Areas where topography prevents the outﬂow of water are called closed basins. With no outlet, all drainage is internal. These rivers do not reach the ocean, and water escapes only by evaporation or percolation. Closed basins may be extensive: the Great Basin—which is actually a collection of smaller closed basins—for example, covers an area of nearly two hundred thousand square miles, covering most of Nevada and large parts of Utah and Oregon. Or they may be quite small: prairie potholes of the northern plains cover just a couple of acres or less.