Punctuated Equilibrium in River Systems: Quantifying Abrupt Hydraulic Instability Across Water Level Timeseries

Previous work has shown that geomorphic changes can drive changes in channel conveyance capacity that affect flood hazard (Slater et al., 2015; Pinter et al., 2008). However, these prior studies have tended to frame evolving flood hazard as a monotonic trend (in response to gradual channel aggradation or degradation), without detailed consideration of the actual trajectory of change. Here we suggest that the evolution of flood hazard (here represented by changes in water level for a constant discharge) in river systems is sometimes expressed as behaviours consistent with geomorphic punctuated equilibrium (Phillips, 2006), whereby periods of relative stability and/or gradual change are interrupted by abrupt shifts in water levels.

To illustrate this, we present an analysis of the frequency and magnitude of abrupt shifts in water level for constant discharges (so-called specific gauge analysis) using long term (>30 year) records at more than 120 US gauging stations. At each station we first identify linear trends (via Mann-Kendall (MK) testing) in the water level time series, before identifying abrupt discontinuities using a Pruned Exact Linear Time (PELT) algorithm, creating a robust framework for detecting multiple regime shifts within each time series.

Preliminary results reveal that, even for our gauging station study sites – typically considered to be geomorphically stable – both gradual adjustments and abrupt shifts in water level are common across a wide range of return-period flows. We also explore how the prevalence of these instabilities varies in response to driving factors such as sediment connectivity, channel confinement, and flow regulation. These findings have implications for flood risk management, suggesting that static flood maps may be insufficient in dynamic landscapes to represent the actual risks posed to exposed populations and assets.

References:

Phillips, J. D. (2006). Evolutionary geomorphology: thresholds and nonlinearity in landform response to environmental change. Progress in Physical Geography, 30(4), 431-447.

Pinter, N., et al. (2008). Cumulative impacts of river engineering, Mississippi River, USA. Geomorphology, 101(1-2), 147-160.

Slater, L. J., Singer, M. B., & Kirchner, J. W. (2015). Hydrologic versus geomorphic drivers of trends in flood hazard. Geophysical Research Letters, 42(2), 370-376.