TO: Technical Advisory Group (TAG) 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 the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystems (GDEs) Workplan Implementation and the California Environmental Flows Framework
RECOMMENDATION
title
Provide an update on the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystems (GDEs) Workplan implementation with a focus on California Environmental Flows Framework (CEFF) steps.
body
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
Monitoring groundwater dependent ecosystems in the Napa Valley Subbasin Groundwater Sustainability Plan (GSP) occurs through a combination of shallow monitoring wells and remotely sensed vegetation health. Data gaps associated with this monitoring identified the need for additional shallow monitoring wells, which was addressed in 2023 through the installation of dual-completion ISW monitoring wells at 8 additional sites, and the development of the ISW and GDEs Workplan that called for biological field assessments, review of existing data, investigations of stream habitat conditions, and evaluation of listed species. The Workplan was completed in March 2024 and implementation began in May 2024.
The Workplan identified six intensive survey sites: four on the Napa River (Napa River at Calistoga, Napa River at St. Helena, Napa River at Yountville, Napa River at Oak Knoll) and two tributary sites (Sulphur Creek and Bale Slough). Access to four of the sites was obtained for summer 2024 (Napa River at Calistoga, Napa River at St. Helena, Napa River at Yountville, and Sulphur Creek). Access was recently obtained to the Napa River at Oak Knoll site, and access permissions are still in progress for the Bale Slough site.
Surveys conducted in 2024 at each of the sites include:
• Flow connectivity studies;
• Fish habitat and fish population;
• Continuous water quality sampling 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 (July);
• California freshwater shrimp surveys in the Calistoga Reach of the Napa River; and
• Surveys of groundwater-dependent vegetation community health and composition.
Precipitation, as measured at the Napa State Hospital, in Water Year (WY) 2024 was 22.9 inches, approximately 89 percent of average. Temperatures in Napa Valley were higher than average during the growing season. Based on local weather stations, Calistoga, St. Helena, and Oakville experienced 35, 35, and 12 days above 100 degrees Fahrenheit, respectively. Groundwater levels and stream stage were monitored at each site except the Napa River at Calistoga, which does not have an associated shallow groundwater well but does have a stage recorder. Groundwater data from the shallow wells have been downloaded and provided as preliminary data.
In 2024, stream habitat was evaluated at the four intensive survey sites through monthly wet-dry mapping to assess the extent of aquatic habitat and how that changed through time, and water quality measurements including dissolved oxygen (DO) and water temperature. The extent of wet and dry conditions was measured by the Napa County Resource Conservation District (RCD) monthly at each site over approximately one-mile reaches using GPS to note transitions between wet and dry conditions. The Napa River at Calistoga, Napa River at St. Helena, and Sulphur Creek reaches all exhibited increased drying during the surveys. Sulphur Creek had no dry reaches in the June 11 surveys, but had started to dry by July 9, with the downstream-most 57 percent of the reach going dry. In the subsequent surveys, the extent of wet conditions was reduced to approximately the upstream 20 percent of the reach, most of which was upstream of the Subbasin boundary. The Napa River at Calistoga transitioned to isolated pools starting in the August 8 surveys, with wet conditions in only approximately 53 percent of the reach, decreasing to 27 percent of the reach by the October survey. The extent of wet conditions for the Napa River at St. Helena location steadily decreased from no dry reaches on June 18 to only 1 percent of the reach mapped as wet in October 2024. In contrast, the Napa River at Yountville was mapped as having wet conditions on September 5 (when 29 percent of the reach surveyed for the Napa River at St. Helena was dry), and almost 98 percent of the Yountville reach maintained wet conditions on October 22. Longitudinal profiles of the channel bed were surveyed in October 2024 over 1000-2000 feet per intensive survey site.
Stream temperature and DO were measured with Hobo loggers at the four accessible sites. DO levels generally declined as stream temperatures increased. Stream temperatures increased to a maximum daily temperature of 74 to 86 degrees Fahrenheit at the four sites by mid-July to August. DO conditions in the Napa River mainstem were inhospitable to steelhead fry and parr, even at the Yountville site which had continuous surface flow through the September 2024 surveys. Stream temperatures were elevated beyond ideal temperatures for juvenile fish. Previous data collected along the mainstem of the Napa River at ISW sites have shown high water temperatures in the high 70’s degrees Fahrenheit. Both DO and water temperature can be highly heterogeneous in streams, particularly those fed by groundwater like the Napa River, and fish may be able to survive in pockets of highly oxygenated water with low temperature. Stream temperatures in the Napa Subbasin are complex and will be assessed under the California Environmental Flows Framework (CEFF). Stressors experienced in 2024 that directly impact streamflow and stream temperature include the normal (below average) rainfall and higher than average maximum and minimum temperatures.
The Napa County RCD conducted fish population and fish habitat surveys at the four accessible sites. The population surveys included snorkeling the four sites and counting the number and species of fish present and their lifestage, if applicable. The number of steelhead (O. mykiss) were counted directly while the population of other fish were placed in ranges (1-10, 11-50, 50-100, 101-500, and >500). Steelhead fry (fish < 100 mm that hatched this year) were observed at all four sites, with the largest numbers occurring in Napa River at Calistoga (118) and Napa River at Sulphur Creek (26) with significantly less at St. Helena (1) and Yountville (2). Steelhead parr (101-200 mm) are steelhead that emerged from spawning gravels in previous years and have yet to smolt and migrate to the ocean. Small numbers of steelhead parr were observed at Napa River at Calistoga (3) and Sulphur Creek (8). The only observed adult steelhead during the surveys were three adult steelhead observed in Sulphur Creek. Steelhead of all ages observed in Sulphur Creek were at the upstream end of the reach approximately 0.2 miles downstream of White Sulphur Springs Road. Other observed fish include roach, stickleback, largemouth bass, and blue gill.
The aquatic wildlife surveys included surveys for three listed wildlife species: foothill yellow legged frog (Rana boylii), northwestern pond turtle (Actinemys marmorata), and California freshwater shrimp (Syncaris pacifica). Visual encounter surveys (VES) and environmental DNA (eDNA) were used to assess the presence of foothill yellow-legged frogs and northwestern pond turtle at the four accessible sites in May and late July 2024. Foothill yellow-legged frog eggs were observed in the Napa River at St. Helena site in May and detected in the July eDNA samples. Foothill yellow-legged frog tadpoles, young of the year, and an adult were observed at Sulphur Creek. Northwestern pond turtles were observed during the VES at the Napa River at St. Helena and Napa River at Yountville sites in May and detected in the eDNA sample in July at the Napa River at Yountville site.
California freshwater shrimp were surveyed over a 1.5-mile reach of the Napa River near Calistoga from Lincoln Avenue to Greenwood Avenue and a 0.5-mile reach of Garnett Creek from the confluence with the Napa River to Grant Street. During the August 2024 surveys, 105 California freshwater shrimp were observed in pools throughout the Napa River reach. Flow in the Calistoga reach was discontinuous at the time of the surveys with isolated pools separated by dry riffle reaches. The observed shrimp included male adults, female adults, and juveniles, suggesting that the population is actively reproducing. No California freshwater shrimp were observed in Garnett Creek.
Groundwater dependent vegetation composition and vigor were assessed along two transects at each site. These were the only surveys that included the Napa River at Oak Knoll. The data from the GDE health surveys is still being processed. Preliminary results suggest that the GDEs along the channel corridor generally supported oaks at surfaces further above the channel bottom and mixed riparian vegetation on surfaces closer to the channel bottom. In general, many of the GDEs showed signs of water stress during the September surveys, particularly the oaks, which may be due to the timing of the surveys, when soil conditions were dry.
The studies described above will be conducted again in 2025 and expanded to the Napa River at Oak Knoll and Bale Slough sites (pending access approval). Over the next year, Workplan implementation will include further work on the CEFF assessment, including synthesis of the 2024 survey data described above. These monitoring data, together with the 2025 surveys, will be used to develop the ecological management goals at each site. Specific ecological management goals for each site will be assessed based on aquatic and terrestrial species present, their habitat needs during different life stages, and ecological-flow relationships.
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, Groundwater Dependent Ecosystem Monitoring 2024 (Stillwater, November 2024)