Sacramento River
© The Nature Conservancy (Grant Johnson)


Rivers and streams are natural waterways that flow across diverse landscapes starting in mountainous regions and usually terminating in a wetland, lake, or the ocean. Where freshwater rivers meet the ocean, they create estuaries, semi-enclosed coastal water bodies with a rich environment containing brackish water.

Most rivers begin high up in the mountains or hills, where winter snow melts and rain falls. Snowmelt and rainfall can also seep into the ground to form groundwater. Groundwater connects to rivers and streams on a permanent or intermittent basis. In gaining reaches, groundwater contributes flows to the stream. In losing reaches, groundwater is replenished by surface flows. Groundwater commonly contributes all or some of the water flowing down a river, particularly during California’s dry summer and fall months.

During periods of insufficient snowmelt or rainfall, or excessive groundwater pumping, groundwater levels can drop below the elevation of the riverbed, pulling surface flows away from rivers into the ground. Depending on the aquifer conditions, groundwater pumping farther away from the river can still impact groundwater levels below the riverbed. In some cases, sections of rivers and streams can go dry for longer periods of time. Since groundwater is often a source of critical cold water for rivers and streams during hot summer and fall months, habitat and cold water refugia for native fish can be lost.

In places where rivers and streams meet the ocean, estuaries can also depend on groundwater.  Groundwater can be an important source of freshwater to estuaries, particularly in intermittent river systems or where upstream surface water use is substantial.

California has over 500 estuaries in a great diversity of sizes and types, ranging from the Sacramento-San Joaquin Delta Estuary—the largest estuary on the west coast of the Americas—to tiny, intermittent streams reaching the ocean.  Lying at the interface of land, freshwater, and oceans, estuaries host a dynamic range of biological and physical processes. Estuaries are crucial in the exchange of energy and materials between these realms, creating a diversity of habitats that support a variety of species. Estuaries are some of the most productive areas on Earth, providing an important food source for resident and migratory animals. Many species also rely on estuaries for a place to breed or rear offspring, and for migration stopovers.  Some of California's biggest cities—San Francisco, San Diego, and Los Angeles—are located near coastal estuaries, which serve as natural buffer zones against coastal storms and sea level rise for cities and communities. In addition, they are popular recreational destinations for fishing, birding, boating, and hiking.

Estuaries graphical diagram image

Consequences of Intensive Pumping

  • Groundwater depletion can cause reaches of rivers and streams to have greatly reduced flows or even go dry at certain times of year, fragmenting wildlife corridors, impairing fish migration, and reducing or eliminating riverine habitats.
  • Groundwater depletion can increase salinity concentrations in estuaries and impact habitat conditions.
  • Depleted rivers and streams result in less water available for communities, agriculture and other industries that support our economy.
  • Lower groundwater levels can eliminate the input of cold water or reduce the total amount of water flowing in a river or stream. Decreased groundwater input can cause the remaining stream water to warm as it gets shallow. Warmer water can affect wildlife behavior and harm important spawning habitats for salmon and other fish.
  • Decreased groundwater levels in riparian areas and floodplains mean plants and trees must have deeper root systems to reach sufficient water for survival and growth. Some plant and tree species cannot adjust enough to survive; others no longer have the conditions suitable for recruitment, decreasing regeneration of floodplain and riparian forests.

Did you know?

of California’s rivers and streams lose water each year to replace groundwater lost  to pumping.