A slough is a swamp or shallow lake system, usually a backwater to a larger body of water. The Elkhorn Slough NERR is a narrow meandering inlet located off Monterey Bay on California's central coast (Image credit: Elkhorn Slough NERR).A slough (pronounced slew) is typically used to describe wetlands.Sloughs along the edges of rivers form where the old channel of the river once flowed.hile this term is used differently across the country, on the West Coast a slough is referred to as a swamp or shallow lake system, usually a backwater to a larger body of water.