Rivers that Depend on Aquifers: Drafting SGMA Groundwater Plans with Fisheries in Mind

In California, surface waters have historically been regulated as if they were unconnected to groundwater. Yet, in reality, surface waters and groundwater are often hydrologically connected. Many of the rivers that support fisheries such as salmon and trout are hydrologically dependent on tributary groundwater to maintain instream flow. This means that when there is intensive pumping of tributary groundwater the result can be reductions in instream flow and damage to fisheries.

For this reason, stakeholders concerned about adequate instream flows for fisheries in California’s rivers, streams and creeks need to be effectively engaged in the implementation of California’s Sustainable Groundwater Management Act (SGMA).

Consider the Scott River in Northern California, part of the larger Klamath River Basin. Nearby groundwater contributes to the Scott River. When high volumes of groundwater are extracted from nearby wells, it depletes the Scott River’s instream flow with adverse impacts on salmon and steelhead trout. As discussed further in Section VI of this guidebook, this has led to litigation over the application of California public trust law to groundwater extraction affecting Scott River instream flows, and efforts to use SGMA to ensure that groundwater pumping near the Scott River is compatible with the instream flow needs of fisheries. Situations similar to the Scott River surface and groundwater basin are unfolding throughout California.

On a statewide basis, how pervasive is the effect of groundwater pumping on surface water flows? Research by Maurice Hall of the Environmental Defense Fund, utilizing the California Department of Water Resources Central Valley Groundwater Surface Flow Model, provides us with some sense of the magnitude of the problem. In a May 2018 presentation, Hall reported:

What the model showed us is that early in the 1900s, 1940s and 1950s, the Sacramento River received a net inflow from the groundwater of something like 1 million acre feet a year…Since that time, the groundwater levels have gone down, and the amount of water that has flowed into the Sacramento River from the surrounding groundwater has gone down accordingly to the point that when we were doing this modeling around 2010, it appeared that on average, the Sacramento River lost just about as much as it gained from the surrounding groundwater in the valley floor. This is the Sacramento River and all of its tributaries upstream of the Sacramento…So the net effect over that period is there was roughly on average 900,000 acre-feet per year less water showing up in the  Sacramento River at Sacramento.” (Hall and O’Brien).

SGMA was enacted in 2014. Pursuant to SGMA, by June 2017 a groundwater sustainability agency was required to be designated for each groundwater basin in California. Each groundwater sustainability agency in high and medium priority basins must prepare and adopt a Groundwater Sustainability Plan (SGMA Groundwater Plan) by 2020 if the basin is deemed to be in a critical state of overdraft or 2022 for all remaining high and medium priority basins.

Each SGMA Groundwater Plan must detail how the groundwater basin will be managed to avoid overdraft conditions and, importantly for fisheries, to avoid adverse impacts on hydrologically connected surface waters.

Although groundwater sustainability agencies and fishery stakeholders recognize that the groundwater-surface water connection needs to be addressed in SGMA Groundwater Plans, at present there is limited guidance on how to do this. That is, what are the specific types of information, modeling, monitoring, and pumping provisions that should be included in SGMA Groundwater Plans to ensure that groundwater extraction does not cause significant adverse impacts on fisheries? The purpose of this guidebook is to provide such guidance.