Submarine channel systems associated with turbidity currents show characteristic variations in channel width and channel slope with distance from source. The most proximal parts are generally formed from pre-existing canyons but these are followed by channels which show segments with two distinct behaviors: (1) sections where channel width is approximately constant (70% of total length) despite significant changes in slope; (2) sections where rapid changes in channel width are associated with relatively small changes in channel slope (30% of total length). Here, I show that width versus slope relationships are controlled by the Froude number of flows along a channel. For low Froude numbers, channel widths should be approximately constant whilst rapid-width-change segments of submarine channels are accurately reproduced if it is assumed that these segments support near-critical flows. Theoretical analysis of momentum conservation in one-dimensional (i.e., channelized) flows supports this concept of two distinct flow regimes: one in which Froude numbers are low and channel widths are constant and one in which Froude numbers oscillate around unity and channel widths fall rapidly with channel-floor slope.