The Collaborative International Dictionary of English v.0.48:

Rapid \Rap"id\, Rapids \Rap"ids\(r[a^]p"[i^]dz), n. [Cf. F.
   rapide. See Rapid, a.]
   The part of a river where the current moves with great
   swiftness, but without actual waterfall or cascade; sometimes
   called whitewater; -- usually used in the plural; as, the
   Lachine rapids in the St. Lawrence. For boaters on the river,
   it is a place that can be hazardous, with danger of capsizing
   or crashing into large rocks.
   [1913 Webster +PJC]

         Row, brothers, row, the stream runs fast,
         The rapids are near, and the daylight's past. --Moore.
   [1913 Webster] Rapid-fire