It is sometimes spelled "sub-basin" depending on an author's preference, but the predominant form is "subbasin". Some subbasins are referred to as "sags", although that term has a broader definition than a subbasin does.

What does basin mean?

1a : an open usually circular vessel with sloping or curving sides used typically for holding water for washing a new washstand and basin. b chiefly British : a bowl used especially in cooking. c : the quantity contained in a basin spilled a basin of water on the floor.

What does River Basin mean?

A river basin is the portion of land drained by a river and its tributaries. It encompasses all of the land surface dissected and drained by many streams and creeks that flow downhill into one another, and eventually into the Milwaukee River.