River mouth
Where a river meets a larger body of water.
A river mouth is the point where a river empties into a larger water body—another river, a lake, reservoir, bay, gulf, sea, or ocean. As the current slows here, the water loses its ability to carry sediment, so material often gets deposited. How the river water enters depends on its density relative to the receiving water, Earth’s rotation, and any existing movement in the larger body, like tides or seiches.
When river water is denser than the surface of the receiving water, it sinks below the surface, creating an underflow or interflow within a lake. If it’s lighter—common when fresh river water meets the sea—it spreads across the surface as an overflow. Alongside these directed flows, the inflowing water also spreads out by diffusion.
The shift in flow conditions at the mouth can cause the river to drop its sediment load, building landforms like deltas, sand bars, spits, and tie channels. These features reshape the local geography and ecosystems. Along coasts, sand bars and similar formations act as barriers, sheltering nutrient-rich habitats. But damming rivers upstream can cut off the supply of sand and nutrients, leaving the mouth with a deficit.
Because river mouths offer easy travel, port access, and sediment deposits, many towns and cities have been built there. In the United Kingdom, places like Plymouth (mouth of the Plym), Sidmouth (mouth of the Sid), and Great Yarmouth (mouth of the Yare) are named for their location. In Celtic, the terms *Aber* or *Inver* appear in names such as Aberdeen, Abercrombie, Abernethy, Inverness, Inverkip, and Inveraray. Rising sea levels from climate change put these coastal cities at greater flood risk, and sediment starvation in the river makes the problem worse.
- Definition
- Point where a river flows into a larger body of water
- Key process
- Sediment deposition due to current slowing
- Flow types
- Overflow, underflow, interflow
- Influencing factors
- Relative density, Earth's rotation, tides, seiches
- Landforms
- Deltas, sand bars, spits, tie channels
- Cultural significance
- Site of many towns and cities due to travel and ports
Lore & Background
At the river mouth, the change in flow conditions can cause the river to drop any sediment it is carrying, generating landforms such as deltas, sand bars, spits, and tie channels. These landforms drastically alter the geomorphology and ecosystem. Along coasts, sand bars and similar landforms act as barriers, sheltering sensitive ecosystems enriched by nutrients deposited from the river. However, the damming of rivers can starve the river of sand and nutrients, creating a deficit at the river's mouth.
Reader's Guide
River mouths are significant because they are sites of large-scale sediment deposition and allow for easy travel and ports, leading many towns and cities to be founded there. Many places in the United Kingdom take their names from their positions at the mouths of rivers, such as Plymouth, Sidmouth, and Great Yarmouth. In Celtic, the corresponding terms are Aber or Inver, from which come numerous place names such as Aberdeen, Inverness, and Inveraray. Due to rising sea levels as a result of climate change, coastal cities at river mouths are at heightened risk of flooding, and sediment starvation in the river compounds this concern. The motion of river water at the mouth is influenced by relative density, Earth's rotation, and ambient motion such as tides or seiches, with river water either plunging below the surface as an underflow or interflow, or floating along the surface as an overflow.
Did You Know?
- If river water is lighter than the receiving water, it floats along the surface as an overflow.
- Sediment deposition at river mouths can generate landforms such as deltas, sand bars, spits, and tie channels.
- Many UK place names like Plymouth and Sidmouth derive from their positions at river mouths.
- Damming of rivers can starve the river mouth of sand and nutrients, creating a deficit.
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