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 a combination of freshwater inflows, wave action, and littoral sand transport. These natural cycles lead to dramatic shifts in salinity, water levels, and circulation, shaping the estuary's unique physical and ecological conditions.

Big Lagoon supports a variety of habitats, including brackish marshes, mudflats, and open water that provide critical feeding and nursery grounds for fish, waterfowl, and invertebrates. Depending on the timing and duration of breaching events, the lagoon supports both freshwater and marine species, such as steelhead, coastal cutthroat trout, jellyfish, and tidewater goby.

Because of its sensitivity to changes in rainfall, ocean conditions, and sediment transport, Big Lagoon offers valuable insights into coastal processes, estuarine resilience, and ecosystem health. Ongoing water level monitoring helps researchers and resource managers better understand how the system responds to both short-term events, like storms or mouth breaches, and long-term drivers like climate variability and sea level rise.