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.
SGMA Depletion Series Page 05 of 07
The six sustainability indicators — and where ISW sits
The statutory definition (annotated)
"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)
"at any point"
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.
"continuous saturated zone"
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.
"not completely 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.
From depletion to "undesirable results"
Interactive: what does a "significant and unreasonable" threshold look like?
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.
Select beneficial uses present
Reference baseline (pre-2015 streamflow)
Figure 2. Indicative depletion thresholds (% of baseflow) below which each beneficial use is generally considered to be impacted. Values are illustrative — actual thresholds are reach-specific and depend on aquatic species, instream-flow requirements, and senior water rights. After Tolley (2020) and SWRCB (2019).
Who is harmed by depletion?
Surface water rights holders
Depletion of ISW reduces flow available to senior surface water rights downstream. In California's prior-appropriation system, the resulting injury is potentially compensable — and SGMA explicitly aims to not alter surface water rights.
Groundwater-dependent ecosystems (GDEs)
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.
Cold-water fisheries
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.
Tribal & cultural uses
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.
Municipal & ag diversions
Diverters with surface-water sources are directly affected when summer flows decline; this is the most legible and easily monetized harm. Often the lever that brings depletion into litigation.
Recreation & tourism
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.
Open questions every GSA struggles with
Baseline period: SGMA uses 2015 as the start of the regulatory period, but baseflow in many basins had already been declining for decades. Is "depletion" measured against 2015 conditions, pre-development conditions, or some natural-flow reconstruction?
De minimis pumpers: household and small-acreage wells often dominate well counts but represent a small fraction of pumping volume — yet they can be located in critical ISW zones (Zipper et al. 2019 cannabis paper).
Return flows: some pumped water returns as deep percolation and may eventually re-enter the stream as recharge. Should net depletion or gross depletion be the regulated quantity? (Robinson 2019)
Climate non‑stationarity: a baseflow threshold defined under historical precipitation may be unattainable under a hotter, drier climate, even with zero pumping.
Cross-jurisdictional reaches: wells outside the basin can deplete in-basin streams (and vice versa), but GSAs only have authority over wells inside their boundary.
Key references in the project library
Cantor, A. et al. (2018). Navigating groundwater–surface water interactions under the Sustainable Groundwater Management Act. UC Berkeley CGIF.
Reeves, H.W. (2017). Stream depletion through the SGMA lens — Michigan Water Withdrawal Screening Tool and assessment process.
SWRCB (2019). Evaluating stream depletions in small watersheds and headwaters of California (draft staff report).
Robinson, J. (2019). Return flow and well depletion: protecting streams from groundwater impact.
Zipper, S.C., Carah, J.K. et al. (2019). Cannabis and residential groundwater pumping impacts on streamflow and ecosystems in Northern California.
Tolley, D., Foglia, L. & Harter, T. (2020). Streamflow depletion analysis for an intermittent stream, Scott Valley, CA.
DWR (2016/2023). Best Management Practices and Guidance Documents for Sustainable Groundwater Management Act.