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It has been hypothesized that velocity reversals provide a mechanism for maintaining pool-riffle morphology in gravel-bed rivers--an important habitat for salmonids, which are at risk in many places worldwide and that are the focus of extensive environmental legislation in Europe and North America. However, the occurrence of velocity reversals has been controversial for over 3 decades. We present a simple one-dimensional criterion that unifies and explains previous disparate findings regarding the occurrence of velocity reversals. Results show that reversal depends critically on the ratio of riffle-to-pool width, residual pool depth (difference between pool and riffle elevations), and on the depth of flow over the riffle, suggesting that land management activities which alter channel form or divert water from the channel can have negative impacts on the sustainability of pool-riffle habitat in gravel-bed rivers. DOI: 10.1061/(ASCE)0733-9429(2009)135:1(66) CE Database subject headings: Pools; Channel morphology; Open channel flow; One dimensional flow; Stream improvement.