The Tieton Reservoir Road was constructed in 1931 in association with the construction of Tieton Dam, completed in 1936. The road was part of the State Highway system until 1951 when US 12 was constructed. There was a series of agreements between Yakima County and the United States Forest Service (USFS) for maintenance of the road, beginning in 1966. By 1985, the wooden log trestle bridge crossing the South Fork Tieton River was failing, so the County and USFS entered into an agreement to replace the bridge.
South Fork Tieton River’s natural alignment at the crossing was between a spur of weak volcanic rock on the west bank and a slow moving glacial-clay landslide on the east bank. In order to avoid construction on the east bank in the landslide, a river channel was cut through the volcanic rock spur, and the bridge erected over that cut. The natural channel was then filled, and the road was realigned. Unfortunately, immediately downstream of the bridge, the rock spur was vertical, creating a 40 foot waterfall when the reservoir draws down. The final legal agreement between the County and USFS was also never perfected, as the County was supposed to take ownership of the road via easement from the USFS.
This creates a fish blockage from mid-August through May or June in most years. The South Fork Tieton River supports the largest Bull Trout population in the Yakima Basin, and the only tagged out-migrant to be tracked going over the falls was killed by the impact-likely other fish have experienced mortality here as well. In order to conserve Bull Trout, Rimrock Reservoir is held high until August 10th to provide fish passage into the river, later in- migrants cannot access the river after late August, which is still within normal spawning migration timing. The falls are a full blockage to Kokanee from lake and are at least a partial blockage for Rainbow Trout (O. mykiss) in the spring. Both species are analogs for the planned introduction of anadromous sockeye and Steelhead to Rimrock Reservoir and the Tieton River that flows into it.
The project has two components: completion of the 1985 agreement to transfer the Road to Yakima County, and construction of a new bridge near the old channel alignment, to allow fish passage into and out of Rimrock Reservoir over a much longer time frame during the year. The completion of the bridge at this location will be beneficial in 4 ways:
1. Allowing a longer duration passage window to Threatened Bull Trout
2. Increasing overall productivity of the reservoir
3. Preparing the reservoir for anadromous fish reintroduction
4. Improving the flexibility of managing the reservoir level of Rimrock Reservoir, and by extension, all the United States Bureau of Reclamation reservoirs in the Yakima River basin
In most years, the Rimrock Reservoir is held high until the fall, when Spring Chinook spawning occurs in the upper Yakima River, irrigation releases are then shifted to the Tieton River for the remainder of the year. During drought years, this inability to release water earlier from Rimrock Reservoir causes problems for both irrigation and the other Bull Trout populations and their access to spawning grounds in other reservoirs.