Sultan River Floodplain Activation Design

Snohomish PUD contracted for the assessment, modeling, study and design of additional habitat restoration work on the lower Sultan River. The additional restoration was voluntary effort (not a license obligation or mitigation requirement) to investigate furthering the restoration of the lower Sultan River floodplain to assist in creating high quality salmon habitat. This work was to include on-channel and off-channel work. On-channel work analysis included addition of engineered log jam, excavation of channels on an island, and removal of a small inlet plug (KB Island) that was tampering water from entering the side channel network. Off-channel elements included the restoration/creation of an additional side channel to provide more rearing opportunities for salmonids. The work under this grant included the hydrologic modeling to design increased amount of off-channel rearing habitat that was in the historic floodplain of the lower Sultan River. A wetland delineation occurred to identify wetlands in the proposed project alignment to inform design and permitting needs. Initial consultation and applications with permitting agencies occurred, again to further the design based on input from agencies.

After high flow events and further analysis of the proposed project elements, it was determined that some of the items originally proposed under the design grant are no longer needed to move forward to the construction phase. This includes the excavation of additional channels on the island - high flow event in 2020 provided additional wood and gravel on the island; thereby doing naturally what was anticipated to occur through active construction. Additionally, Snohomish PUD used the designs created under this contract and funded the construction work. Based on the designed placement of a engineered log jam along the upper extent of the grant project, the project would have created less desirable results in Side Channel 2 (the area opposite of the log jam) and hindered the continued extensive use by spawning chum salmon. As such, those three elements are currently not being sought for construction funding under the new grant cycle.

The restoration/creation of an additional off-channel habitat of 1,900 lineal feet was designed. The contractor also developed a based of design report and updated contract specifications for that channel.

The goal of this project was to expand upon prior efforts to design the enlargement of the lateral migration of the river into off channel areas to effectively increase the value and utility of these areas for rearing primarily for juvenile Chinook and coho salmon. The intent and objective of the design was to route flow in a manner that will ensure the persistence of these areas. The degree to which the high flow channel remains seasonally wetted was informed by modelling. A defined outlet and return to the river ensures that rearing fish are not trapped or stranded when flows recede. A secondary goal of this project was to increase the dynamism of the main channel and expand the range of spawning habitats available to anadromous fish in the lower river.