Rimrock Lake FMO Habitat
Rimrock Reservoir Adfluvial Populations
Rimrock Reservoir and Clear Lake are the only water storage reservoirs in the Yakima Basin that were not natural lakes prior to impoundment (Figure 1). 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 Reservoir 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 Reservoir 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 Reservoir 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 Reservoir is drafted heavily during the irrigation season and the reservoir pool remains depleted for much of the winter. Unlike the other impoundments, Rimrock Reservoir 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 Reservoir 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[1]. 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 Reservoir 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. Drawdowns of Rimrock are also associated with fewer redds the following year in the South Fork Tieton River and Indian Creek (Dayan and Haskell 2024[2]). 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 Reservoir 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 Reservoir bull trout populations may benefit from relatively higher levels of prey than those in other reservoirs. A 2003 report[3] 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[1] 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[1]) 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 limnological study of the five major Yakima Basin reservoirs, authors found that Rimrock Reservoir exhibited weaker and shorter-lived stratification compared to the other reservoirs (Lieberman and Hiebert 2003[3]). A narrow thermocline developed in the summer (5-15m depth) but dissipated by September due to water withdrawals, wind action, and tributary input. By the end of September, 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 releases exported nutrients downstream but plankton was retained 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[3].
Threats
Connectivity
Fish Passage Barriers
Threat Severity: Significant
Tieton Dam blocks upstream passage into Rimrock Reservoir, leaving Rimrock Reservoir 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 Reservoir 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[4]).
Passage from Rimrock Reservoir 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.
Access to the South Fork Tieton River was reduced when a bridge on Forest Service Road 1200 was constructed years ago. The natural channel of the river was relocated to flow under the bridge through a notch blasted out of bedrock. A waterfall begins to form at this location when the reservoir is drafted below 131,000 acre-feet. It is believed to become impassable for bull trout attempting to migrate upstream when the pool volume drops below 127,000 acre-feet (Thomas 2001). An informal agreement is in place with the USBR to maintain at least this minimum pool through August 10 of each year, the date Yakima basin biologists agree that the majority of bull trout have likely made their spawning migration into the South Fork Tieton River. Under current operations, heavy drafting of Rimrock Reservoir does not begin until early September and the reservoir has held more than 127,000 acre-feet on August 10 (average 177,155) for the 32 years since modern-day operations (i.e., flip-flop) were initiated in 1981. Previous to this at least part of the spawning migration may have been blocked from entering the South Fork Tieton. Yakima County is leading the effort to address the USFS Rd. 1200 bridge issues for protection of bull trout and for operational flexibility. Construction was planned for 2025 to return the South Fork Tieton River to its natural channel, but there have been funding shortfalls. Completing this action should do away with the seasonal waterfall, but will also remove the August 10th restriction on pool level.
The effects on post-spawn bull trout that inevitably descend this waterfall on their outmigration back to Rimrock Reservoir are unknown. The effects on adult bull trout who start their upstream spawning migration later than August 10 is unknown but likely significant considering that many of the bull trout populations in the Yakima Basin have a bi-modal distribution spawning run with some migrating upstream in the early summer and some just before spawning.
Entrainment
Threat Severity: Unknown, likely significant
Rimrock Reservoir bull trout are entrained through 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 from three years of entrainment monitoring in the early 2000s. James (2002b[5]) estimated that the number of bull trout entrained ranged from 46–87 in 2001 and Hiebert et al. (2002[6]; 2003[3]) 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[7]) 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.
In conjunction with the 2005 construction of the Tieton hydroelectric project, a fish salvage effort was conducted and 37 bull trout were captured in the stilling basin directly below Tieton Dam (Ackerman 2005[8]). 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[9]). Two fish collected by trap and haul efforts in the stilling basin in 2020 were assigned to the South Fork Tieton population.
Dewatering due to flow management
Threat Severity: Unknown
Water storage operations in the Yakima River Basin are managed to draft water from the upper reservoirs first (Cle Elum, Keechelus, and Kachess respectively). When storage is reduced to the minimum, the USBR initiates "flip-flop" where flows are reduced from the upper reservoirs and increased out of Rimrock Reservoir through Tieton Dam to support late-season agriculture. Under this management regime, Rimrock Reservoir 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 the North Fork Tieton River, South Fork Tieton River, 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 pose fish passage issues until the pool refills 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.
Other
Land-use Issues
Forestry
Threat Severity: Unknown, likely insignificant
Commercial timber harvest has occurred in proximity to Rimrock Reservoir 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
USFS grazing allotments exist in proximity to Rimrock Reservoir, including the Soup Creek allotment and the Tieton allotment. The Soup Creek allotment is a cattle lease that encompasses the majority of land north of Rimrock Reservoir but stops to the east of Indian Creek up on the ridge. The Tieton allotment is also a cattle lease that encompasses the majority of the land south of Rimrock Reservoir, including in the South Fork Tieton watershed on both sides of the river. The Conrad Meadows allotment is currently (as of 2025) vacant, but is available for lease to cattle grazing.
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 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 drive through both Indian Creek and the North Fork Tieton River. OHV impacts to South Fork Tieton River are unknown. A larger discussion on motorized access to the reservoir bed is needed to address this potentially significant threat.
Research has shown motor boats increase turbidity, pollution, and potential for invasive species propagation (Whitfield and Becker 2014[10]). Pollution from motorized boats and OHV use in Rimrock is unquantified and therefore impacts to bull trout are unknown.
Angling is addressed in its own category below.
Roads and Development
Threat Severity: Unknown, 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. To the west of Silver Beach resort is USFS managed Indian Creek Campground. While this campground is not situated directly in the floodplain of the creek, it is a short ~0.25 mile walk to Indian Creek. Recreational rock dams are often seen and dismantled in this area adjacent to the campground. Rimrock Reservoir Resort is privately owned and located east of the rest of the facilities already mentioned. It has several annual RV sites, a boat ramp, and a marina.
Mining
Threat Severity: Unknown, likely insignificant
A small number of historic borrow pits are located near Rimrock Reservoir, and there are several small historic mines higher in the Rimrock Reservoir 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 Reservoir and there is evidence of introgression with bull trout in Indian Creek, the North Fork Tieton River, and the South Fork Tieton River. From 2012-2024, the U.S. Fish and Wildlife Service has identified 15 hybrids during trap and haul work. Seven of these fish were introgressed with North Fork Tieton River origin fish, one with an Indian Creek fish, and one with a South Fork Tieton River fish. The remaining hybrids could not be assigned. Brook trout have not been observed in the South Fork Tieton River during demographic surveys, although there are no physical barriers preventing immigration from Rimrock Reservoir before the 30 foot barrier falls ~14 miles upstream.
Historically stocked brook trout in several headwater high lakes are known to be naturally reproducing populations and are considered likely source populations within the Rimrock basin.
Clear Creek, a tributary to Clear Lake, is a known hot spot for brook trout.
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. Kokanee are relatively abundant in Rimrock Reservoir and likely provide much of the prey base for bull trout; they spawn extensively below Clear Creek Dam and in lower Indian Creek. The Washington Department of Fish and Wildlife does not stock kokanee in Rimrock Reservoir, thus only natural origin kokanee are present.
Disease
Threat Severity: Insignificant
Disease issues have not been noted in Rimrock Reservoir FMO habitat.
Water Quantity and Quality
Flow issues/dewatering
Threat Severity: Unknown
When Rimrock Reservoir is drawn down, especially below the 30,000 AF recommendation, bull trout are concentrated in a smaller area which may impact turbidity, temperature, and ecological interactions.
Fish passage issues are discussed above under the Connectivity section.
Current and modeled future temperature conditions
Threat Severity: Unknown, likely significant
Lieberman and Hiebert (2003[3]) observed temperature stratification only during August in their study period (Figure 2). They noted the stratification profile is also "weaker" than other Yakima Basin reservoirs due to lake morphometry, hypolimnetic discharges, wind action, and inflow temperatures of the North Fork Tieton River and the South Fork Tieton River. Temperatures recorded on September 26, 2000 were consistently near 14 C from the surface to the bottom.
The morphometry of Rimrock Reservoir may preclude bull trout from having adequate depth-temperature refuge during the warmer months, and critical periods like pre- and post- spawning. Warmer temperatures and droughts due to climate change will exacerbate unfavorable temperature conditions in Rimrock Reservoir. A 2022 study showed that when possible, adfluvial bull trout select deeper lake habitat with temperature less than 10 C (Taylor 2022[11]).
Other changes in hydrology
Threat Severity: Unknown, likely significant
There has been an increase in large runoff events in Rimrock Reservoir tributaries which may impact egg incubation, juvenile rearing conditions, and sediment levels in spawning areas.
Fisheries Impacts
Angling Regulations/Fisheries Use/Poaching (Recreational)
Threat Severity: Unknown, likely significant
Angling in Rimrock Reservoir is common and bull trout are caught incidentally. A creel survey is needed to quantify potential impacts. Restrictive fishing regulations are in place on the Rimrock Reservoir tributaries, however, poaching is known to occur. South Fork Tieton and Indian Creek are closed to fishing from the mouth to the barrier falls throughout the year, and include the portion of these streams that flow across the reservoir bed when reservoir levels are low. The North Fork Tieton River has less restrictive regulations and is open from the Saturday before Memorial Day through August 15th under selective gear rules including the portion flowing across the reservoir bed at low lake levels. Selective gear rules limit fishing to no bait allowed and flies or lures with single barbless hooks only.
Management/Monitoring (Research)
Threat Severity: Unknown, likely insignificant
There is not any ongoing monitoring of fisheries in Rimrock Reservoir. About 45% of bull trout caught below Clear Creek Dam during USFWS Trap and Haul are Indian Creek and South Fork Tieton River origin fish that migrate up the North Fork Tieton River to the base of Clear Creek Dam. After construction of the Clear Creek Dam fish ladder, these fish will be able to volitionally pass upstream.
Other Threats
None noted.
Summary of Primary Limiting Factors and Threats
The primary threats to bull trout in Rimrock Reservoir 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. Evidence of brook trout introgression to all three populations indicates brook trout are a significant threat. Climate change and the overall decline in available water contribute to increased water temperature, decreased habitat availability, and increased competition for resources. Threats related to recreation include OHV use in the reservoir bed and rock dam construction where the lower end of tributaries flow across the reservoir bed. Poaching is thought to have some impact on bull trout abundance.
Bull Trout Recovery Actions for Rimrock FMO (BTAP) - INSERT POWER APP IFRAME
- Rimrock FMO Action #1: Provide Connectivity at Tieton Dam
- Rimrock FMO Action #2: Reduce Entrainment of Bull Trout Through Tieton Dam
- Rimrock FMO Action #3: Manage Reservoir Pool Levels to Promote Connectivity at Tributary Confluences (New Action 2025)
- Rimrock FMO Action #4: Evaluate and Manage Ecological Dynamics in Rimrock Reservoir (New Action 2025)
Update Notes
This page was created during the 2025 BTAP update and includes information from the 2012 BTAP and other sources. Approved by the BTWG in February 2025.
References
- ↑ 1.0 1.1 1.2 Mongillo, P., and L. Faulconer. Yakima Fisheries Enhancement Study. Washington Department of Fish and Wildlife, 1980, https://ybfwrb.org/wp-content/uploads/2017/10/Mongillo_Faulconer_1980.pdf.
- ↑ Dayan, David, and Craig Haskell. Deep Drawdowns at Rimrock Reservoir Are Associated with Decline in Bull Trout Redd Counts the Following Year. USFWS, 2024, https://ybfwrb.org/wp-content/uploads/2025/02/2024_11_14-Rimrock-Drawdown-White-Paper-final.pdf.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Lieberman, D., and S. Hiebert. A Limnological Study of Keechelus, Kachess, Cle Elum, Bumping, and Rimrock Lakes in the Yakima River Basin. 2003, https://ybfwrb.org/wp-content/uploads/2025/01/Lieberman-and-Hiebert-2003_YakimaReservoirReportDRAFT_Figures.pdf.
- ↑ Eitemiller, Douglas. Bull Trout in Andy Creek (Rimrock Lake). 3 Sept. 2024.
- ↑ James, P. W. Rimrock Lake Fish Entrainment Study 2001: Final Report. Central Washington University, 2002, https://ybfwrb.org/wp-content/uploads/2017/10/James_2002b.pdf.
- ↑ Hiebert, S., et al. Fish Entrainment from Rimrock Reservoir, Tieton River, Washington 2002. Bureau of Reclamation, 2003, https://ybfwrb.org/wp-content/uploads/2017/10/Hiebert_et_al_2003.pdf.
- ↑ Underwood, K., and S. P. Cramer. Simulation of Human Effects on Bull Trout Population Dynamics in Rimrock Reservoir, Washington. 2007, pp. 191–207, https://ybfwrb.org/wp-content/uploads/2017/10/Underwood_Cramer_2007.pdf.
- ↑ Ackerman, N. K. Fish Salvage from the Tieton Dam Stilling Basin, Tieton River, Washington. S.P. Cramer and Associates, Inc., 30 Nov. 2005, p. 13, https://ybfwrb.org/wp-content/uploads/2017/10/Ackerman_2005.pdf.
- ↑ Courter, I., and J. Vaughan. “Hydropower Operations Reduce Bull Trout Entrainment Mortality at Tieton Dam.” Hydro Review, vol. 30, no. 5, 2011, https://ybfwrb.org/wp-content/uploads/2017/10/Courter_Vaughan_2011.pdf.
- ↑ Whitfield, A. K., and A. Becker. “Impacts of Recreational Motorboats on Fishes: A Review.” Marine Pollution Bulletin, 2014.
- ↑ Taylor, Aimee. Spatio-Temporal Movement Patterns of Sub-Adult Adfluvial Bull Trout. 2022. Central Washington University, https://ybfwrb.org/wp-content/uploads/2023/01/Spatio-temporal-Movement-Patterns-of-Sub-adult-Adfluvial-Bull-Tro.pdf.