Rimrock Lake FMO Habitat

Rimrock Lake Adfluvial Populations

Rimrock Lake is the only one of the five major storage reservoirs in the Yakima Basin that was not a natural lake prior to impoundment. The reservoir holds 198,000 acre-feet of water at full pool with a surface area of 2,562 acres. Tieton Dam was constructed on the Tieton River in 1925 about 21 miles upstream of the confluence with the Naches River. The dam is a complete barrier to upstream migration, isolating three local populations of bull trout that now reside in the lake and spawn in upstream tributaries. These local populations most likely originated from native fluvial bull trout in the Tieton River.  Construction of the Tieton Dam forced bull trout in these local populations to adopt an adfluvial life-history pattern, and each population uses Rimrock Lake as FMO habitat. One of these populations spawns in Indian Creek, one spawns in the South Fork Tieton River, and the third spawns in the North Fork Tieton River above Clear Lake, which was impounded with the construction of Clear Creek Dam in 1914. All three populations The Indian Creek and South Fork Tieton River populations use Rimrock Lake as primary FMO habitat while the North Fork Tieton population also uses Clear Lake. All undoubtedly benefit from the abundance of kokanee salmon and relatively high lake productivity. <-- Citation?? The Indian Creek and South Fork Tieton River populations are the healthiest most abundant in the Yakima Basin. The North Fork Tieton River population has only moderate abundance. The demographics of the North Fork Tieton River population are still unknown but this population is likely impacted by marginal passage at Clear Creek Dam.

In addition to blocking upstream passage, Tieton Dam entrains fish through its outlet works. Adults and subadults residing in the reservoir are entrained annually through the outlet works of the dam as Rimrock Lake is rapidly drawn down to provide for irrigation demands downstream in the late summer and early fall. Bull trout mortality as a result of entrainment has been documented. Although some survive entrainment, they are permanently displaced from the lake and their natal streams. The fate of most of these fish remains unknown. There is some evidence of entrained fish spawning with Naches River fluvial bull trout populations, although the rate of gene flow has not been well characterized. but genetic distinctions between populations do not indicate a high level of gene flow.

The three Rimrock local populations are moderately genetically differentiated from one another (M. P. Small et al., 2009, p. 13). The level of differentiation within Rimrock is high considering the close proximity of the three spawning tributaries, shared usage of Rimrock Lake as FMO habitat, historic lack of barriers between all three tributaries, and present-day lack of barriers between South Fork Tieton River and Indian Creek. These findings have been broadly interpreted to support the conclusion that there is high site fidelity, and correspondingly, low migration between populations and demographic independence. However, metrics of genetic connectivity are easily misinterpreted as indicators of demographic connectivity when population sizes are large or small (Gagnaire et al., 2015, p. 770; Lowe & Allendorf, 2010, p. 3042).  Populations can maintain high differentiation despite substantial migration in the case of low effective population size, and while natal site fidelity is high within Rimrock, it is not complete. Small numbers of individuals collected in each of the Rimrock spawning tributaries originate from one of the other Rimrock populations and other individuals demonstrate admixture suggestive of past gene flow among Rimrock populations (M. Small et al., 2011, p. 17; M. P. Small & Martinez, 2011, p. 6).

Rimrock Lake FMO Habitat Overview

FMO habitat for the following bull trout populations is in Rimrock Lake. Shoreline development is low density, thus it is not believed to influence habitat quality; water sports activities on the surface of the reservoir are not likely an issue either given the preference of bull trout for deep water. Like all of the storage reservoirs in the Yakima Basin, Rimrock Lake is drafted heavily during the irrigation season and the reservoir pool remains depleted for much of the winter. Unlike the other impoundments, Rimrock Lake can be nearly emptied as it was in 1979 when just 30 acre-feet of water remained. The effects of extremely low carryover storage on Rimrock Lake bull trout have not been quantified but have been a concern for many years. In the midst of the 2001 drought year SOAC, WDFW, the USFWS and the Yakama Nation submitted recommendations to USBR to maintain at least 30,000 acre-feet in the reservoir based on the work of (Mongillo 1982). Although the Rimrock pool ultimately declined below this level in 2001 it has not since. In 2024, another drought year, the Rimrock pool was drained to less than 14,000 acre-feet. Local biologists documented entrainment and mortality of numerous kokanee salmon during this low-pool period. (Effects on Bull Trout unknown)

Bull Trout Recovery Actions for Rimrock FMO (BTAP)

• Rimrock FMO Action #1: Provide connectivity at Tieton Dam
• Rimrock FMO Action #2: Manage operations to reduce entrainment at Tieton D9+