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: Update on ISW and GDE monitoring and perspectives from outside of Napa County
RECOMMENDATION
title
Receive a presentation on the status of the Interconnected Surface Water and Groundwater Dependent Ecosystems Workplan implementation with a focus on applying the California Environmental Flows Framework in the Napa Valley Subbasin and describing the process for developing ecological management goals.
body
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
As part of the implementation of the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystems (GDEs) Workplan: Napa Valley Subbasin, the Technical Team is using the California Environmental Flows Framework (CEFF) to characterize aquatic and terrestrial GDEs and assess environmental flows in the Subbasin. The goal is to complete the science-based sections of CEFF (Sections A and B) by the end of 2025 to inform updates to the Sustainable Management Criteria for the Periodic Evaluation. A key component of CEFF is to identify overarching ecological management goals, including streamflows to maintain ecosystem health. These goals are a function of the species and lifestages present, their ecological needs, the physical system (e.g., water year type, temperature, water year trends), and the degree to which the ecological needs can be met (i.e., whether flows would be sufficient given the constraints of the physical system). Flow constraints under variable conditions, including climate extremes and ISW conditions with and without groundwater pumping will be characterized and assessed using the Napa Valley Integrated Hydrologic Model (NVIHM). At the February TAG meeting, we will present preliminary ecological management goals based on evaluation of the literature for the Napa River and similar watersheds and results of the 2024 ISW and GDEs monitoring for the Subbasin. We will also describe the process to refine these goals and develop stream reach-specific objectives as more data is collected and the groundwater model is updated 2025.
Napa River Watershed Context
Previous Napa River Watershed (Watershed) studies found that most spawning and rearing habitat for steelhead occurs upstream of the Subbasin, where stream temperatures are cooler and flows are perennial. Because they rear in the Watershed for one to two years before returning to the ocean, steelhead are particularly sensitive to summer habitat conditions, which are generally unfavorable in the Subbasin. The stream reaches in the Subbasin, including the mainstem Napa River, are typically too warm (or go dry) during the summer and likely support passage to upstream and downstream habitat rather than summer rearing. The degree to which successful rearing occurs in the lower reaches may vary by water year type and climatic conditions (i.e., temperature, previous water year types).
To provide context for important considerations as we develop ecological management goals and stream reach-specific objectives, Dr. Jacob Katz (California Trout) will present an overview of salmonid rearing in California and watersheds similar to the Napa River Watershed. His studies have shown that a key factor in salmonid survival in the ocean to return to their natal streams to spawn is access to food-rich floodplains and wetlands that allows the fish to grow sufficiently large to survive. One such wetland is the Napa River wetlands downstream of the Subbasin. Dr. Katz also will discuss steelhead life strategies in similar watersheds in the California coastal ranges where the valley bottoms are primarily passage corridors to more stable year-round flows and more favorable water temperatures in the uplands.
2024 Monitoring
The 2024 ISW and GDEs monitoring occurred on four of the six sites outlined in the ISW and GDEs Workplan: Napa River at Calistoga, Napa River at St. Helena, Napa River at Yountville, and Sulphur Creek. Access to the remaining two sites (Napa River at Oak Knoll and Bale Slough) was obtained after the 2024 field season; these sites will be surveyed in 2025.
Surveys conducted in 2024 at each of the sites included:
• Flow connectivity studies;
• A test of the effectiveness of infra-red drone imagery to identify wet and dry reaches and document stream temperature;
• Fish habitat and fish population (June and July);
• Continuous water quality measurements for temperature and dissolved oxygen;
• Two surveys for aquatic wildlife (one in May and one in July);
• eDNA sampling for northwestern pond turtle and foothill yellow-legged frog (May);
• California freshwater shrimp in the Calistoga Reach of the Napa River; and
• Groundwater-dependent vegetation community health and composition.
Two of the sites had long stretches where the stream went dry starting in July (Sulphur Creek and the Napa River at St. Helena). The Napa River at Calistoga site transitioned to isolated pools starting in July, while the Napa River at Yountville was flowing for most of the surveyed length (approximately one mile). Dissolved oxygen levels generally declined as stream temperatures increased and by mid-July to August oxygen conditions in the Napa River mainstem were inhospitable to steelhead fry and parr, even at the Yountville site which had continuous surface flow. Stream temperatures were elevated enough to stress fish but were generally sublethal. The exception was the Napa River at St. Helena where the maximum weekly average temperature was 79 degrees Fahrenheit (26 degrees Celsius), which is potentially lethal to rearing steelhead based on the literature. Dissolved oxygen levels were low at all four sites, nearly always within the range that is stressful to rearing salmonids but declining to potentially lethal levels (3 mg/L) for at least some period at every site. At each study site, the continuous temperature and dissolved oxygen measurements represent conditions at a point location rather than the reach; fish may utilize sections of the reach where temperature and dissolved oxygen were more suitable, if they exist.
Steelhead juveniles were observed at two of the sites, the Napa River at Calistoga and Sulphur Creek, although fry in Sulphur Creek were at the upstream end of the surveyed site just outside of the Subbasin, where flow, temperature, and dissolved oxygen were more suitable for steelhead. The Napa River at Calistoga site also supported California freshwater shrimp. Foothill yellow-legged frog eggs (and positive eDNA result) were observed at Napa River at St. Helena, and tadpoles and an adult were observed at Sulphur Creek. Northwestern pond turtles were observed at the Napa River at St. Helena and Napa River at Yountville sites.
The 2024 monitoring generally supported previous observations in the Napa River Watershed and taken together suggest that the Napa River mainstem was unlikely to support juvenile rearing due to extensive dry reaches, high stream temperature, and low dissolved oxygen. Juvenile steelhead were observed in the Calistoga reach and in the perennial section of Sulphur Creek (outside of the Subbasin). In 2025, snorkel surveys later in the summer should be considered to assess potential survival of juveniles observed in the late spring when flows are higher.
The 2024 hydrology and climate exhibited slightly below average precipitation but very high summer temperatures. The frequent high temperatures likely increased stream temperatures at the study sites. The effect of groundwater pumping on surface flows (and the extent of flowing conditions at each site) is being evaluated and will be presented to the TAG in Fall 2025.
Preliminary Ecological Management Goals
Based on the 2024 monitoring results, previous observations, and relevant literature, preliminary ecological goals for the study sites include:
• Napa River at Calistoga: maintain isolated pools to support California Freshwater Shrimp habitat and support steelhead rearing if water temperatures are sufficient (i.e., wet years with cool summer temperatures).
• Napa River at St. Helena: maintain flowing conditions (generally in the late spring/early summer depending on temperature) to support emergence of foothill yellow-legged frogs as well as support northwestern pond turtle.
• Napa River at Yountville: support habitat for northwestern pond turtle.
• Sulphur Creek: maintain flowing conditions to support emergence of foothill yellow-legged frogs (generally in the late spring/early summer depending on temperature) and allow rearing steelhead to move upstream into perennial reaches prior to the stream going dry.
These preliminary goals are based on one year of data at four of our six monitoring sites. They will be refined based on the results of the 2025 monitoring coupled with results of the updated groundwater model. The science-based portions of CEFF will be completed by the end of 2025 to support refinement of the sustainable management criteria in 2026. The CEFF analysis will be included as an appendix to the 2027 Groundwater Sustainability Plan Periodic Evaluation (i.e., five-year update). The Adaptive Management principles will be applied as new monitoring results are obtained to adjust ecological management goals and flow recommendations as appropriate.
Discussion questions with Dr. Katz:
• What should be considered to maintain and improve steelhead populations in the Napa River Watershed and what should be considered for stream reach-specific ecological objectives?
• How are steelhead habitats and life history strategies similar or different in other watersheds?
• How do water year types (wet, average, dry) or changing climate patterns affect steelhead habitat and resilience in other rivers?
• How do you expect steelhead and their habitat to adjust to climate change?
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 County Groundwater Sustainability Agency, Interconnected Surface Water and Groundwater Dependent Ecosystems and Identifying and Developing Ecological Management Goals (Stillwater and Katz, March 2025)