A slough (pronounced "slew") is a swamp or shallow lake system, usually a backwater to a larger body of water.
A slough is typically used to describe wetlands. Sloughs along the edges of rivers form where the old channel of the river once flowed. These areas are also referred to as oxbows because they tend to form at a bend in the old river bed, making them look like the horns of an ox when viewed from the air.
Along the West Coast, sloughs are often named for the quiet, backwater parts of bays and therefore, they are part of the estuary, where freshwater flows from creeks and runoff from land mix with salty ocean water transported by the tides.
There are two sloughs in NOAA's National Estuarine Research Reserve System (NERRS). Elkhorn Slough is located just north of Monterey, California. Elkhorn Slough is known as a popular birding destination with more than 135 species of birds including six that are listed as threatened or endangered. South Slough NERR is located in southwestern Oregon. South Slough provides important habitat for salmon, great blue herons, bald eagles, migrating ducks, elk, sea otters, oysters, and crabs.
Ocean Facts - What is a Slough? (NOAA National Ocean Service)