Safeguarding California's Streams

Groundwater keeps rivers flowing during dry summers providing water for people, recreational oppotunities for communities.

Groundwater is the source of drinking water for millions of people across the state of California.

Groundwater provides essential water supplies for farms to sustain food production and agricultural economies. 

Groundwater contributions to streams is essential for sustaining fish and wildlife habitats especially in the dry season.

Groundwater is a drought resilient water supply that serves as a buffer against the impacts of drought when sustainably managed.

Improving sustainable management of groundwater resources ensures that groundwater will be available in future droughts to meet the needs of communities and wildlife. Groundwater is a primary source of drinking water for communities, contributing up to 60% of the state’s water supply in dry years. Avoiding impacts of new wells on existing groundwater users is therefore essential for enhancing water security and building climate resilience for drinking water, agriculture, and the habitats of fish and wildlife, especially with increasing drought frequency and severity. 

By making certain that new wells don’t undermine existing surface water rights, permitting guidelines can benefit current water users. Wells can deplete streamflow that is otherwise allocated to existing water rights. Properly designed well ordinances can ensure that these rights are upheld, help prevent conflicts between water users, and promote fair use of water resources. With sustainable management, local governments can support agricultural activities, maintain ecological balance, and provide reliable water supplies for communities. This protection also helps in preserving the natural flow regimes of rivers and streams, which are vital for the health of aquatic ecosystems.

Healthy creeks support clean water and thriving ecosystems, which benefit local residents through improved public health and recreational opportunities. They reduce flood risks and recharge groundwater, helping communities adapt to drought and climate variability. When creeks are restored and protected, they also foster civic pride and stewardship, strengthening connections between people and their environment.

Healthy rivers provide social, economic, and environmental benefits that communities value. Many ecosystems, such as wetlands, rivers, and lakes, rely on groundwater to sustain their flora and fauna. By managing groundwater levels sustainably, counties can meet conservation goals as well as regulatory requirements related to protected species and their habitats. Protection of groundwater dependent ecosystems also supports recreational resources, promotes biodiversity, and enhances the resilience of ecosystems to environmental changes. 

Addressing gaps in the Sustainable Groundwater Management Act which does not apply to the majority of the state’s streams. While the vast majority of groundwater withdrawals occur within SGMA basins, most of the state—and the majority of the state's streams—are not included.3 Avoiding impacts of new wells—whether inside a SGMA basin or not—is essential to managing groundwater levels sustainably, protecting groundwater dependent ecosystems, and promoting long-term water security and resilience against droughts and climate change impacts.

Identify and map public trust resources within the county where streamflow and groundwater dependent wildlife or ecosystems are at risk of negative impacts from additional well pumping. 

Key initial steps in updating a well ordinance to protect public trust resources linked to surface waters are to: (1) identify the specific public trust uses supported by streamflow—such as fishing, navigation, and ecosystem health; and (2) map where these resources are located. While public trust protections are most clearly tied to navigable waters, local governments should include other surface waters that support sensitive or highly valued habitats, regardless of navigability. Several counties that have undertaken efforts to update their ordinances so far have focused on salmon and steelhead as their primary public trust resources to determine which streams and watersheds to assess for potential impacts from new wells. The logic for focusing on these fish, in addition to being state-listed species, is that they are present in all times of the year and flows that meet their life-history needs are likely to meet those of other species and support general ecosystem health. 

Informational resources: TNC mapped California’s groundwater dependent ecosystems and high-priority rivers. This map and other resources can be easily accessed through the Groundwater Resource Hub.

Identify protective streamflow thresholds for selected public trust resources to guide development of management goals, assessment of existing conditions, and establishment of protective standards. 

To protect public trust resources and meet conservation goals, it’s important to define when a reduction in surface flow would significantly harm key species or habitats. For new wells, this means identifying how much streamflow is needed to support healthy populations and ecosystem functions. Streamflow needs are complex because they naturally change with the seasons and vary from dry to wet years. However, this variability in flow—its timing, amount, and duration—is critical for freshwater ecosystems; many species have adapted key life processes, such as migration and spawning, to these patterns. Setting ecological flow thresholds is also essential for evaluating current conditions (Step 3) and for creating standards and permitting rules (Step 4) that will be included in the well ordinance framework (Step 5).

Informational resources: TNC’s Natural Flows Database can help assess current stream conditions and flow needs even when there are no stream gages or data. TNC and partners developed the California Environmental Flows Framework to provide protective flow criteria and help managers determine how much water streams need for nature.

Assess existing conditions of streamflow and groundwater resources to determine if current streamflow meets ecologically protective thresholds, including demands from existing water rights for both surface and groundwater in all water year-types. 

This assessment helps determine whether streamflows are meeting the ecological thresholds identified in Step 2 and how they align with management goals. This process is key to identifying when, where, and how much streamflow might still be available to support new wells without harming ecosystems. To evaluate the risk of new wells, it’s important to have accurate data on streamflows across different types of water years. While some healthier streams may be able to handle small impacts, others already stressed by diversions, pumping, or land-use changes may be too impacted for additional unmitigated groundwater development.

Informational resources: The California Environmental Flows Framework provides information on existing streamflow conditions in some locations. TNC’s Gage Gap tool provides an easy way to find streamflow gage data. To assess existing water rights, the SWRCB’s California Water Accounting, Tracking, and Reporting System is a computer database developed to track information on water rights in California. For northern coastal watersheds, the Water Availability Tool is another source for information that can help assess whether there is water available for diversion without impacting existing water rights. 

Develop groundwater availability maps and additional resources to aid in assessment and permitting decisions based on the degree of risk to public trust resources associated with further groundwater development throughout the county. 

Counties can provide essential information and tools to support planning and permitting using the information from Steps 1–3 to identify areas where the addition of wells would not have a significant impact on a stream, depending on type, number, and size. In areas where additional wells pose significant risks, limits on pumping, mitigation requirements, and other measures can be developed. The process of developing groundwater availability maps and tools is complex and requires collecting data, analyzing them spatially, and assessing potential for streamflow depletion using hydrologic models.

Informational resources: To model the impacts of wells on streamflow across large geographies, TNC is developing an approach that pairs high- and low-resolution modeling tools. Low-resolution models (e.g., Analytical Depletion Function) require less data, time, and cost to develop and may be appropriate for large areas where depletion risks are low. Higher resolution models (e.g., USGS MODFLOW) might be necessary where the risk of streamflow depletion impacts on streamflow are higher.

Develop well ordinance, policy, and permitting guidelines that avoid negative impacts to communities, nature, and existing water users. 

Cities and counties working to update their well ordinances can draw inspiration and practical insights from similar efforts by local governments throughout California. Valuable approaches include engaging stakeholders, forming technical advisory committees, integrating land-use planning, and establishing clear structures for policy development. To address uncertainty and data gaps, the permitting framework should be transparent, be adaptable to new information, and include clear reporting requirements to support compliance and the protection of public trust resources.