rapidnes

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 ]