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 implementation and 2025 Monitoring Results
RECOMMENDATION
title
Provide an update on the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystem (GDE) Workplan implementation with a focus on 2025 monitoring results.
body
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
Monitoring groundwater dependent ecosystems for Napa Valley Subbasin Groundwater Sustainability Plan (GSP) implementation occurs through a combination of shallow monitoring wells and remotely sensed vegetation health. To address data gaps and better characterize the interconnection between surface water and groundwater dependent ecosystems, the NCGSA installed additional dual-completion ISW monitoring wells and developed a workplan that included biological field assessments, review of existing data, investigations of stream habitat conditions, and evaluation of listed species. The Interconnected Surface Water and Groundwater Dependent Ecosystems Workplan (Workplan) was completed in March 2024 with implementation beginning 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).
Surveys conducted in 2025 include:
• flow connectivity studies;
• fish population and fish habitat;
• continuous water quality monitoring for temperature and dissolved oxygen;
• up to four visual encounter surveys for aquatic wildlife;
• environmental DNA (eDNA) sampling for northwestern pond turtle and foothill yellow-legged frog;
• California freshwater shrimp surveys in the Calistoga Reach of the Napa River;
• point count surveys and acoustic monitoring for birds; and
• surveys of groundwater-dependent vegetation community health and composition.
Water year 2025 precipitation at Napa State Hospital was 21.6 inches, approximately 84% of average. However, WY 2025 was a much wetter year in the northern portion of the basin due to a focused, high-intensity atmospheric river in November 2024 where 17 inches of rain fell over 3 days recorded at the Petrified Forest OneRain station. Groundwater levels were monitored continuously at the Napa River at Oak Knoll, Sulphur Creek, and Bale Slough sites. Continuous groundwater levels were not recorded at the Napa River at St. Helena and Napa River near Yountville sites due to pressure transducer malfunction. A new monitoring well was installed at the Napa River at Calistoga site in July 2025, and groundwater level data collection began in October 2025. Stream stage data were recorded at the Napa River at St. Helena and Napa River at Oak Knoll sites. Stage data collected at the other four sites will be tied into a reference elevation for conversion to water surface elevation in WY 2026.
In 2025, stream habitats were evaluated at the six intensive survey sites using monthly wet-dry mapping to assess changes in the extent of aquatic habitat through time. At each site, the extent of wet and dry conditions was measured over approximately one-mile reaches using GPS to note transitions between conditions. The Napa River near Yountville and Napa River near Oak Knoll sites were flowing throughout 2025. The Napa River at Calistoga, Napa River at St. Helena, Sulphur Creek, and Bale Slough reaches all dried during the surveys, though the timing and extent of drying varied at each site. The Napa River at Calistoga reach began transitioning to isolated pools by early July, but the reach was wetter overall in 2025 (70% of the reach was wet throughout summer and fall) than in 2024 (30% of the reach was wet throughout summer and fall). The Napa River at St. Helena site began to dry downstream of Stonebridge Park by early August and was almost completely dry from the Pope Street bridge to Dowdell Lane by October. Like Calistoga, the site stayed wetter throughout the dry season in 2025 (35% of the reach was wet in October) than in 2024 (1% of the reach was wet in October). Sulphur Creek began to dry near the downstream end of the site in June, and the reach within the groundwater basin was almost entirely dry by August. The section of the Sulphur Creek reach immediately upstream of the basin boundary was flowing year-round. Dry conditions at the upstream end of the reach in July-October are attributed to dewatering associated with the Sulphur Creek Fish Passage Restoration Project. Bale Slough transitioned to isolated pools in June and was completely dry by August. A longitudinal profile of the Bale Slough channel bed was surveyed in June 2025.
Dissolved oxygen and stream temperature were monitored with HOBO loggers at all six sites. Dissolved oxygen levels generally declined as stream temperatures increased. By mid-July-August, oxygen conditions were generally stressful for steelhead. Stream temperatures were elevated but generally below lethal thresholds. Both dissolved oxygen and 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.
Napa County RCD conducted fish population surveys at all six 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 greater than 500). Steelhead were present at four of the six snorkel sites surveyed in May and June 2025. Juvenile steelhead were most abundant on Sulphur Creek in May, with much smaller numbers at the other sites. Juvenile Chinook Salmon were observed at every site and anecdotally observed throughout the Napa River watershed in 2025. Chinook salmon were most abundant at Calistoga where hundreds of fish were observed. Whether fish of either species survived beyond June is unknown. Other observed fish include roach, stickleback, Sacramento sucker, largemouth bass and bluegill. The RCD also conducted fish habitat typing surveys at Oak Knoll and attempted a habitat typing survey at Bale Slough, but the site was too dry to accurately map habitat units. The Bale Slough survey will be conducted in 2026.
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 and eDNA were used to assess the presence of foothill yellow-legged frogs and northwestern pond turtles at the six sites in April-June 2025, with follow-up surveys in July at Sulphur Creek and Napa River at Oak Knoll. Foothill yellow-legged frogs were found only in tributaries to the Napa River-tadpoles were found in Dry Creek (upstream of the Napa River at Oak Knoll site), and egg masses and tadpoles were found in Sulphur Creek. Northwestern pond turtles were observed at the Napa River at Oak Knoll site and detected from eDNA sampling at the Napa River at St. Helena 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 October 2025 surveys, 117 California freshwater shrimp were observed in pools throughout the Napa River reach, and 6 were observed in Garnett Creek. 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, with a similar population to the 2024 surveys.
Fifty-nine (59) unique avian species were identified in spring point count surveys and/or acoustic data across five of the six sites (access to Bale Slough was still pending during spring survey dates). One special-status species (northern yellow warbler [Setophaga aestiva]) was identified in the acoustic data collected at Sulphur Creek and the Napa River at St. Helena.
Groundwater dependent vegetation composition and vigor were assessed along two transects at each site. 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. GDEs at the mainstem Napa River typically showed minor vegetation stress, particularly in mature trees. Vegetation at Sulphur Creek was primarily upland and riparian herbaceous species not reliant on groundwater. No vegetation stress was apparent at Bale Slough, but riparian restoration plantings at the site are irrigated.
Flow connectivity surveys, water quality monitoring, and fish population surveys will be conducted again in 2026. Although originally scheduled for 2024 and 2025, the first bird surveys were conducted in 2025, and a second set of bird surveys will be conducted in 2026. In addition, a second set of wildlife surveys will occur in Bale Slough and the Napa River at Oak Knoll. To date, the 17.28 inches of precipitation at Napa State Hospital through WY 2026 has exceeded very dry conditions (9.52-15.57 inches) that would trigger supplemental surveys. This summer, the GSP technical team will develop the California Environmental Flows Framework assessment for the Napa Valley Subbasin using the results of the 2024 and 2025 surveys.
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 Monitoring 2025 Update (Stillwater, April 2026)
B. Stillwater Sciences, Napa Resource Conservation District, Napa County, and Luhdorff & Scalmanini, Consulting Engineers. 2026. Napa Valley Subbasin Interconnected Surface Water and Groundwater Dependent Ecosystems Monitoring, 2025. Available at: https://www.napacounty.gov/DocumentCenter/View/42177/ISW-and-GDE-Monitoring-2025