Sustainable Groundwater Management: What We Can Learn from California’s Central Valley Streams Reference This publication is a summary of detailed report: The Nature Conservancy. 2014. Groundwater and Stream Interaction in California’s Central Valley: Insights for Sustainable Groundwater Management. Available at: http://www.scienceforconservation.org/ Acknowledgements The full study was prepared by RMC for The Nature Conservancy The study was made possible by the generous support of the Water Foundation, an initiative of Resources Legacy Fund. Photo on cover: Seasonal vernal pools surface amidst lush grasslands and blue oak woodlands at Dye Creek Preserve, part of the Lassen Foothills project where restorative land management and conservation-compatible ranching techniques are administered by The Nature Conservancy on behalf of the state of California. © Ian Shive B SUSTAINABLE GROUNDWATER MANAGEMENT: WHAT WE CAN LEARN FROM CALIFORNIA’S CENTRAL VALLEY STREAMS Across the Tulare Basin, San Joaquin Basin and Introduction Sacramento Valley these changes have differed in mag- roundwater is intimately connected to surface water, nitude, but share a similar trend. In areas with hydraulic Gwhich has profound implications for sustainable connection between groundwater and surface water, water resource management. California has historically increases in groundwater extraction continue to cause overlooked this important interaction and as a conse- declines in groundwater levels that reduce stream flow. quence, decisions about groundwater extractions have generally failed to address the resulting impacts to sur- Our report found that as groundwater production grew face flows and aquatic ecosystems such as rivers, threefold, surface water was seriously depleted in the wetlands and springs. This has contributed to a loss of Central Valley. This region, which accounts for 20 approximately 95 percent of the historical wetlands and percent of all groundwater pumping in the United river habitat in California’s Central Valley.1 States, has now lost nearly all of its wetlands and river habitat. Our modeled results indicate that over 80 With the passage of the Sustainable Groundwater percent of the valley’s rivers lose more water today Management Act (SGMA), groundwater sustainability than they did in their relatively natural state. By the agencies across the state will soon be required to end of our study period, the valley’s rivers were losing manage groundwater resources to avoid causing unde- almost 1.5 billion gallons of water each day—that is sirable results to groundwater levels and interconnected enough water to supply 2.5 times the water needs of groundwater and surface water. These groundwater Los Angeles. In addition, groundwater aquifers contain levels and areas of interconnection support ground- 6.5 trillion gallons less water now than they did at the water-dependent ecosystems2 (GDEs). Therefore, an start of the study period. important first step in sustainable groundwater man- agement is to understand how groundwater pumping The results of our study pre-date the extended drought impacts surface water, including streams, and GDEs. that began in 2011 and it is likely that the drought has To build the case for ecosystem protections now found exacerbated stream depletions. In addition, our study in SGMA, The Nature Conservancy completed a study illustrates that the effects of groundwater pumping in 2014 to illustrate how groundwater pumping is can take years—even decades—to recover. This means affecting streams and rivers in California’s Central that the full extent of the impacts of groundwater Valley. The report, entitled Groundwater and Stream pumping during the drought will continue to plague Interaction in California’s Central Valley: Insights for us for many years. 3 Sustainable Groundwater Management , uses an inte- These findings have troubling implications not only grated hydrologic model to reconstruct the historical for the health of our ecosystems, but also for the sur- impacts of groundwater use on groundwater levels face water right holders. If groundwater pumping and stream flow conditions. The results from that continues to increase, it will become even more chal- detailed study are summarized here. lenging to ensure that surface water is available for Our study focused on the state’s Central Valley the cities, industries, agriculture and plants and ani- because of its importance in California’s overall water mals that rely on surface water systems. supply. We used a model developed by the Department Sustainable groundwater management requires that of Water Resources (DWR) to simulate the Central we acknowledge the critical connection between Valley’s hydrologic conditions during the years from groundwater and surface water. In addition, in man- 1922 to 2009. aging this connection, we must acknowledge the protracted time period it can take for groundwater 1 The Bay Institute (1998) From the Sierra to the Sea: The Ecological History of the San Francisco–Bay Delta Watershed. extractions to impact stream flow. The best tools we 2 Groundwater dependent ecosystems are “terrestrial, aquatic and coastal have to sustain our important groundwater supplies ecosystems that require access to, replenishment or benefit from, or otherwise rely on subsurface stores of water to function or persist.” Howard and are to proactively manage and monitor groundwater Merrifield (2010) Mapping Groundwater Dependent Ecosystems in California. use and to invest heavily in groundwater recharge. PLoS ONE 5(6): e11249. doi:10.1371/journal.pone.0011249 Available at: http://www.scienceforconservation.org/ Implementation of SGMA provides the impetus to 3 The Nature Conservancy. 2014. Groundwater and Stream Interaction in change our approach and to integrate management California’s Central Valley: Insights for Sustainable Groundwater Management. Available at: www.scienceforconservation.org of groundwater and surface water. SUSTAINABLE GROUNDWATER MANAGEMENT: WHAT WE CAN LEARN FROM CALIFORNIA’S CENTRAL VALLEY STREAMS 1 groundwater basins and sub-basins across California, Background the largest of which are the Sacramento Valley, San Joaquin Basin and Tulare Basin that underlie the The Interconnection between Groundwater and Central Valley. Stream Flow Streams and rivers in the Central Valley typically flow ost of California’s groundwater occurs in mate- over sediments that are connected to underlying aqui- rial deposited by streams, called alluvium. M fers. Because the sediments that make up the bottoms Alluvium consists of coarse deposits, such as sand of these stream channels are porous, water can flow and gravel, and finer-grained deposits such as clay back and forth between the streams and the under- and silt. The coarse and fine materials are usually lying aquifer. coalesced in thin lenses and beds that were deposited by streams. In this environment, coarse materials such A range of groundwater–surface water interconnec- as sand and gravel deposits usually provide the best tions are found in basins in the Central Valley. When source of water and are termed aquifers; the fin- groundwater levels in the surrounding sediments are er-grained clay and silt deposits are relatively poor high relative to the streams, groundwater flows from sources of water and are referred to as aquitards. the aquifer into the streams, contributing to the stream California’s groundwater basins usually include one flow. This condition is known as a gaining stream— or a series of alluvial aquifers with intermingled aqui- streams gain surface flows from high groundwater tards. DWR has delineated more than 500 alluvial levels. In some cases, this groundwater inflow keeps FIGURE 1: Groundwater and Stream Interaction in Alluvial Aquifers Groundwater basins in California are predominately alluvial or “valley-fill” groundwater aquifers. These aquifers are made up of unconsolidated or loosely-cemented sediments that have been deposited over long periods of time in valleys. These deposits, sometimes thousands of feet deep, are usually underlain by more solid, and less permeable, rocks that make up the geologic floor of the valley and the surrounding hills or mountains. The sediments that make up the valley-fill are deposited in interwoven layers and veins that vary widely in particle size, from cobbles and gravel, to sands, to clay. The water in these aquifers resides in, and moves through, the pore spaces between the sediment particles. Water moves more easily through sediments of larger particles, and moves very slowly, if at all, through sediments of finer particles, like clays. Gaining Stream—Where rivers or streams run across valley floors underlain by valley-fill aquifers, there will inevitably be exchange of water between the streams and the underlying and surrounding aquifers. If surrounding groundwater levels are higher than the water levels in the river, the river will “gain” water from the surrounding groundwater. This is called a “gaining” reach of stream. This groundwater inflow is often a large portion of the flow in streams after precipitation events have passed. This is often the natural Gaining Stream—Groundwater flows into the condition of streams, since streams are stream, increasing surface flows commonly the major discharge location for groundwater flow. Losing Stream—Pumping of groundwater draws down the groundwater levels near the pumping well, and multiple wells can lower groundwater levels over large regions of the aquifers. If groundwater levels are