Fluvial processes and streamflow variability: Interplay in the scale-frequency continuum and implications for scaling

This paper explores the links between channel/floodplain morphometry, streamflow variability, and sediment transport across a wide range of scales and frequencies of discharge. On the basis of extensive analysis of observations from a climatologically and geologically homogeneous region in the midwestern United States, we provide evidence that streamflow statistics are significantly affected by the scale-dependent channel/ floodplain interactions, which in turn are controlled by (and at the same time actively participate in defining) the dominant fluvial processes at a given scale. More specifically, we document that (1) the channel cross-sectional geometry exerts a strong control on the frequency distributions of both daily and maximum annual discharges; (2) the frequency of exceedance of bank-full discharge is scale dependent (particularly, channels draining large areas flood less often but stay overbank longer than channels draining small areas); (3) the critical area at which the variability of floods with scale changes from increasing to decreasing associates with the scale at which the fluvial regime changes from net-erosional to net-depositional and the floodplain gets well established due to its increased frequency of occupation by the flow; and (4) scaling in suspended sediment load reflects the scaling in channel and floodplain morphometry and depicts the signature of the aforementioned fluvial regime transition. The observation is made that maximum annual floods are composed of two distinct populations, one from below and one from above bank-full flows, and that the quantile at which this transition occurs depends on scale. On the basis of this observation, the notion of statistical multiscaling of floods is reexamined.