A new hypothesis predicts that in mobile-bed river channels, interactions between the channel hydraulics and bed configuration prevent the Froude number (Fr) from exceeding 1 for more than short distances or periods of time. Flow conditions in many steep, competent streams appear to be close to critical. Froude numbers of steep (slope ≈ 0.01) sand-bed streams with considerable freedom to adjust boundaries oscillate between 0.7 and 1.3 over 20- to 30-s cycles, with an average of 1.0 at the channel thalweg. Critical flow in these streams is maintained by the interaction between the mobile bed and free water surface at high Fr, which results in a cyclical pattern of creation and destruction of bed forms. Field observations support that a similar mechanism of flow–bed form interaction constrains Fr ≤ 1 in active-bed braided gravel rivers, step–pool streams, laboratory rills, lahar-runout channels, and even some bedrock channels. Empirical and analytical results show that as slope increases, competent flows tend to asymptotically approach critical flow. An assumption of critical flow would dramatically simplify paleohydraulic flow reconstructions and modeling of flow hydraulics in high gradient streams.