TO: Technical Advisory Group for the Napa County Groundwater Sustainability Agency
FROM: Brian D. Bordona, Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby, Natural Resources Conservation Manager
SUBJECT: Presentation on Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystem (GDE) Workplan and CEFF Assessment
RECOMMENDATION
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Provide an update on the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystem (GDE) Workplan implementation and the CEFF assessment.
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Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
The Napa Valley Subbasin Groundwater Sustainability Plan (GSP) and the Interconnected Surface Water and Groundwater Dependent Ecosystems Workplan (Workplan) both recommended using the California Environmental Flows Framework (CEFF) to assess ecosystem instream flow needs in the Napa Valley Subbasin (Subbasin). CEFF is divided into three sections. Section A includes identifying ecological management goals and the functional flow components (e.g., maintain suitable habitat conditions for juvenile steelhead rearing) required to support the ecosystem functions that correspond to each management goal. CEFF Section B includes developing ecological flow criteria for flow components by developing detailed conceptual models relating the functional flow components to ecological management goals and quantifying flow-ecology relationships while taking non-flow factors (e.g., channel incision, physical habitat, and potentially stream temperature) into account. In Section C, which will be completed after implementation of the Workplan, ecological flow requirements are considered along with other water users to develop environmental flow recommendations. The assessment focuses on dry-season baseflow (generally June through October), which is strongly tied to groundwater management in the Napa Valley Subbasin and crucial for supporting special-status aquatic species.
Four listed species that occur in the Subbasin (steelhead, foothill yellow-legged frog, northwestern pond turtle, and California freshwater shrimp) depend on dry-season baseflow for at least part of their lifecycle. The presentation to the Technical Advisory Group focuses on steelhead and foothill yellow-legged frog at three of six intensive sites (Napa River at St. Helena, Napa River at Oak Knoll, and Sulphur Creek) analyzed as part of the Workplan. The ecological goals for each site were tied to observations of species presence and habitat condition during Workplan implementation. Observations suggest that the channel goes dry during normal years at both the St. Helena and Sulphur Creek sites. In contrast, the Napa River at Oak Knoll generally remains flowing during normal years but may transition to isolated pools or dry conditions during droughts.
The hydrology at the six intensive survey sites outlined in the workplan was evaluated using the California Natural Flows Database (CNFD) and the Napa Valley Integrated Hydrologic Model (NVIHM) and compared with field observations and USGS gage data, where available. CNFD is a statistical model that uses hydrologic data from across the state from the period prior to dam construction to develop a statistical model of the range of natural flow conditions throughout California. At all six intensive sites, the range of dry-season baseflows were modeled using the NVIHM with and without pumping and compared with the CNFD and, at two of the sites, with USGS gage data. In general, CNFD predicts slightly lower dry-season baseflow in the mainstem Napa River than the NVIHM without pumping, and higher dry-season baseflow in the two tributary sites than the NVIHM. Other studies have shown that CNFD struggles in alluvial basins where baseflows are dominated by groundwater.
As part of CEFF Section B, detailed conceptual models relating flow to ecological management goals were developed for each species of interest. Steelhead spend 1-3 years in the watershed before migrating to the ocean and therefore must spend at least one dry-season in the watershed. Their survival depends on having suitable habitat, including sufficient water, cool temperatures, and sufficient dissolved oxygen. In contrast, foothill yellow-legged frogs lay eggs in streams during the spring as flows are receding and require sufficient time for the eggs to hatch and tadpoles to metamorphose into subadults that can leave the stream and find other water sources. The time from laying eggs to metamorphosis varies based on stream temperature with shorter times for warmer temperatures.
Monitoring as part of the Workplan implementation observed juvenile steelhead in Sulphur Creek in both years and in smaller numbers at the Napa River at St. Helena site. Two adult steelhead and zero juveniles were observed in the Oak Knoll site in 2025, suggesting the site may act as a migration corridor and may not support juvenile rearing. At all three sites, temperature and dissolved oxygen were stressful for juvenile steelhead in 2024 and 2025, which likely limits growth and survival. While the degree to which stream temperature and flow at these sites are correlated is not fully understood, stream temperatures can be high early in the dry season (May and June) when flows are slightly higher than July. Both mainstem sites had relatively limited habitat diversity, with long pools and little cover. Sulphur Creek has much more frequent gravel bars and a diversity of high and low velocity areas that juvenile steelhead prefer and is closer to colder water than the mainstem sites. Monitoring in 2024 and 2025 suggests that stream temperature and dissolved oxygen are much more suitable for juvenile steelhead just upstream of the Sulphur Creek site near the confluence with Heath Creek, where flows are perennial.
Most of the foothill yellow-legged frog observations were at the Sulphur Creek site where there is more abundant suitable habitat (including slow velocity zones) for egg masses and tadpoles. One foothill yellow-legged frog egg mass was observed at the Napa River near St. Helena site near the confluence of Sulphur Creek in 2024 but no frogs, tadpoles, or egg masses were observed in 2025. For most of the reach the edge habitat created by bar and pool morphology is not present in the St. Helena site. Foothill yellow-legged frog habitat was poor at the Napa River at Oak Knoll site regardless of flow and the positive eDNA result at this site was attributed to tadpoles observed upstream in Dry Creek where the habitat was better.
Preliminary dry season baseflow ecological flow recommendations were developed based on monitoring results previously collected in the basin (including stream temperature data), data collected as part of the Workplan implementation, results of the NVIHM on the effects of groundwater pumping on streamflow and the CEFF analysis described above. Based on modeling and observations to date, it is assumed that stream temperatures remain stressful for salmonids during dry-season baseflow even in the event of no groundwater pumping. The ecological flow recommendation for the Napa River at St. Helena site is to maintain flowing conditions through mid-July to support foothill yellow-legged frog metamorphosis. The degree to which foothill yellow-legged frog use varies at this site from year to year is unknown. Additional surveys could help to determine if egg masses, one of which was observed in 2024, occur during most years, and to refine the timing of flow needs.
Because dry-season, low-flow water quality and habitat conditions are poor regardless of flow, there are no flow recommendations to support juvenile steelhead for the Napa River at Oak Knoll. No juvenile steelhead or foothill yellow-legged frogs have been observed at the Napa Oak Knoll site, and both habitat and water quality are poor. Northwestern pond turtles occur at the site but do not require specific flow in the dry-season. Because no special status species at this site rely on dry-season baseflow, no preliminary dry-season baseflow flow recommendations are developed for this site. Although there are no flow recommendations at Oak Knoll, groundwater near Oak Knoll may affect flows in Dry Creek, which supports foothill yellow-legged frogs, although this interaction requires additional study.
For the Sulphur Creek site, maintaining flow to support foothill yellow-legged frog metamorphosis (through July) is important for their success. Whether frogs emerge earlier in the season during dry years or later in the season during wetter, cooler years is not known. Continued frog surveys at this site would help to address this uncertainty. Because flows are dependent on inflow from outside of the groundwater subbasin, rather than local groundwater, there are no flow recommendations at this time.
Maintaining intra-basin connectivity in the Napa River during dry-season baseflow is important to allow adult and juvenile steelhead to migrate to better habitat within the watershed, typically outside of the Subbasin. At the mainstem sites, this connectivity is important while tributaries within the Subbasin are still flowing throughout their length and migrating fish can access better habitat. The timing of tributary disconnection varies within the Subbasin but observations and the NVIHM results without groundwater pumping suggest that larger tributaries become intermittent between May and July. In 2026, fish population surveys will occur twice at each site to better understand fish movement during the dry season and further analysis of hydrologic data will be used to assess flows to support migration to be included in the periodic evaluation. At all sites, continued monitoring of fish and frog presence, water quality, groundwater elevation, and flow connectivity will be used to refine preliminary minimum ecological flow requirements.
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: The proposed action is not a project as defined by 14 California Code of Regulations 15378 (State CEQA Guidelines) and therefore CEQA is not applicable.
SUPPORTING DOCUMENTS
A. Napa California Environmental Flows Framework Assessment Progress Update (Stillwater Sciences, June 2026)