4K. A stream mouth is the part of a river where the river flows into another river, a lake, a reservoir, a sea or an ocean. The water from a creek can enter the receiving body in a variety of different ways. The motion of the brook mainly depends on the relative density of the rivulet compared to the receiving water and any ambient motion in the receiving water, such as tides or seiches. Gully canal runlet river seaside sandbank beach seaside coastal seashore waterside.