Walla Walla River Lampson Site Modifications

Rio ASE was hired to assess and implement a side channel design that builds off an earlier design completed by the Confederated Tribes of the Umatilla Indian Reservation (CTUIR) and the Walla Walla Basin Watershed Council (WWBWC). The earlier project included the construction of a 3,000-foot-long side channel and adjacent floodplain habitat along the Walla Walla River upstream of Milton-Freewater, Oregon. Since its initial construction, the side channel inlet has experienced deposition and aggradation from both coarse- and fine-grained sediment. Annual maintenance was required to keep this side channel open, providing a perennial connection. Rio’s design created additional side channel length and redundant inlets beginning at a pool located upstream of the existing side channel inlet. This new inlet will provide a more consistent water surface elevation and reduce the chance for bedload entrainment within the inlet thereby providing greater functionality throughout the year for improved long-term use.

This project consisted of a wetlands analysis, permitting support, and design from conceptual through final construction drawings. The WWBWC is looking toward implementation in 2020. Future tasks for Rio are likely to include assistance with contractor bidding and selection, construction observation, and as-built drawings.

The purpose of this project was to provide a perennial connection to the 2011 constructed side channel; increase and enhance mainstem, floodplain, and side channel habitat for spring Chinook salmon, steelhead, and bull trout; and enhance riparian habitat for wildlife. Secondary goals included protection of landowner’s infrastructure and property from flooding and erosion. The design includes floodplain grading, in-stream large wood structures, and a newly constructed side channel segment.