Some dynamic aspects of river geometry

Natural alluvial river channels, in contrast to most regime or equilibrium canals, are characterized by variable streamflows. This paper relates channel geometry to measures of streamflow variability as distinct from mean discharge or flow magnitude. A relationship between mean channel width and the mean and coefficient of variation of channel‐forming discharges is derived. For the given mean discharge it is shown that the more variable the flow, the narrower the Channel is in the mean. To a great extent, this relationship is confirmed by data on alluvial channels in the Missouri River basin. Variability of water discharges proves to be an important factor influencing natural river geometry. This initial approach to the understanding of the dynamics of river geometry leads to the construction of a simple simulation model in which a series of channel forming discharges is generated and corresponding changing channel geometry is calculated. Results from the simulation indicate (1) the channel has, in effect, a kind of truncated memory; i.e., the impact of an existing channel exists so long as the prevailing discharge does not exceed the present channel capacity; (2) the effect of memory or the impact of the existing channel on the mean channel geometry is opposite to that of flow variability (i.e., the memory tends to keep the channel geometry larger in the mean); (3) the simulated channel width, as expected, increases abruptly at high discharges, declining over time along a die‐away curve until the advent of a recorded high flow.