A river mouth is the part of a river where the river flows into a larger river, a lake, a reservoir or an ocean.
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Water motion
The water from a river can enter the receiving body in a variety of different ways. The motion of the river mainly depends on the relative density of the river compared to the receiving water and any ambient motion in the receiving water, such as tides or seiches.
If the river water is denser than the surface of the receiving water, the river water will plunge below the surface at the plunge curve. The river water will then either form an underflow or an interflow within the lake.
If the river water is lighter than the receiving water, as is typically the case when fresh river water flows into the sea, the river water will float along the surface of the receiving water as an overflow.
Alongside these advective transports, inflowing water will also diffuse.
Landforms
At the mouth of a river, the change in flow condition can cause the river to drop any sediment it is carrying. This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels.
Cultural influence
Many places in England take their names from their positions at the mouths of rivers, for example Plymouth (River Plym), Sidmouth (River Sid) and Great Yarmouth (River Yare).