A river estuary is the interface between the ocean and fresh water. Estuaries are important as nurseries for anadromous fish and other marine species such as crab and rockfish. The length of an estuary is determined by terrestrial topography, size of the river and its opening to the sea. Typically, fresh water rides over heavier salt water and forms what is referred to as a "lens" of interacting liquids. Mudlflats, sandbars, and riparian areas are often associated with estuaries.