What is a backwater?
In river terms, a backwater is a former side channel or river arm that has been cut off from the main flow, so water barely moves through it. Many backwaters stay connected to the river at one end. The Dutch call this kind of one-end-connected arm a strang. The word backwater is also used for water held back by a dam or weir; this article covers the river-landscape meaning.
Backwaters form in several ways. In a natural river the flow splits around an island to form a side channel; if one end later silts up, the side channel becomes a backwater. A meander that the river cuts through can leave its old loop behind as an oxbow, and straightening a river for navigation leaves severed bends in the same way. Because so little water flows through, a backwater gradually fills with silt and vegetation and can end up as marsh.
Recognising it
- A long, often curved body of water on the floodplain beside the present river.
- Usually open to the river at one end and closed at the other.
- Still or very slow water, with reeds and water plants along the banks.
In practice
- Expect changing water levels. A connected backwater fills when the river rises. Do not plan buildings or storage right at its edge.
- Allow for soft ground. Silting backwaters often hold thick layers of mud and peat. Get a ground investigation before building paths, jetties or bank walls.
- Check consents first. Work in or near a river and its floodplain generally needs permission from the relevant environmental or water authority, and backwaters are often in protected habitats.
- Treat it as an asset. River restoration schemes in the Netherlands and elsewhere reconnect or re-dig backwaters because they give fish, small aquatic animals and water plants a sheltered habitat.
Common mistake: treating a connected backwater as an isolated pond. Its level rises and falls with the river, including in flood season.
Related terms
- Re-meandering
- Groundwater seepage
- Emergency dike
- Dike monitoring
- Flood spillway
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