A criterion is given for the stability of steady uniform flow in open channels. When this number exceeds one, the flow is ultra‐rapid, roll waves form, and the flow cannot be steady. This criterion is compared with several which have been proposed, and is judged to be more comprehensive and exact. An empirical formula for Chezy's “C” in ultra‐rapid flow is derived from data already published by the author.
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Ralph W. Powell (1948) studied this question.