Bonanza City Stream & Floodplain Project, P2

Phase II of the Bonanza City Stream & Floodplain Project continued the work of rehabilitating river function, complexity, and floodplain connectivity of the Yankee Fork Salmon River, which was historically dredge-mined. The 0.85 mile section of river within the project area was obliterated in the early 1950’s by the Yankee Fork Gold Dredge and has since been unable to recover. Under Phase II, new meandering sections of river channel were built within the newly constructed (Phase I) floodplain area. Large wood structures, riffle-run-pool sequences, and floodplain access were installed for the purpose of restoring appropriate sinuosity to the currently straightened and simplified channel. The outcome of Phase II, specifically, was the construction of new stream channel segments in the western portion of the project area, connection/enhancement of existing side-channels within the eastern portion of the project area, construction of a temporary bypass channel to be used during the upcoming phase of the project, collection and installation of trees for large woody debris structures, and initial additions of potted plants, riparian pole cutting, and native vegetation seed.

Specific actions included: modification, enhancement and construction of 0.74 total stream miles from June-September of both 2019 and 2020. This included 0.41 miles of new stream construction within the floodplain that had been built during Phase I (PCSRF project 006 17 SA). The remaining 0.33 miles of channel work included adding large woody debris structures, boulders, and cover logs to existing mainstem and side-channels within the project area. During construction of the new channel segments, 750 yards of spawning gravel was added to 0.06 total stream miles. Four pools were excavated in the new channel sections and at least six pools are expected to form in the future due to LWD structure scour. A total of 80 LWD structures were installed in the newly constructed channels and the existing stream channels. LWD structures varied in complexity, and included: apex, standard margin, and angled margin jams, deflector logs, sweeper logs, unanchored cover logs, and Beaver Dam Analogs (reported as other engineered structures in metrics). 0.27 miles of newly constructed streambank were stabilized using brush bank and roughened edge methods, which used small woody material and native dredge substrate. A combination of 0.98 streambank miles and 3.0 riparian acres were treated with new riparian vegetation plantings. 1.4 acres within the project area were treated with additional dredge tailings removal. 1.5 acres of upland sites were treated with topsoil amendments and new upland vegetation plantings. Spotted knapweed within the project area was pulled by hand in the few areas where it was found.

The above project actions and outcomes met the planned anticipated biological objectives of the project with a couple of notable exceptions. Some physical aspects of the project results and implementation efficiency varied from what was planned due to a turbidity event in June 2019. Excavation activities in the floodplain resulted in turbid groundwater flowing approximately 840 feet subsurface before seeping into the Yankee Fork River at the down-valley end of the project site, resulting in a visible turbidity plume in the Yankee Fork. To avoid further turbidity events, a 0.03 mi section of the project area did not receive the planned instream actions or structure placement. Species of upland plants were modified from the original plan to include additions of spruce and Douglas fir and to eliminate the use of snowberry, serviceberry, and rose shrubs based upon lessons learned on success of plants from the West Fork Yankee Fork project.

An amendment was completed in July 2019 to add funds in order to accomplish Phase II revegetation elements. Revegetation efforts would have been delayed until the 3rd phase of the project due to higher than anticipated bi

The ESA-listed populations that will benefit from the project include: Snake River Spring/Summer Chinook Salmon – Upper Salmon River MPG – Yankee Fork Population and Snake River Basin Steelhead – Salmon River MPG – Upper Salmon population. Chinook Salmon and Snake River Steelhead are known to utilize the project area for spawning, juvenile rearing, and as a migration corridor to access upstream spawning habitat.

Phase II objectives include building new sections of river channel ‘in the dry’, where at the end of the project the river will have a level of sinuosity and complexity that more closely resembles the historic conditions that the Yankee Fork chinook salmon and steelhead populations evolved to thrive in. The newly constructed sections of channel will have pools for rearing juvenile salmonids, glides for spawning adults, slower water velocities that are more suitable for all age-classes, and complexes of large and small wood to restore levels of cover. These critical fish habitat elements are almost completely lacking in the existing river channel.