Reach 4B, Eastside Bypass and Mariposa Bypass Channel and Structural Improvements Project – Constraints and Opportunities for Lower San Joaquin River Restoration

Reach 4B is a 32.5 mile stretch of the San Joaquin River in Merced County, California.  The reach begins at the Sand Slough Control Structure (River Mile 168.5), extends to the Mariposa Bypass (Sub-reach 4B1), and further to the Eastside Bypass confluence (Sub-reach 4B2).  Until recently, all flows reaching the Sand Slough Control Structure were diverted to the flood bypass system, leaving Reach 4B1 perennially dry for more than 40 years (with the exception of agricultural return flows and occasional flood events).

Reach 4B restoration actions will provide a fish migration corridor (emphasizing adult and juvenile Chinook salmon) as well as salmonid rearing habitat only during cooler months and the outmigration period. It will also provide habitat for both native and introduced resident fish species in some years.  In this presentation, we discuss:

1. potential flow and fish routing concepts, including migration barriers and in-river and floodplain rearing habitat;
2. evaluation and qualification of existing in-river and bypass environmental factors, based on available pathways; and
3. constraints and opportunities to provide migration and rearing success through the reach.

We will also provide the background for developing a potential comparison of existing and proposed conditions with respect to habitat and flow requirements to establish a relative suitability index for each sub-reach.   Suitability will be evaluated by identifying and characterizing environmental factors in each sub-reach. Potential factors may include the number and type of structural migration barriers, habitat suitability for migration and in-river rearing during critical time periods, and estimated energy expenditure of fish migrating through a given sub-reach. Qualifying the importance of key factors within individual sub-reaches to determine the overall suitability of a particular route will also be discussed.