A tract of gently sloping land by a river that periodically overflows depositing clay, silt, or gravel which has been brought downstream where the water slows down: Mike and his family decided to pitch their tents on the alluvial plain near the river because it was flat and sandy.

The waste plains look desolate and barren, however they may be rich in minerals that have been washed down by the river creating an alluvial plain.