Los Peñasquitos Lagoon, on San Diego’s northern edge beside Del Mar and Torrey Pines, is a coastal marsh and bar-built estuary shaped over thousands of years as a deep bay gradually became a brackish marsh and then a salt marsh. Fed by Carroll, Carmel, and Los Peñasquitos creeks, it long supported Kumeyaay shellfishing and later lay near Rancho Los Peñasquitos, the first land grant within modern San Diego County. Railroads, Highway 101, Interstate 5, and urban runoff later altered drainage, tides, salinity, and habitat. Today its wetlands and preserve remain ecologically important, with limited public access and ongoing restoration to protect water quality and tidal exchange.