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 wave action and sediment 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, eelgrass beds, and mudflats, which provide critical feeding and nursery grounds for fish, waterfowl, and invertebrates. 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.