The Big Lagoon estuary, located just north of Humboldt Bay on California's north coast, is a coastal lagoon separated from the Pacific Ocean by a narrow sand spit. Spanning approximately 1,600 acres, it is a classic example of a bar-built estuary, where the mouth periodically opens and closes due to freshwater inflows, wave action and littoral sand transport. These cycles create dynamic changes in salinity, water levels, and circulation, shaping the estuary's unique physical and ecological conditions.

Big Lagoon supports diverse habitats, including brackish marshes and mudflats, which provide critical feeding and nursery grounds for fish, waterfowl, and invertebrates. Both freshwater and marine species occupy the lagoon, depending on timing and duration of breaching cycles, including steelhead, coastal cutthroat trout, jellyfish, and tidewater goby. As a sensitive system, Big Lagoon is highly responsive to changes in freshwater inflows, ocean conditions, and climate variability, making it a valuable site for studying coastal processes, estuarine resilience, and ecosystem health