with the bed downstream of the rapids. Very young streams flowing across solid rock may be rapids for much of their length. Rapids cause water aeration of the stream or river, resulting in better water quality.

For a rapid to form, a necessary condition is the presence of a gradient, which refers to the river or stream's downward slope. When a river has a larger gradient, the water flows downhill faster. Gradients are typically measured in feet per mile or metres per kilometre. This impacts the