Site C seining for fish, July 1, 1999,

Photo: USFWS

Grasslands Bypass Project Contaminants Monitoring

Historically, farmers in the Grasslands area of the western San Joaquin Valley have discharged subsurface agricultural drainwater through wetland channels in the San Luis National Wildlife Refuge Complex to the San Joaquin River. This drainage contains elevated concentrations of selenium, salt, boron and other trace elements.

Bay/Delta Diagram

To convey this drainwater more directly to the San Joaquin River, bypassing wetland channels, a portion of the San Luis Drain was reopened in September 1996 as the Grasslands Bypass Project. The San Luis Drain has been modified to allow discharge through Mud Slough, a natural waterway that traverses through the San Luis National Wildlife Refuge Complex and a section of the North Grasslands Wildlife Area.

Analyses completed to date indicate that selenium concentrations have declined in the fish and invertebrates of Salt Slough, the principal wetland channel from which drainwater has been removed by the Project.

After the San Luis Drain was reopened and began discharging into Mud Slough, selenium concentrations in the most common fish (mosquitofish and inland silversides) both upstream and downstream of the discharge point rose substantially during the first six months of operation of the Grassland Bypass Project. These concentrations have more recently declined to levels that are approximately the same as pre-project conditions.

Continued biological monitoring is needed to insure that the project will not have a net negative effect on the ecosystems of the San Joaquin Valley.


All the Grassland Bypass Project monitoring data and results of our biological monitoring can be found in the annual reports maintained by the San Francisco Estuary Institute at the following link: