Shearer, I had the same question when reading Heise's reply. I didn't realize that it was singular. I looked it up and here is what it says, although I'm not sure which one they (CRSA) emphasizes:
(1) a part of a river where the current is very fast
(2) characterized by speed; moving with or capable of moving with high speed; "a rapid movment"; "a speedy car"; "a speedy errand boy"
www.cogsci.princeton.edu/cgi-bin/webwn2.1(3) A rapid is a section of a river where it loses elevation over a relatively short distance (that is, the stream gradient is locally steepened), causing an increase in water flow and (usually) turbulence. A rapid is a hydrological feature between a run and a cascade. A rapid is recognized by the river becoming shallower and having rocks exposed above of the flow surface. As flowing water splashes over and around the rocks, air bubbles become mixed in with it and the water takes on a white colour,
en.wikipedia.org/wiki/Rapid
