Depletion of interconnected surface water (ISW) is one of six sustainability indicators under California's Sustainable Groundwater Management Act. It is also the indicator most often called out by DWR as inadequately addressed in submitted Groundwater Sustainability Plans.
"Interconnected surface water refers to surface water that is hydrologically connected at any point by a continuous saturated zone to the underlying aquifer and the overlying surface water is not completely depleted."
— Cal. Water Code § 10721(i); 23 CCR § 351(o)
If any part of the reach has a continuous saturated zone to the aquifer at any time of the year, it is ISW. Seasonally connected reaches count.
Excludes the disconnected regime (page 01). Once a vadose zone is established below the streambed, the reach is no longer ISW under the statute — although the surface water can still be depleted.
A reach with zero flow is not ISW for that period. So the act of running a stream dry through pumping can remove a reach from the ISW inventory rather than triggering a violation — a known critique of the regulatory framing.
Try selecting different beneficial uses for an ISW reach. The chart shows representative depletion thresholds that have been proposed in the literature or used in submitted GSPs. There is no single "correct" threshold — each beneficial use has its own physical and ecological tolerance.
SGMA frames ISW analysis around beneficial uses and users of interconnected surface water. The categories below are the ones most often considered in California GSPs — not an exhaustive list, and not an assertion that any of them is being harmed in a given basin. Whether depletion rises to a significant and unreasonable level for a particular user is a reach-specific question that depends on magnitude, timing, and the user's tolerance.
Depletion of ISW can reduce flow available to senior surface water rights downstream. In California's prior-appropriation system, any resulting injury is potentially compensable — and SGMA explicitly aims to not alter surface water rights.
Riparian forests, wet meadows, springs, and seeps depend on shallow water tables. A water-table decline of even a few feet can dry out phreatophytes (cottonwood, willow, alder), with cascading effects on shade, bank stability, and aquatic habitat.
Salmonids depend on (a) sufficient summer baseflow to keep pools wetted, (b) cool groundwater discharge to maintain thermal refugia, and (c) hyporheic exchange for incubating eggs. Even moderate depletion can compress habitat dramatically.
Traditional fishing, ceremonial, and gathering uses depend on flowing water and on the species that flowing water supports. Many tribal reservation boundaries and federal reserved water rights are tied to historical streamflow.
Diverters with surface-water sources can be directly affected when summer flows decline; this is the most legible and easily monetized impact when it occurs. Often the lever that brings depletion into litigation.
Kayaking, swimming, fishing access, and scenery all depend on visible flow. Less easily quantified but often politically important — many SGMA basins overlap with recreation-dependent economies.