Tomales Bay, located along the San Andreas Fault on California’s Marin County coast, is a long, narrow tidal estuary spanning about 15 miles (24 km) and averaging 1 mile (1.6 km) in width. This unique coastal system connects to the Pacific Ocean at its northern end and is influenced by tidal exchange, freshwater inflows, and seasonal climate patterns.
With a Mediterranean coastal climate—cool, foggy summers and mild, rainy winters—Tomales Bay supports diverse habitats, including eelgrass beds, salt marshes, and mudflats. These ecosystems provide critical feeding and nursery grounds for fish, shellfish, and migratory birds along the Pacific Flyway. The estuary is also home to thriving oyster farms, which rely on the bay’s nutrient-rich waters.
Tomales Bay plays a key role in studying coastal hydrodynamics, sediment transport, and climate change impacts, as rising sea levels and watershed changes continue to shape its ecology. Protected within Point Reyes National Seashore, the estuary remains an important site for research, recreation, and conservation.
Real-Time Water Level
Understanding Data on this Page
Temperature: This sensor is out-of-water during low low tides. The temperature measured during those times when it is "dry" will be air temperature readings, and will differ significantly from water temperature.
Water Level: The real-time water level data displayed on this page is collected in Inverness and referenced to the NAVD88 datum (North American Vertical Datum of 1988). This means the values represent water level elevation relative to a fixed reference point, not the actual depth of the water at a given location.
Please note that this differs in both location and datum from NOAA tide predictions for Tomales Bay. NOAA forecasts are typically based on MLLW (Mean Lower Low Water) as their reference point and are generated for the mouth of Tomales Bay. Because NAVD88 and MLLW use different vertical benchmarks, the water levels shown here will not directly match standard tide predictions. Additionally, since the two locations experience different hydrodynamic forces, the timing of water level changes will also differ.
Since water level impacts vary by location, we encourage users to take time to observe and understand how the plotted values relate to their specific site conditions. By doing so, you can better interpret the data for your needs, whether for navigation, flooding awareness, or ecological observations.