Salt marshes are almost always part of an estuary. An estuary is an enclosed body of water that is affected both by freshwater and by seawater. Freshwater comes into the estuary through the large river system, and seawater comes in from the sea through an inlet. Salt marshes are best developed in areas where these conditions are met. Some animals and plants are especially adapted for estuarine conditions and live their whole lives in the salt marsh. Other animals just visit the marsh for a while, then move back into the sea or the rivers with the tides.