This site was established for the review of a beta version of a graphic visualization describing groundwater dependent ecosystems (GDEs) in California. It was developed for the Groundwater Team of the Nature Conservancy’s California Water Program. It is intended to provide an introduction to GDEs and demonstrate how they can be affected by excessive groundwater withdrawal. Comments should be directed to Melissa Rohde,

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Groundwater Dependent Ecosystems

What are they, why are they important, and how can they be protected?

© Barbara Wampole

We may not be able to see groundwater, but it’s vital to life in California.

Groundwater is critically important to people, supplying 40% of California’s water supply in normal years and up to 60% in times of drought.1 It is also essential in supporting many types of plants and animals, from those found in mountain and desert springs to coastal wetlands.

Groundwater can supply all or some of the water needed to sustain life. Across California, many communities rely partially, or even entirely, on groundwater to supply their water needs. Ecosystems—ecological communities of interacting plants and animals—similarly rely on groundwater. Groundwater is important in California for both human and natural communities because we face long, dry summers and droughts with little rain to replenish surface flows. Groundwater keeps streams, springs, and seeps across California flowing after the rainy season ends. It also keeps deep-rooted plants alive, even when there is no water flowing at the surface.

But with lower groundwater levels from drought and unsustainable pumping practices, springs and groundwater wells are going dry in some places, and important ecosystems are losing access to groundwater, risking collapse.

What is a Groundwater Dependent Ecosystem (GDE)?

GDEs are ecosystems comprised of plant and animal communities that require groundwater to meet some or all of their water needs. GDEs are specifically defined in California under the Sustainable Groundwater Management Act (SGMA) as “ecological communities or species that depend on groundwater emerging from aquifers or on groundwater occurring near the ground surface.”2

Why are GDEs important?

GDEs exist in all of California’s 515 groundwater basins and represent some of California’s unique native habitats, supporting biodiversity and protecting rare or endangered species. GDEs provide valuable functions that benefit people, such as purifying water, sustaining pollinator habitat, reducing flood risk, supplying water and base flow in rivers (the flow that is not augmented by rainfall or surface runoff), and providing hunting, fishing, boating, and other recreational opportunities. Despite the importance of GDEs, recognition of groundwater's contribution to GDEs in California has been historically overlooked.

What can we do to protect our remaining GDEs?

California pumps more groundwater than any other state and ranks ninth highest among nations world-wide.3 California extracts 14.2 million acre-feet of groundwater on average every year (enough water to fill California’s largest reservoir, Shasta Lake, three times).4 Over the past century, groundwater pumping has chronically lowered groundwater levels in many areas across the state—notably the Central Valley—shrinking rivers and wetlands, and in some cases, drying them out completely.5 This lowering of groundwater levels is compromising water supply, degrading water quality, and destroying GDEs.

Recent efforts empower California to reverse a century-long trend of decline. In 2010, the first comprehensive study to identify and map GDEs was published.6 The data from this study has been updated, giving local communities a starting point to identify and map GDEs in their areas. In 2014, California passed the Sustainable Groundwater Management Act of 2014 (SGMA), which requires local communities to identify their GDEs and consider impacts to GDEs in groundwater management decisions.

California has a new opportunity to sustainably manage our vital groundwater resources, benefiting our water supply and preserving our GDEs.

GDEs are Diverse

California’s GDEs encompass a wide range of different natural communities.


Wetlands are lands submerged seasonally or year-round by groundwater or surface water, where plants have adapted to grow in shallow water or saturated soils.

Occurring in coastal areas or inland, water in wetlands can be fresh, brackish (somewhat salty), or salty. Examples include mountain meadows, marshes, vernal pools, and swamps, among others. Wetlands are highly productive and support a wide range of aquatic organisms including mammals, fish, birds, invertebrates, and plants. Wetlands benefit people by cleaning water, protecting communities from flooding, removing carbon from the atmosphere, and stabilizing shorelines.

Wetlands receiving water from groundwater tend to stay wet longer than those fed only by surface water, providing more stable conditions for natural communities. However, when groundwater levels are lowered, the connection to the wetland is lost, water becomes unavailable, and valuable natural habitats shrink.

Consequences of Intensive Pumping

  • Plants and animals that depend on wetlands lose the habitat they need to survive and can die.
  • Migratory birds and other animals lose a necessary water, shelter, and feeding source, disrupting large-scale animal movement, such as the Pacific Flyway, one of the most important global migration pathways for birds.
  • Water quality deteriorates.
  • Hunting, fishing, birding, boating, and other recreational opportunities are lost.
  • Availability of water for human use decreases, negatively affecting domestic, industrial, and agricultural use.
of native wetlands in California’s Central Valley have already been lost to land use changes, water diversions, unregulated groundwater pumping, and climate change.7


Estuaries are semi-enclosed coastal water bodies that exist where freshwater rivers meet the ocean and create a unique environment containing brackish water.

© Douglas Steakley

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.8 Estuaries are some of the most productive areas on Earth, producing or collecting high amounts of organic material that provide an important food source for resident and migratory animals. Many animals also rely on estuaries for a place to breed or rear offspring, and for migration stopovers.

California has over 500 estuaries in a great diversity of sizes and types, ranging from the Sacramento-San Francisco Delta Estuary—the largest estuary on the west coast of the Americas—down to tiny, intermittent streams reaching the ocean. For many of these abundant smaller estuaries, groundwater can be an important source of freshwater, particularly in our intermittent river systems or where upstream surface water use is high. Estuaries occur in some of California's biggest cities: San Francisco, San Diego, and Los Angeles. These coastal estuaries play an important role in creating a natural buffer zone against coastal storms and sea level rise for our cities and communities. In addition, they are popular recreational destinations for fishing, birding, boating, and hiking.

Rivers and Streams

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. Along this journey, river and stream flows interact with groundwater, by either replenishing it and/or receiving additional surface flows.

Most rivers begin their journey high up in the mountains or hills, where winter snow melts and rain falls. Yet snowmelt and rainfall can also seep into the ground to form groundwater. Groundwater connects to rivers and streams on a permanent or intermittent basis. When groundwater levels are higher than the flowing river, groundwater adds to the river increasing its flow. In arid climates found in some parts of California, groundwater commonly contributes all or some of the water flowing down a river, particularly during the dry summer and fall months.

When there are dry conditions, insufficient rainfall or excessive groundwater pumping, groundwater levels can drop below the elevation of the riverbed. Depending on the aquifer conditions, groundwater pumping occurring farther away can still impact groundwater levels below the riverbed. In some cases, impacts from groundwater pumping can also be delayed, meaning that groundwater levels can continue to drop after pumping has stopped. These impacts can cause river flows to be depleted as surface flows sink and less surface water is available to replenish groundwater. In some cases, rivers and streams can go dry in certain segments and/or for longer periods of time as compared to their natural state. This is problematic because groundwater is often a source of much needed cold water for rivers and streams during the hot summer and fall months. Areas where groundwater flows into streams and rivers provide habitat and refuge to many native fish that need cold water to survive, such as salmon and steelhead trout.

Consequences of Intensive Pumping

  • Because rivers and streams frequently carry our water supplies, depleted rivers result in less water available to support communities, agriculture, and our economy.
  • Groundwater depletion can cause reaches of rivers and streams to have a greatly reduced flow, or even to go dry at certain times of year. This can fragment wildlife corridors, impair migration of fish, such as salmon and steelhead trout, and reduce or eliminate riverine habitats.
  • Groundwater often provides summer and fall base flows in streams and rivers. Depletion of groundwater can drastically lower base flows and decrease habitat availability during the months of the year when species are at greatest risk of stress from high water temperatures.
  • Decreased groundwater levels in riparian areas and on floodplains mean that trees and other plants must have deeper root systems to reach sufficient water for survival and growth. Some trees and other plant species cannot adjust quickly enough to survive. Other species no longer have the conditions necessary for recruitment, decreasing the regeneration of floodplain and riparian forests.
  • 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.
3/4 of California’s rivers and streams lose water each year to replace groundwater lost to pumping.5

Caves and Aquifers

Caves are naturally occurring, hollow underground spaces beneath the surface of the Earth that support habitats entirely reliant on groundwater.

© Photographer Name, Year

Springs and Seeps

Springs and seeps are places where groundwater emerges from underground onto the Earth’s surface.

Springs and seeps supply surface flows of water from groundwater aquifers. While these flows are often limited, they may support critical GDEs, including endangered, rare and/or endemic species of mammals, plants, mollusks, fish, and insects that have no other source of water and sometimes no other alternative habitat. Some of the most important springs are found in deserts, and serve as the only watering hole for hundreds or thousands of acres. These springs constitute important watering holes and stopover points for wildlife on the move, including large mammals such as the desert bighorn sheep.

Many springs and seeps are particularly sensitive to even slight declines in groundwater levels and can permanently disappear when groundwater pumping decreases regional groundwater levels. Springs with substantial flow can serve as the headwaters for our streams and rivers, directly sending groundwater downstream. Smaller springs and seeps, especially in arid areas, may support a vibrant but limited riparian corridor before disappearing. In California desert areas, springs and seeps as well as their riparian areas provide islands of habitat for migratory birds. Groundwater from springs and seeps can be colder than surface water, providing important or unique habitat conditions such as the spawning habitat for salmon found in the Shasta Valley. In other places, springs and seeps can be warm or hot because the groundwater is heated under the Earth’s surface, such as in the volcanic area at Lassen National Park or along the Amargosa River in the Mojave Desert, where groundwater from older and deeper reserves provide uniquely important habitat conditions.

Consequences of Intensive Pumping

  • Lower groundwater levels can reduce or eliminate springs and seeps. Resident fish, plants, and invertebrates can lose their habitat and die.
  • In some cases where unique plants or animals rely on a seep or spring, the loss of this habitat can cause an entire species to go extinct.
  • Migratory birds and animals lose a necessary water source.

Terrestrial Vegetation

Groundwater dependent plants require groundwater to be close enough to the Earth’s surface so that their roots can access water.

Vegetation dependent on groundwater access it through their roots. If those levels change rapidly, plants lose access to groundwater and can die. Gradual decreases in groundwater levels sometimes allow plants grow deeper roots to access lower groundwater or to adapt to less water. However, groundwater declines often favor opportunistic invasive plants that displace and degrade natural habitats.

There are two types of groundwater dependent plants—shallow rooted and deep-rooted plants. Shallow-rooted plants rely on groundwater being close to the Earth’s surface and can often be found in groundwater discharge areas such as mountain meadows or coastal wetlands. These shallow-rooted plants are adapted to having their roots submerged in groundwater. Deep-rooted plants (“Phreatophytes”) are not submerged in groundwater and access groundwater in shallow aquifers.

Shallow-rooted and deep-rooted plants respond very differently to declines in groundwater levels. Shallow-rooted plants die immediately when groundwater recedes and soils become dry. However, deep-rooted plants such as mesquite trees are somewhat more resilient to groundwater decline since they are able to extend their rooting systems. Yet they will die out eventually if the depth to groundwater is too far or the rate of groundwater decline is too fast.

Consequences of Intensive Pumping

  • Vegetation becomes stressed and may die.
  • Local animals lose their habitat and must relocate or die.
  • Reduced seedling survival, impaired recruitment, and a reduction in total vegetation.
  • Recreation opportunities are lost.
  • Invasive plants can take over, eliminating native habitat, and create a nuisance or increased fire risk.

California’s Groundwater Dependent Ecosystems are unique, diverse and important.

By maintaining sufficient groundwater levels and the critical connections between groundwater and surface water, we can meet the needs of both humans and GDEs. Protecting Groundwater Dependent Ecosystems requires a science-based understanding of where they are located, how they rely on groundwater, and how changing groundwater conditions can impact them. Routine monitoring can help answer these questions and to ensure they are not adversely impacted by human use of groundwater.

© The Nature Conservancy