A backwater refers either to a body of water in a main river which is backed up by an obstruction such as the tide or a dam, or to a branch of a main river which runs alongside it before rejoining it.Merriam Webster Dictionary
When a section of a river is near the coast or another feature that sets its base level, then the section which is influenced by the conditions at its mouth is termed a backwater. If a river flows into a lake or sea, this is the region in which the slope of the river decreases because the lower water flux permitted at the mouth causes the water to back up. Where the river outlet is strongly affected by tides, this cyclic change in base level changes the portion of the river that is a backwater. As a result fresh and salt water may become mixed to form an estuarine environment.
If a river has developed one or more alternative courses in its evolution, then one channel is usually designated the main course and secondary channels may be termed backwaters. The main river course will usually have the fastest stream and will for example be a main navigation route, whereas backwaters may be more shallow and flow more slowly.
References
Southard, John B. (2006). "Chapter 5: Open-Channel Flow". An Introduction to Fluid Motions, Sediment Transport, and Current-generated Sedimentary Structures. MIT OpenCourseWare. http://ocw.mit.edu/NR/rdonlyres/Earth--Atmospheric--and-Planetary-Sciences/12-090Fall-2006/CD91BF88-9591-4A08-87C3-1352F2FF0CFD/0/ch5.pdf. Retrieved 16 March 2010.
