Oxbow lake (thing)

This occurs after constant erosion around a semicircle in a river. The eroded earth is deposited where the water is slowest, and eventually the erosion cuts straight through, while the deposits block the original stream.

ASCII art diagram of how an oxbow lake is formed:

   original river
      _------_
     / .----. \
    ( (      ) )
     \ \    / /
-----' /    \ '-----
------'      '------

The circle exists as part of a meandering stream, following the lowest land.

 erosion and depostion
     .--------.
    /  .----.  \
    \  \    /  /
     \  \  /  / ,-- deposition at these corners
-----'   )(   '-----
--------'  '--------
         \/
          \
           erosion occurring
           on both sides

As a result of the pattern of erosion and deposition, the shape of the stream morphs by expanding at the top and contracting at the bottom.

     separation  .-- oxbow lake
     .--------. /
    /  .----.  \
    \  \    /  /
     \_/    \_/ ,--- cut off (due to deposits)
--------------------
-------------------- 
         \
          straighened stream (due to erosion)

Eventually, a strong storm will come along and force the river over it's edges, where it will find the shorter straight path across the curve easily due to the heavy erosion. The deposits will finally accumulate enough on the sides of the river to cut off the curve to form an oxbow lake.