Some streams do not flow in a straight course. They meander, forming an "S" shape, or half loops as they move from side to side.

Sometimes the distance between the loops is shortened as the stream cuts deeper into the ground. The stream eventually cuts a new, shorter channel (cutting off, for example, the open part of the top loop in the "S" so that the stream course now looks more like a J) and abandons the meander loop entirely. 

Eventually the openings to the loop are silted in, and the old meander is completely isolated from the new stream course. The old meander is called an oxbow, because of its characteristic shape. Oxbows can be wet or dry. When they fill with water, they are called oxbow lakes. A slough (pronounced sloo), is a side channel of a stream. It flows slowly through swampy ground.