Rimrock Lake FMO Habitat

Rimrock Lake Adfluvial Populations

Rimrock Lake and Clear Lake are the only water storage reservoirs in the Yakima Basin that were not natural lakes prior to impoundment. Rimrock 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. Local populations adopted an adfluvial life history with the construction of Tieton Dam and each population uses Rimrock Lake as foraging, migration and overwintering (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. 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.

Based on redd counts, the Indian Creek and South Fork Tieton River populations are probably the most abundant in the Yakima Basin, but South Fork Tieton bull trout have been in obvious decline since 2009. Indian Creek has seen high variability in redd counts with no observable trend. The North Fork Tieton River population has low to moderate abundance.

Rimrock Lake FMO Habitat Overview

The construction of Tieton Dam in 1925 inundated the Tieton River valley where the North Fork and South Fork Tieton rivers converge including a large meadow complex called McAllister Meadows. The area is now a 10-mile long reservoir, bordered by Highway 12 and Tieton Reservoir Road (USFS 1200). The total depth of the reservoir isn't more than 165 feet at full pool. Today, shoreline development is low density, thus it is not believed to influence habitat quality; water sports activities on the surface of the reservoir have an unknown impact on bull trout.

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 Reclamation's Systems Operation Advisory Committee (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 and Faulconer 1980. Although the Rimrock pool ultimately declined below this level in 2001, it remained at or above the recommended level until 2024. In 2024, a drought year, the Rimrock pool was drained to less than 12,000 acre-feet.

In addition to blocking upstream passage, Tieton Dam entrains fish through its outlet works. Some adult and subadult bull trout residing in the reservoir are entrained through the outlet works of the dam. Rimrock Lake is rapidly drawn down to provide for irrigation demands downstream in the late summer and early fall which could result in increased entrainment rates. 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 (See Rimrock Lake Populations for more info). Local biologists documented entrainment and mortality of numerous kokanee salmon during the 2024 low-pool period (effects on Bull Trout are unknown). Trap and Haul efforts at Tieton Dam have mostly failed with only two fish collected since 2020. The stilling basin is deep and turbulent, making fish collection difficult.

The Rimrock Lake bull trout populations may benefit from relatively higher levels of prey than those in other reservoirs. A 2003 report by Lieberman and Heibert documented greater nutrients in Rimrock compared to the other major storage reservoirs in the Yakima River basin. However, Mongillo and Faulconer 1980 pointed out that a lack of anadromous fish runs and large reservoir fluctuations reduce littoral habitat, potentially disrupting zooplankton populations and thus trophic interactions.

Habitat Monitoring

Mongillo and Faulconer (1980) measured water quality parameters and zooplankton densities for Rimrock Reservoir. They found higher nutrient retention when compared with the other Yakima Basin reservoirs. Seasonal dynamics in zooplankton density and biomass were noted, including increased zooplankton density during the warmer months

In a 2003 study, Lieberman and Hiebert limnological study of the five major Yakima Basin reservoirs, authors found that Rimrock lake exhibited weaker and shorter-lived stratification compared to the other reservoirs. A narrow thermocline developed in early summer (5-15m depth) but dissipated by September due to water withdrawals. By the fall, the reservoir was nearly isothermal. The shape of the reservoir contributes to more frequent mixing when compared to the other reservoirs, leading to a relatively uniform nutrient distribution in the water column. They also found that water released primarily exported nutrients downstream but retained plankton within the reservoir. Overall nutrient concentrations were higher than other reservoirs, including increased nitrogen and phosphorus, supporting greater productivity in the form of Cholorphyll-a. Surface temperatures ranged from 7.8 C in May to 20.2 C in August. In contrast to the other reservoirs, this report states that nutrient levels in Rimrock classify it as mesotrophic to eutrophic during certain times of the year, especially in late summer. Bottom temperatures were cooler but less stable than other reservoirs due to frequent mixing in Rimrock (6 C in May, to near-surface temperature by fall). On September 26, 2000 the bottom temperature was 14 C.

Threats

Connectivity

Fish Passage Barriers

Threat Severity: Significant

Tieton Dam blocks upstream passage into Rimrock Lake, leaving Rimrock Lake only accessible to adfluvial bull trout that already reside there. Reclamation and partners are in the early phases of designing and construction fish passage facilities at Tieton Dam.

Barriers exist on the lower end of tributaries to Rimrock Lake such as Bear, Hart and Russel Creeks; these are small and (except for Bear) steep tributaries that are unlikely to provide significant habitat to bull trout even if accessible.

In 2024, WSDOT removed a barrier culvert at the mouth of Andy Creek; bull trout were subsequently observed using the Creek (Eitemiller 2024).

Passage from Rimrock Lake upstream into Clear Lake is blocked by Clear Creek Dam, which is discussed in detail in the North Fork Tieton Population page and in the Clear Creek Dam Fish Passage page. Reclamation and partners completed designs for a new fish ladder and cold water intake pipe and construction began in 2024.

Entrainment

Threat Severity: Unknown, likely significant

Rimrock Lake bull trout are entrained in the outlet works of Tieton Dam. The population-scale impact of entrainment on the three Rimrock bull trout populations that reside in the lake has not been quantified. Estimates have been derived on the number of fish entrained. Three years of entrainment monitoring occurred in the early 2000s. James (2002b) estimated that the number of bull trout entrained ranged from 46–87 in 2001 and Hiebert et al. (2002; 2003) estimated the entrainment loss to be 145 bull trout in 2002 and 140 in 2003. None of the 9 bull trout captured in the 2002 study by Hiebert et al. survived entrainment. Underwood and Cramer (2007) used data from these entrainment studies, as well as James’ population data to create a life cycle model that was used to simulate effect of entrainment on long-term population dynamics.

During the 2005 construction of the Tieton hydroelectric project a fish salvage effort was conducted; 37 bull trout were captured in the stilling basin directly below Tieton Dam (Ackerman 2005). Additional evidence of entrained bull trout surviving and migrating within the Yakima Basin comes from a single fish that was captured and sampled at Roza Dam in 2004. This fish genetically assigned to the Indian Creek population. Courter and Cramer subsequently reported on higher survival estimates for entrained bull trout post-construction of the hydroelectric project, with estimated mortality reduced from 85% to 45% (Courter and Vaughan 2011).

*Add any other data and info on Tieton Dam entrainment

Dewatering due to flow management

Threat Severity: Unknown

Water operations in the Yakima River Basin are managed to draft water from the upper reservoirs first (Cle Elum, Keechelus, and Kachess respectively). When storage in those places is reduced to the minimum, the USBR initiates "flip-flop" where flows are reduced from the upper reservoirs and increased out of Rimrock Lake through Tieton dam to support late-season agriculture. Under this management regime, Rimrock Lake maintains a relatively high pool throughout the summer season. In low-water years, "flip-flop" may occur earlier than usual, like in 2024 when USBR began drafting from Rimrock on August 20th. After "flip-flop" the pool level drops quickly to reveal the river channels of North Fork Tieton, South Fork Tieton, and Indian Creek, which may end up flowing through the dry reservoir bed for miles. The full extent of these channels are not monitored consistently for connectivity and may experience fish passage issues until the pool fills enough in the following spring.

In dry years, the reservoir is drawn down to low levels that likely increase entrainment rates, reduce water quality and result in unnaturally high fish densities. This may impact the growth and survival of overwintering bull trout in the reservoir.

Land-use Issues

Forestry

Threat Severity: Unknown, but likely insignficant

Commercial timber harvest has occurred in proximity to Rimrock Lake between 2001-2020, particularly on the North side of the lake near Russel Ridge and in tributary watersheds like Andy Creek and Russell Creek. Harvest also recently occurred on the peninsula between Tieton Dam and the confluence of the South Fork Tieton River. Potential sediment or other impacts are unknown.

Agriculture and Grazing

Threat Severity: Unknown, but likely insignificant

Grazing allotments exist in proximity to Rimrock Lake....?** Confirm once Gene replies.

Recreation

Threat Severity: unknown, likely significant?

The Rimrock Grand Prix motorcycle race occurs at Silver Beach Resort each fall, with most of the racecourse on the dry reservoir bed. Concerns have been raised about impacts on bull trout. WDFW and other partners have worked closely with the Dust Dodgers motorcycle club to minimize impacts on bull trout habitat during this event. The club has built bridges, signed routes, etc. (See Indian Creek Action #1). In 2023 BTTF technicians were on-site during the race to help keep racers out of waterways, but reported that signage was great and everyone was respectful of the rules. The 2012 and 2017 BTAP primary concern for Indian Creek Bull Trout was the motocross race, however, as of 2024 the race organizers and local biologists have taken yearly measures to prevent driving through the streams on that particular day and the threat of the race has been minimized.

Aside from the sanctioned race, when the reservoir is low enough, vehicles driving through both Indian Creek and North Fork Tieton River occurs regularly. OHV impacts to South Fork Tieton River are largely unknown. A larger discussion on motorized access to the reservoir bed is needed.

Water sports on the surface of the reservoir aren't likely to impact bull trout except through angling which is addressed below. Pollution from motorized boats and OHV use in the reservoir is unquantified and therefore impacts are unknown.

Roads and Development

Threat Severity: Unknown, but likely insignificant

Highway 12 and Tieton Reservoir Road (USFS 1200) are both paved and are adjacent to the reservoir for the majority of the shoreline, thus don't contribute dust or sediment like an unpaved road would. Effects of pollution from road associated run-off have not been quantified and are likely insignificant. Other development includes a number of recreational sites and camping areas - both developed and dispersed. Silver Beach Resort near the West end of the lake is the largest development on the shoreline and includes a marina, boat launch, campground, lodge, and market.

Mining

Threat Severity: Unknown, but likely insignificant

A small number of historic borrow pits are near Rimrock Lake, and there are several small historic mines higher in the Rimrock Lake watershed. No associated impacts on bull trout have been identified.

Other

No other land use issues have been identified.

Ecological Interactions

Brook Trout

Threat Severity: unknown, likely significant

Brook Trout have been observed in tributaries to Rimrock Lake and there is evidence of introgression with Bull Trout in Indian Creek and the North Fork Tieton River. Clear Creek is a known hot spot for brook trout. Brook Trout were stocked in several high lakes in headwaters to Rimrock Lake in the past. South Fork Tieton is not known to contain brook trout, although there are no physical barriers preventing immigration from Rimrock Lake before the 30 foot barrier falls ~14 miles upstream.

Other Invasive Species

Threat Severity: insignificant

Other invasive species have not been observed.

Diminished Prey Base

Threat Severity: unknown, likely significant

Construction of Tieton Dam in 1925 blocked passage to anadromous fishes including salmon, steelhead and lamprey. In addition, pool fluctuation is extensive, likely affecting littoral zone productivity and prey base. (I thought Kokanee were stocked yearly but didn't see Rimrock on any of the 2023 or 2024 stocking reports... Marc??). Kokanee are relatively abundant in Rimrock lake and liekly provide much of the current prey base for bull trout in the lake; they spawn extensively below Clear Creek Dam and in lower Indian Creek.DEVELOP LAKE TEMP SECTION

Disease

Threat Severity: insignificant

Disease issues have not been noted in Rimrock Lake FMO habitat.

Water Quantity and Quality

Flow issues/dewatering

Threat Severity:

Water quantity issues are discussed above under the Connectivity section.

Current and modeled future temperature conditions

Threat Severity: unknown

What is the total depth of Rimrock Lake? When pool is minimum, is there still depth (temperature) refuge below the thermocline??

Other changes in hydrology

Threat Severity: unknown

Fisheries Impacts

Angling Regulations/Fisheries Use/Poaching

Threat Severity: unknown, likely significant

Angling in Rimrock Lake is common and Bull Trout are caught incidentally. A creel survey is needed to quantify potential impacts.

Fisheries Management/Monitoring

Threat Severity: unknown, likely insignificant

There is not any ongoing monitoring of fisheries in Rimrock Lake. Bull Trout that utilize Rimrock FMO are frequently caught below Clear Creek Dam during USFWS Trap and Haul.

Other Threats

None noted.

Summary of Primary Limiting Factors and Threats

The primary threats to Bull Trout in Rimrock Lake FMO are related to connectivity and include the lack of passage and entrainment through Tieton Dam. Some issues occur at low pool, including lack of cover and reduced passage in spawning tributaries that flow across the inundation zone. Threats related to recreation include OHV use in the reservoir bed and rock dam construction in where the lower end of tributaries flow across the reservoir bed. Angling is thought to have some impact on Bull Trout abundance.

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 Dam

Update Notes

This page was created during the 2025 BTAP update and includes information from the 2012 BTAP and other sources.