watershed. Information on conditions, threats and actions for this shared FMO habitat are detailed in the Kachess Reservoir FMO Habitat page.

Population Genetics

Despite a short distance of ~4 miles between the two spawning tributaries, samples from Kachess River bull trout and Box Canyon Creek bull trout had a high level of genetic variance between populations (Small et al. 2009). The genetic differences were stronger than populations that share Rimrock Reservoir FMO habitat where the spawning tributaries are separated by a similar geographic distance. The Kachess and Box Canyon effective population sizes are the lowest in the basin (Small et al. 2009) and the populations in Kachess Reservoir spawn in different locations at different times due to environmental conditions, like the annual dewatering in the Upper Kachess River. Spatial and temporal differences in spawning, in addition to small effective population size (e.g. genetic drift) likely maintain the genetic differences between these populations (Small et. al 2009).

Gold Creek is geographically closer than many of the other Yakima Basin populations, but the samples from Gold Creek were still highly differentiated from the Box Canyon and Kachess River populations. Dams have prevented movement between these populations since the early 1900's, but the genetic structure of the upper Yakima populations pre-dates dam construction, probably because of dewatering in Gold Creek and the upper Kachess River limited the effective population sizes, and thus increased genetic drift (Small et. al 2009). Ongoing genetic isolation probably continues to increase genetic differentiation between the Kachess Reservoir populations and the Gold Creek population.

Connectivity and thus the potential for genetic exchange with downstream populations was reduced by the construction of Kachess Dam in 1912. Recent work by USFWS (Beebe et. al 2024) indicates entrainment continues to occur in the Yakima Basin, and it is well-documented at Keechelus Dam. Only five bull trout have been encountered by USFWS below Kachess Dam, with one of those originating from Gold Creek. With the extirpation of the Teanaway population and numerous downstream habitat limitations, it is unlikely entrained fish from the Upper Yakima River populations are exchanging genetics with Naches fluvial bull trout populations. However, there have been occasional sightings of bull trout in the Yakima River Canyon and at Roza Dam, so it is not impossible. Fish entrained through Kachess and Keechelus dams are able to spawn in the mainstem Yakima River between Easton Reservoir and Keechelus Reservoir. There have been redd surveys conducted in this reach in several years, but it is difficult to quantify bull trout redds in this reach due to other fall spawning salmonids occurring at the same time.

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Population Monitoring

Between 2020-2022 USFWS and a CWU graduate student conducted an acoustic telemetry study of bull trout spatiotemporal movements in Kachess Reservoir which described the behavior and home range of sub-adult adfluvial bull trout. The tagged individuals were rescued as fry from the Upper Kachess River, reared at the Yakama Nation La Salle facility, and released at sub-adult sizes into the reservoir. Results showed that temperature was the main predictor of fish depth, and that bull trout in the reservoir tended to avoid spending time in water warmer than 10°C for extended periods. Once the temperature stratification in the reservoir broke down in mid-October, sub-adult bull trout exhibited daily vertical migrations between 0 - 45 meters depth on average. The home ranges varied for individual fish, some made wide ranging movements throughout the reservoir, and some remained in a small ~1 km sq area throughout the study (Taylor 2022).

The USFWS continued the above acoustic telemetry study and discerned differences between a non-drought (2022) and a drought year (2023). The study found that although The Narrows becomes a complete fish passage barrier when Kachess Reservoir water surface elevation drops below 2199.5 ft. (USBR and Ecology 2016), it can also become a thermal barrier beforehand that restricts fish between Big and Little Kachess basins and limits access to spawning tributaries in Little Kachess. The study found that during 2022, fish moved across The Narrows in both directions, but in 2023, most tagged fish moved south of The Narrows by mid-summer and no crossings occurred after July. Fish used in the study were 36 juvenile Bull Trout rescued from the upper Kachess River and reared at La Salle over the winter (Hamilton et al., 2025).

Shared Threats

Find shared threats to the Kachess bull trout Populations under the Kachess Reservoir FMO Habitat page.

Shared Recovery Actions

Recovery actions for individual populations and shared FMO are highlighted on the corresponding pages.

Update Notes

This page was created during the 2025 BTAP update and was populated with some information from the 2012 BTAP.