Sloughs (pronounced “slews”) are shallow lakes or swamps. Generally they serve as backwaters – or a stagnant part of a river – and are consequently located at edges of rivers where a stream or other canal once flowed. 

Bends in old riverbeds are common locations for sloughs, leaving them curved and frequently referred to as “oxbows.” In California and other parts of the West, sloughs often double as estuaries – or coastal areas where fresh water mixes with salty ocean water – if they are backwaters for bays.

The Sacramento-San Joaquin Delta is crisscrossed with sloughs with names such as Lost Slough and Steamboat Slough, a reminder of how the Delta once served as a transportation conduit between the Bay Area and the Central Valley.

Elkhorn Slough, near Monterey, is one of two sloughs the in the National Estuarine Research Reserve System (NERRS). Elkhorn Slough is known for its rare biodiversity, hosting hundreds of bird species, 6 of which are threatened or endangered. Many sloughs, such as the Revolon Slough near Los Angeles, endure water quality damage resulting from litter and trash, putting habitats at risk and reducing their possible recreational, agricultural and groundwater recharging functions.