A boil is turbulence in a river resulting from such causes as deep holes, channel changes, or underwater obstructions. It can also be a large upheaving of moving water in a river. In Life on the Mississippi, Mark Twain writes of how he came to master the language of the water: “Those tumbling ‘boils’ show a dissolving bar and a changing channel.” A sand boil is a land term that refers to sand in a low mound, produced by liquefied sand pushing up to the ground surface.

Arthur Sze