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. 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 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. All undoubtedly benefit from the abundance of kokanee salmon and relatively high lake productivity. <-- Citation / update?? The Indian Creek and South Fork Tieton River populations are the most abundant in the Yakima Basin, although they may still be in decline. The North Fork Tieton River population has only moderate abundance.

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 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 (See Rimrock Lake Populations for more info).

Population Monitoring

Include reference to radiotelemetry studies (Mizell and Anderson, 2008, 2015)

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 who is 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 12,000 acre-feet. Local biologists documented entrainment and mortality of numerous kokanee salmon during this low-pool period. (Effects on Bull Trout unknown)

David Dayan analysis? Any other new data from tagged fish to show entrainment?

Moved from Indian: Before impoundment Indian Creek was a tributary of the North Fork Tieton River. It becomes one again each year when the reservoir is drawn down and the North Fork Tieton River flows across the dry bed of the reservoir, usually between late September and mid-October. An extensive meadow complex known as McAllister Meadows, near the confluence of Indian Creek and the North Fork Tieton River, was inundated after impoundment in 1925.

Threats

Connectivity

Fish Passage Barriers (Check/update FDSI)

Threat Severity: Significant

Tieton Dam blocks upstream passage into Rimrock Lake, leaving Rimrock Lake only accessible to adfluvial bull trout that already reside there.

Entrainment

Threat Severity: Unknown, likely significant

*Add 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 September 1st. 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.

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. A large 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.

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??). There is a natural spawning population of Kokanee in Indian Creek.

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:

Fisheries Impacts

Angling Regulations/Fisheries Use/Poaching

Threat Severity: unknown, likely significant

Angling in Rimrock Lake is common and it is likely that 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

Low Population Resiliency

Threat Severity:

Extrinsic Vs. Intrinsic small population threats?

Other

New 2024 Suggestions: (see google doc)

-Varial Zone passage (AT: addressed above in connectivity?)

-Small pop dynamics / effective population size (need for more discussion?)

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 spawning tributaries. 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 D9+