Surface Storage

Surface storage offers a mixed bag of benefits and impacts.  On the one hand, dams can provide water storage, flood control, hydropower, and recreation; on the other hand, dams can disrupt ecosystems, impact native species populations, block sediment transport, and cause water quality problems.  The mix of benefits and impacts is highly dependent on where and how a reservoir is built.

Toolbox Main Page IconEver since the Gold Rush days, building dams and canals has been California’s answer to dealing with its unpredictable hydrology.  More than 1,400 dams and thousands of miles of canals and aqueducts have been built to move water from the mountains where it falls to the farms and cities where it’s used.  These dams capture and store runoff for use during drier times, provide flood control for downstream communities, and generate hydropower.  However, construction of these dams has come with serious ecological costs, among them a decline in abundance of native species, fragmented ecosystems, degradation of downstream habitat, and water quality impacts.

California has nearly 200 reservoirs with a capacity of 10,000 acre-feet or more; their combined surface storage capacity is more than 41 million acre-feet.  Some of the state’s largest reservoirs are major components of the state and federal water projects.  However, there are many smaller reservoirs that serve local or regional needs for a multitude of purposes.

Most of the state’s reservoirs were built in the early to mid-twentieth century with only one or two basic purposes, such as water supply or flood control.  However, with the onset of environmental regulations in the 1970s, over time the role of surface storage has evolved beyond these primary purposes to include other objectives such as water quality management, ecosystem management, and recreation.  New regulations and legislation have added new constraints and objectives, requiring many reservoirs to operate differently then what was envisioned when they were built.  So at the same time as competing demands for water have increased, the operational flexibility of various surface water systems has decreased.

The majority of the state’s dams were constructed more than 40 years ago.  In recent decades, construction has slowed; only six new water supply reservoirs were constructed from 1980 to 2000, and only three have been completed since 2000.  Some surface storage reservoirs have been enlarged since 2000.


There are two types of surface storage reservoirs:

Reservoirs which are formed by building a dam across a river, such as Shasta Dam on the Sacramento River, Oroville Dam on the Feather River, and Friant Dam on the San Joaquin River.  These dams also typically provide flood protection.

Reservoirs which are built not built across a waterway, but instead at particular location and are filled by conveying water and pumping it into the reservoir; these reservoirs are referred to as “off-stream” reservoirs.  Examples in California are San Luis Reservoir near Los Banos, Los Vaqueros Reservoir in the East Bay, and Diamond Valley Lake in Southern California.  Because off-stream reservoirs are not blocking a flowing river or stream, they generally have less environmental impacts.


It should be noted that simply building more surface water storage does not necessarily equate to more water supply; there must be a water source from which to fill it.  The white paper, Integrating Surface Water Storage into California’s Changing Water System in November of 2014 found that at most, California’s largest water systems could potentially use between 5 and 6 million acre-feet of storage capacity; the potential is limited by a lack of additional streamflow needed to fill an expanded storage system.

In the paper, the authors explained why doubling the size of a reservoir does not double water deliveries: “Water deliveries are ultimately limited by the amount of water flowing into a reservoir. A small reservoir in a watershed with variable inflows will greatly improve regular water deliveries. But, as the reservoir size increases, compared with the amount of inflow available to fill the reservoir, the available storage space is filled less and less frequently, which means that the each additional increment of added storage capacity provides less and less water supply benefit.”

The paper also determined that the ability to utilize additional water storage varies greatly with location, water conveyance capacity, and how well the storage is integrated with the other components of the water system.

It’s also important to note that surface storage has limited value during long droughts.  Reservoirs capture water during wet times, reserving some for dry years.  As years progress during extended droughts, these reserves simply become depleted.


Conjunctive management involves moving water from reservoirs and storm runoff during wet periods into groundwater basins for use during dry periods.  By managing surface water and groundwater storage conjunctively, the study found that the greatest benefit can be achieved, expanding annual water delivery to as much as 20 percent of the increase in storage capacity.


Dams provide a wide range of economic, environmental, and social benefits.  Surface storage reservoirs store water from the wet winter season for use in the dry summer and fall months.  Dams provide flood control, preventing the loss of life and property caused by flooding by impounding floodwaters and then controlling the release to the river. Dams can also provide renewable electricity in the form of hydropower and a wide range of recreation benefits, such as boating, skiing, and camping.

Surface storage is an essential component of a conjunctive management program by controlling the timing and volume of water used to recharge groundwater basins.  Surface storage can also help facilitate water transfers.


The benefits of surface storage can come with some considerable impacts.  Building a dam across a river or stream fragments the ecosystem and can impede or prevent fish and wildlife migration.  In California, estimates are that dams block much of the spawning habitat in the Central Valley.  Habitat loss and degradation has been implicated in the decline of native species, particularly salmon.

The relatively stable flows released from reservoirs can be damaging to native species and vegetation that evolved with variable flows and the change in timing of those flows can disrupt life-cycle processes that are cued to seasonal flow patterns.  The reduced occurrence of floodplain inundation impacts native species that depend on floodplain habitats for feeding and growth.

Dams also block the movement of sediments as rocks, sand, and other natural materials build up at the mouth of the reservoir instead of dispersing with the flow through the channel.  The loss of this sediment can impact downstream habitats by causing erosion of channel bed and banks and loss of spawning gravels for salmon and trout.  Reduced sediment delivery from rivers to coastal areas also leads to accelerated beach erosion.

Dams can also degrade water quality as organic material that would have otherwise been washed downstream now builds up in the reservoir; as it decomposes, it consumes large amounts of oxygen, which can lead to algae blooms and oxygen-deprived dead zones incapable of supporting aquatic species.

Generally speaking, because off-stream reservoirs are not blocking a natural watercourse, they tend to have less environmental impacts.

New surface storage projects are required to address impacts under the application of various laws, regulatory processes and statutes, among them the public trust doctrine, the California Environmental Quality Act (CEQA), the National Environmental Policy Act (NEPA), and federal and state clean water act regulations.  Mitigation of environmental impacts is required by avoiding, minimizing, rectifying, or compensating for negative impacts.


With climate change projected to bring more extreme floods and deeper droughts, expanding surface storage capacity can be an effective climate change adaptation strategy.  Increased storage capacity can give greater flexibility to capture increased runoff and provide additional flood protection, and the ability to store water from wetter years for use in drier years will help address climate variability.

However, climate change could potentially complicate reservoir operations.  Most on-stream surface storage reservoirs also serve a flood protection role, and a shrinking snowpack and smaller spring runoff will bring with it the complexity of managing reservoirs for competing purposes such as flood control, water supply, and hydropower.


In November of 2014, California voters approved Proposition 1, the Water Quality, Supply, and Infrastructure Improvement Act, a $7.5 billion water bond for investments in the state’s water management systems. The bond included $2.7 billion to pay for public benefits related to new water storage projects, both surface storage, and groundwater storage.

The California Water Commission is administering the Prop 1 funds through a program called the Water Storage Investment Program (WSIP).  In July 2018, the Commission approved maximum conditional funding amounts for eight projects that collectively would increase the state’s water storage capacity by 4.3 million acre-feet. The projects range from expanding existing reservoirs to boosting groundwater storage to building new storage.

The eight projects based on the public benefits their projects will provide, such as flood control, ecosystem improvement, water quality improvement, emergency response and recreation.

Even though initially approved, there are additional requirements, including obtaining final permits, finalizing environmental documents, execution of contracts for the administration of public benefits, and acquiring other funding commitments for the costs not covered by the Prop 1 funds.  Once complete, the Commission will hold a final award hearing.

For more information on the Water Storage Investment Program, click here.

Click here to download the regional/local storage resource management strategy from the California Water Plan.
Click here to download the CalFed surface storage resource management strategy from the California Water Plan.


Helpful documents and websites …

State agencies …

For the latest reservoir conditions …

Large reservoir projects currently under consideration …

This is not necessarily a complete list.

More on water storage from Maven’s Notebook …

View all posts on water storage by clicking here.

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