Across the watershed | U.S. EPA (2023)

On this page:

• Across the watershed
• EPA's Watershed Approach to Protecting Water Quality
• Important areas of the watershed
  • San Francisco Bay
  • Das Sacramento-San Joaquin River Delta
  • The Sacramento River watershed
  • The San Joaquin River watershed
  • The Tulare Lake Basin watershed

Across the watershed

The San Francisco Bay Delta watershed spans more than 75,000 square miles and includes the largest estuary on the west coast of North and South America. It also contains the world's only inland delta.

The watershed extends nearly 500 miles from the Cascade Range in the north to the Tehachapi Mountains in the south, and is bounded by the Sierra Mountain Range on the east and the Coast Range on the west. Almost half of California's surface water begins as rain or snow that falls into the Continental Divide and flows downstream through the Golden Gate Strait into the Pacific Ocean.

In addition, the watershed provides a primary source of drinking water for 25 million Californians, irrigation for 7,000 square miles of agriculture, and includes important economic resources such as California's water supply infrastructure, ports, deepwater shipping canals, major highway and railroad corridors, and power lines. In the delta in particular, declining water quality and increasing demand for limited water resources are the subject of intense review and planning to protect this precious resource for the future.

The watershed includes a variety of freshwater, brackish water, and saltwater habitats. Several endangered and threatened aquatic species can be found here including delta smelt, steelhead, spring run chinook salmon, winter run chinook salmon and others. Two-thirds of California's salmon pass through these waters, and at least half of the state's Pacific Flyway migratory waterbirds rely on the region's wetlands.

EPA's Watershed Approach to Protecting Water Quality

The EPA addresses water pollution from thewatershed approach, a comprehensive framework for addressing water resource challenges. The effects of pollution and stressors (such as dams, water diversions, and invasive species) in one part of an aquatic system can adversely affect other parts of the system. Water quality restoration is most effective when pollution sources, stressors, and solutions are identified for the entire watershed.

The watershed approach facilitates the participation of interested stakeholders, streamlines the application of environmental laws, and encourages the local responsibility needed to achieve and maintain improvements in water quality.

Water quality problems are more fully characterized when a diverse group of stakeholders invest in identifying sources and solutions. Scientific and technical information can be generated with the guidance of stakeholders and used to identify options to address the most pressing tipping point vulnerabilities. The community investment generated through this process is important to ensure that water quality improvements are sustained over time.

Important areas of the watershed

The San Francisco Bay Delta Watershed consists of several major waterways, including the Sacramento and San Joaquin Rivers and their tributaries. Where these two major rivers meet near Sacramento, a large inland delta forms where the river water pools before flowing through Carquinez Strait into San Francisco Bay.

In addition to the major rivers and delta, the watershed also includes Suisun Bay, San Pablo Bay, San Francisco Bay, and the Golden Gate Strait, where the entire watershed empties into the Pacific Ocean.

San Francisco Bay

The San Francisco Bay watershed covers 4600 square miles of which the bay covers 1600 square miles. It is the largest Pacific estuary in America. Home to over 7 million people, the Bay Area is one of the most densely urbanized areas in the country.

San Francisco Bay includes San Pablo Bay, Central Bay, and South Bay. The tributaries that flow into the bay include the Petaluma, Napa, Guadalupe Rivers, and numerous smaller streams and creeks.

Historically, San Francisco Bay had about 300 square miles of tidal marsh with 6000 miles of channels and 12 square miles of shallow pan. By the 1850s, the bay was navigable as far away as San Jose. After the gold rush of 1850, large amounts of sediment from upstream erosion and hydraulic mining flowed into the bay, and the surrounding tidal wetlands were diked for salt extraction, hayfields made or filled in, reducing the size of the bay by up to a third .

The bay is home to over 500 species of fish and wildlife, 20 of which are endangered or threatened with extinction. The bay and its surrounding water systems host millions of migratory birds each year as they cross the Pacific Flyway.

Das Sacramento-San Joaquin River Delta

For thousands of years, the Sacramento-San Joaquin River Delta (Delta) was a vast plain of seasonal flooding that created 1,000 square miles of brackish tidal marsh, narrow, branching tidal channels, and freshwater marshes.

Following European settlement in the 19th century and water supply, agriculture, and urban development in the 20th century, these original delta habitats were replaced by sub-sea level, dike-protected islands and simplified, deep, wide, armored waterways.

Today, there are about 1,100 miles of levees in the delta, protecting 625 square miles of productive agricultural land and 500,000 residents in small, rural communities and urban areas.

The mixing of freshwater and saltwater (brackish water) in Suisun Bay and the delta provides essential habitat for migratory fish such as salmon and native fish such as delta smelt and striped bass.

The Sacramento River watershed

The Sacramento River Watershed is approximately 27,500 square miles and covers most of Northern California. The watershed is bounded on the east, north, and west by mountains including the Sierra Nevada, Cascade, and Coast Ranges.

The Sacramento River is the largest river in the Continental Divide and the longest river in California. It begins in the Cascade Range and flows south over 400 miles through the flat Great Central Valley where it meets the San Joaquin River and forms the delta before flowing west into the Pacific Ocean. The tributaries that feed into the Sacramento River include the Pit River, Butte Creek, Feather River, and American River.

The lands of the Sacramento River watershed are diverse, ranging from snow-capped peaks to low-lying agricultural areas. There are large areas of forest covering mountain slopes, more than 3000 square miles of agriculture and many small urban areas. The Sacramento metropolitan area covers more than 350 square miles, has a population of 2.5 million people, and is the largest urban area in the Continental Divide.

The water flows in the Sacramento River watershed were greatly altered by gold mining in the 19th century, land conversion from grasslands to agriculture, the development of water supplies in the 20th century, and urbanization of farmland.

The San Joaquin River watershed

The San Joaquin River Watershed is approximately 15,600 square miles and is sandwiched between the Sacramento River Watershed to the north and the Tulare Basin Watershed to the south. The San Joaquin River watershed is bounded by the Sierra Nevada mountains on the east and the Coast Range mountains on the west.

The San Joaquin River is the second longest river in California. It begins in the high mountains of the Sierra Nevada and flows west approximately 100 miles, then turns north and flows for 260 miles where it joins the Sacramento River. The tributaries that feed into the San Joaquin River include (from south to north) the Fresno, Chowchilla, Merced, Tuolumne, Stanislaus, Calaveras, Mokelumne, and Cosumnes rivers.

Water flows in the San Joaquin River have been significantly altered with dams and diversions removing 95% of the water at Friant Dam from the river. These diversions result in the San Joaquin River being dry for more than sixty miles of its course. Some sections of the San Joaquin receive minimal amounts of agricultural and urban runoff. The Delta-Mendota Canal was built to replenish the water in the San Joaquin River by transporting water from the Sacramento River to the Mendota Pool where it is channeled to the San Joaquin River Canal and agricultural users.

The land area in the San Joaquin River watershed is diverse, ranging from snow-capped peaks to agricultural areas below sea level. There are large tracts of forest covering mountainsides, more than 3000 square miles of agriculture in the valley, and a human population of 2 million people living in major urban centers of Stockton and Fresno, small towns, and rural communities.

The Tulare Lake Basin watershed

The Tulare Lake Basin lies south of the San Joaquin River watershed and is bordered by the Sierra Nevada Mountains to the east, the Tehachapi Range to the south, and the Coast Range to the west. Major rivers in the Tulare Lake Basin come from the Sierra Nevada mountains and include the Kings, Kaweah, Tule, and Kern rivers. The smaller rivers of the Sierra Nevada include Deer Creek, White River and Poso Creek.

Prior to 19th-century European settlement and 20th-century water development, the Tulare Lake Basin was defined by four large lakes (about 800 square miles in wet years) and large wetlands (about 625 square miles) that periodically flow into the San Joaquin River Divide River drained . Today, the great lakes and wetlands have been replaced by irrigated agriculture, rural and urban development. Rivers draining the Tulare Basin have no natural surface waterway from the Continental Divide. Water moves in and out of the Tulare Lake Basin through channels through precipitation and water diversion.

Hydrology and Hydrography of the Tulare Lake Basin: A Summary of the Movement of Water and Aquatic Speciesis a very useful account describing how water flowed in and out of the Tulare Basin prior to large-scale water development and how water moves within the Continental Divide today. Detailed maps show waterways, canals, direction of flow and locations of diversions. The hydrology of the four major river systems, the Kings, Tule, Kaweah, and Kern rivers, is provided, as is a description of aquatic habitat, aquatic organisms, and the ability of aquatic organisms to move in and out of the Tulare Basin watershed .