SWAMP - Intensive Rotational Basin Program Information

The Northeast Basin has three major watershed areas, which drain to the Sacramento-San Joaquin River Delta (Delta) downstream of Vernalis. The southernmost watershed area is the Calaveras River watershed, which is approximately 377,709-acres. During the irrigation season the majority of the flow from the Calaveras River below New Hogan Reservoir is diverted into Mormon Slough for agricultural use before returning the tail water to the river upstream of its confluence with the SJR Federal Deep Water Ship Channel in Stockton.

The second watershed in this sub-area is the Mokelumne River Watershed, approximately 908,294-acres. The Mokelumne, like the other eastside rivers, flows cool, high quality water from Camanche Reservoir. The Mokelumne receives discharges from various urban and agricultural sources before flowing into the SJR Delta near New Hope Landing.

The third major watershed in the Northeast Basin sub-area is the approximately 501,373-acre Cosumnes River Watershed. The Cosumnes is one of the few rivers in California that does not have a major in-stream impoundment. There are several small drinking water reservoirs on tributaries of the Cosumnes. The Cosumnes River is impacted by several sources including rural and urban communities, range cattle, vineyards and other agricultural inputs. During the summer months, the Cosumnes is normally dry from the Highway 16 Bridge in Rancho Murieta to its confluence with the Mokelumne River near Mokelumne City.

Sampling in this basin occurred between January 2002 and December 2002. Approximately 20 sites were monitored, twice a month for field parameters (electrical conductivity (EC), pH, temperature, dissolved oxygen (DO) and turbidity) and bacteria (total coliform and E. coli). Additional spot analyses were conducted for toxicity, nutrients, trace elements, and total suspended solids as funding became available.

The Eastside Basin contains the three largest SJR tributaries, in terms of flow: the Merced, Tuolumne, and Stanislaus Rivers, as well as the Littlejohns Creek and Lone Tree Creek Watersheds. Headwaters of the rivers originate in the Sierra-Nevada, with downstream flows in each river regulated by major reservoirs. Below the reservoirs, the eastside rivers have varying discharges and withdraws from municipalities and agriculture before flowing into the SJR and eventually the Delta.

The southernmost tributary in the Eastside Basin is the Merced River. The Merced River Watershed covers approximately 883,115-acres, and contributes approximately 15-percent of the lower SJR flow.

The next major tributary to the north of the Merced River is the Tuolumne River. The Tuolumne River Watershed is approximately 1,195,493-acres and contributes approximately 27-percent of the total flow of the lower SJR.

The Stanislaus River is the northern most river in the Eastside Basin. The Stanislaus River Watershed contributes approximately 18-percent of the total lower SJR flow at Vernalis from its 737,372-acres.

North of the Stanislaus River are the Littlejohns Creek and Lone Tree Creek Watersheds, cumulatively covering 371,861-acres. Lone Tree and Littlejohns Creeks are mainly used for agricultural supply and return flows, as well as flood control for the Farmington Flood Control Basin during extreme high-water events. Water is stored in Salt Springs Valley and Woodward Reservoirs then released as needed for irrigation and flood control. Lone Tree and Littlejohns Creeks merge southeast of Stockton to form French Camp Slough. French Camp Slough then flows into the SJR just upstream of the Deep Water Ship Channel south of Stockton.

In addition to the major tributaries in the Eastside Basin, there is an area of about 305,000-acres that drains directly to the lower SJR via a series of irrigation and drainage canals on the eastside of the San Joaquin Valley. These canals contain water from a variety of sources including agricultural surface returns, urban runoff, treated municipal wastewater, ground water, and natural stream flows. The area draining directly to the SJR has three major sections. One large section between the Merced and Tuolumne Watersheds, one smaller area in the north between the Stanislaus and Tuolumne Watersheds, and one to the south between the Merced River Watershed and the Southeast Basin. These laterals and drainage canals contribute approximately 4-percent of the total flow in the lower SJR.

Sampling in the Eastside Basin occurred between January 2003 and March 2004. Approximately 30 sites were monitored twice a month for field parameters (EC, pH, temperature, DO and turbidity) and bacteria (total coliform and E. coli). Additional spot analyses were conducted for total suspended solids, total organic carbon (TOC), partial minerals, trace elements, and toxicity, as funding permitted.

The Westside Basin encompasses the watersheds of the westside creeks and is approximately 386,000-acres, contributing 6-percent of the total SJR flow. Land use in this sub-area is predominantly agriculture including; confined animal facilities, row crops and orchards, there are also several small municipalities. Creeks in this area are naturally ephemeral but valley floor sections are kept running for most of the year with irrigation supply and return water. Water in this sub-area is of relatively poor quality, is high in salts, and comes from several different sources including the Delta-Mendota Canal (DMC), pumped ground water, and diversions from the SJR.

The third sub-area is the Southeast Basin, which reaches from the SJR in the South up to the watershed divide between Bear Creek and the Merced River in Merced County to the North. The SJR upstream of the Mendota Pool, an in-stream impoundment, is typically dry for most of the year due to agricultural diversions. Most of the water in this sub-area enters at the Mendota Pool which receives agricultural supply water from the DMC and some upstream releases during extreme rainfall events. Most of the water released from the Mendota Pool and irrigation return flows are diverted out of the lower SJR at Sack Dam for irrigation supplies. The lower SJR is usually dry from Sack Dam until near where it reaches the Eastside Bypass and Bear Creek, which are the main SJR tributaries that drain this sub-area. Including agricultural supply and return flows, this sub-area accounts for approximately 23-percent of the SJR flow at Vernalis.

Grassland Bypass Project Data collected from October 1995 through December 2016 by the Central Valley Regional Water Quality Control Board is available in CEDEN (California Environmental Data Exchange Network).

Please see the Sacramento-San Joaquin River Delta webpage for more information.