Napa project ties environmental-flow monitoring to groundwater thresholds for steelhead and frogs

A monitoring and modeling team applying the California Environmental Flows Framework to the Napa Valley subbasin reported results showing large year-to-year variability in fish counts and preliminary model estimates of pumping impacts on summer baseflow.

Christian Brodrick, geomorphologist at Stillwater Sciences, presented two years of monitoring (2024–2025) from six intensive sites in the Napa system and described how the CEF approach is being used to set ecological goals and inform minimum thresholds in the groundwater sustainability planning context.

Field findings and counts: Brodrick said surveys documented sharp differences between 2024 and 2025. In Sulphur Creek, juvenile steelhead counts were "fewer than 10" in 2024 and "more than 200" in 2025; at Calistoga the team recorded "more than 2,500 Chinook salmon juveniles in Calistoga in 2025." The team also used eDNA, visual encounter surveys, habitat mapping and a StreamWatch citizen-science wet/dry mapping program to characterize seasonal connectivity.

Hydrology and modeling approach: The project combined multiple hydrologic tools. Brodrick said the Napa Valley Integrated Hydrologic Model (LSCE) is being used to represent summer baseflow magnitude (monthly time steps) and that model scenario runs indicate groundwater pumping reduces flow by roughly 2–5 cubic feet per second (cfs) at Saint Helena and about 5–9 cfs at Yamhill. He recommended pairing the model outputs with USGS gage data and the natural flows database to capture different flow components.

Ecological goals and species implications: Using a scoring system informed by local monitoring, the team proposed site-specific ecological goals: maintain summer groundwater within riparian rooting zones to support riparian vegetation; preserve upstream and downstream fish passage; maintain rearing habitat and temperatures suitable for steelhead and Chinook where present; and sustain conditions that allow foothill yellow-legged frogs to complete egg-to-adult development where those frogs occur. Brodrick noted trade-offs: the warm conditions harmful to steelhead can accelerate frog development, and ephemeral tributaries that dry annually may suppress bullfrog predators and benefit foothill frogs in some locations.

Uncertainties and next steps: Brodrick emphasized that the work is ongoing. The team will quantify groundwater–flow–habitat relationships for steelhead and frogs, continue annual fish monitoring and periodic frog surveys, and refine thresholds based on population and temperature relationships. He flagged remaining uncertainties about whether reduced groundwater flux alone can maintain cool summer temperatures and about the effects of upstream incision and legacy channel modification on habitat.

The team recommended continuing monitoring, refining model parameterization (including hyporheic conductivity where relevant), and using scenario-model comparisons (pumping on/off) to assess the relative benefits of management options.