Adjustment of river channel morphology to modes of climate variability

River networks are typically treated as conduits of fixed discharge conveyance capacity in flood models and engineering design, despite knowledge that alluvial channel networks adjust their geometry, conveyance, planform, extent and drainage density dynamically over time in response to shifts in the magnitude and frequency of streamflows and sediment supply. Although it is recognized that climate patterns have consistent effects on streamflow distributions in different regions of the world, and that channel morphology can be influenced by climate variability, a systematic association between large-scale climate patterns conducive to wetter-/drier-than-average conditions and changes in channel conveyance has not been established in observational records.

To explore the relationship between river channel conveyance/geometry and three modes of climate variability with different periodicities (the El Niño-Southern Oscillation, Atlantic Multidecadal Oscillation, and Arctic Oscillation) we use two-, five- and ten-year medians of channel measurements, streamflow, precipitation and climate indices over seven decades in 67 United States rivers. These temporal aggregates provide more robust estimates of average channel form than individual field measurements by reducing the noise in the data and accounting for lag times in channel response to changing streamflow/sediment regimes. The use of identical aggregation periods for all variables provides a common baseline to assess whether climate variability is reflected in average conditions of precipitation, streamflow, and thereafter, channel morphology.

Overall, we find that in two thirds of these rivers, channel capacity undergoes coherent phases of expansion/contraction in response to shifts in catchment precipitation and streamflow, driven by climate modes with different periodicities. The physical responses of the river networks suggest that they may be viewed as dynamic, breathing systems that expand and contract over interannual to multidecadal timescales in synchrony with regional climate. In due course, it may even be possible to assign quality markers to individual streamflow gauging stations according to the sensitivity of their cross-sections to climate variability.