7.3 Project Area 21 (River Mile 31.5 to 30.3)

Project Area 21 (PA-21) is located within the active channel and floodplain from RM 31.5 to the bridge at RM 30.3.

7.3.1 Site Description

7.3.1.1 Channel Characterization

The channel through PA-21 is characterized as a single-thread, plane-bed channel. Overall, the channel is highly impacted by human activities (likely dredging and straightening) and the presence of infrastructure. A majority of the channel is wide and shallow with little complexity. Local rapids and pools are associated with the presence of boulders and rock weirs. Two short split flow sections are present between RM 31.1 and 30.7, where a moderate amount of LWD has accumulated and formed relatively deep pools. Downstream of RM 30.7, the channel may have been dredged historically; the channel bed is several feet below the floodplain surface (Photograph 7-3). Throughout the project area, the right bank flows along the toe of the valley wall. Much of the left bank is lined by levees, riprap, and spoil piles that confine the channel against the valley wall. An access road located at approximately RM 30.9 also confines the left bank at the downstream end of a long dredge- poil levee that lines the tributary that enters the river at approximately RM 31.0. Several rock weirs and rock structures throughout, and bedrock near RM 31.2, hold the channel grade hroughout the project area. Three large rock weirs with deep plunge pools and one large rock barb are located between RM 30.7 and 30.6. Irrigation pumps are located at approximately RM 31.15, 30.9, and 30.5. Instream habitat is limited by lack of complexity and by hydraulic conditions that result in high velocities and prevent the retention of LWD and sediment. Through much of the project area, the channel is wide and shallow. Pools and instream cover were generally limited to the locations of large weirs. A majority of the weirs appeared to be passable by adult fish but may present difficulty for juvenile passage. The straight, confined conditions likely result in high instream velocities during spring runoff and floods, and there were very few opportunities for fish to seek refuge.

7.3.2 Conceptual Project Actions

Restoration actions for PA-21 involve opening new flow pathways and widening the floodplain corridor by setting back levees, armoring, and spoil piles. Additionally, LWD will be placed throughout the project area, including supplementary placements at existing rock structures. The levees and spoils located on the right bank between approximately RM 30.95 and 30.9 would be setback to the edge of the low-lying floodplain, which will require relocation of the pump and access road. Spoils and riprap along the right bank between RM 31.5 and 31.25 will be set back into the low-lying floodplain. Surface water extraction points may be converted to groundwater sources to limit disturbance associated with these practices.

7.3.2.2 Biological Benefits

Setting back armoring and easing channel confinement will decrease velocities along the face of the bank to create low-velocity channel margin habitat, and decrease overall channel velocities during high flows. Adding complexity to the project area via LWD will provide hydraulic diversity and refuge in the mainstem, significantly improving habitat conditions and carrying capacity for juveniles. In the sh