The Lagoon was once a narrow inlet of the ocean where at low tide Terminal Creek flowed across tidal flats. At high tide, the inlet was filled by the ocean.
Construction of the Causeway in the 1920s dammed the flow of Terminal Creek, forming the freshwater/brackishwater lagoon.
Conditions in the Lagoon change with the seasons. In fall, winter and spring, heavy precipitation sends vast amounts of cold freshwater gushing over Bridal Veil Falls just upstream of the Lagoon. In these seasons, the Lagoon is full of cold oxygenated water that flows quickly into the Bay. Even when high tides allow seawater to flow into the Lagoon under the Causeway bridge, it is quickly flushed back into the bay by the strong freshwater flow from Terminal Creek.
In effect, in these seasons, the Lagoon is a small freshwater lake. Conditions at this time of year are excellent for juvenile salmon, stickleback, cutthroat trout, crayfish and a vast array of aquatic insects.
In summer, there is little to no rain and Terminal Creek becomes a trickle, releasing small amounts of cool, oxygenated freshwater into the Lagoon. Saltwater that flows into the lagoon from the Bay at high tide remains in the lagoon and mixes with the creek’s freshwater. As a result, in summer, the Lagoon is a brackish water lake whose waters drain leisurely into the Bay. Many aquatic creatures find it difficult to live in brackish water, however crayfish and some aquatic insects thrive in these mildly salty waters.
The year-round abundance of aquatic plants and small aquatic creatures attract mallard, swans, mergansers, grebes and Canada geese.