Oxbow Lake

Oxbow Lake, also cut-off lake or mortlake, a crescent-shaped lake formed in a section of a winding curve (meander) of a river channel that no longer carries the main river flow. Oxbow lakes occur on flood plains with well-developed and sinuous river meander systems (see River Features). In the bulbous course of the meander, the outside concave bend of the river suffers the fastest current and so the greatest erosion. The inside convex curve (the point bar or slip-off slope), where the current is slower, receives alluvium deposits. As the outside concave bend erodes, the velocity of the current increases and a pool is created adjacent to the eroded bank.

Deposition continues on the point bar causing the river bank to change position and the meander to move slowly and laterally downstream. The river (especially when engorged by flood) cuts across the narrowing neck of land between the meander bends, shortening and straightening its course. The looped path is abandoned and sediment is deposited on the banks of the river’s new course.

Blocked off from the course of the main river, the former meander loop becomes an oxbow lake lying alongside the parent meander system. The pool left by the river and the overflow from the flooding (including sediment) combine to fill the horseshoe-shaped hollow of the oxbow lake. A marsh may form, and, in time, depending on local drainage, the lake may infill with deposits and vegetation and a meander scar form. Oxbow lakes and meander systems occur worldwide, for example, in the rivers Wear and Cuckmere in England and the Mississippi in the United States. In Australia they are known as billabongs.