An assessment of overbank flow-statistics within the context of environmental flow rules

Characterizing diversity in river flow regimes is crucial in informing river flow management guidelines for protecting freshwater ecosystems. In partly-confined reaches, river exhibit substantial lateral and vertical interaction when flow inundates its floodplain patches. A knowledge gap exists at the intersection of environmental flow assessment, overbank exceedance flow dynamics, and real-world policy application. This study aims to bridge that gap through a characterisation of overbank exceedance flows. Incorporating overbank exceedance flow days – a threshold hydrological phenomenon, and its variability can be significant information to restore the interaction between channel and floodplain. Our work to significantly expand the knowledge of flow processes that illuminate the stream-floodplain linkages with only a modest increase in data needs is a particular novelty. We investigated recent changes in the overbank exceedance flow in the Sheepscot River in Maine, USA. Our results indicate that: (a) there is a weak correlation between the commonly used Indicators of Hydrological Alteration (IHA) metrics and overbank exceedance flow characteristics, (b) overbank exceedance flow time series are non-stationary, (c) the stream flow maintenance rules (as per Maine’s instream flow standards encoded in its Chapter 587) when based simply on seasonal median flow estimates significantly underrepresents the overbank flow exceedance phenomenon. Further, through a joint analysis of the volume of overbank exceedance flows and number of overbank exceedance days, we identified emerging seasons of overbank exceedance phenomenon. Sensitivity analyses using ±10–20% adjustments to the bankfull threshold demonstrated that the observed seasonal trends and regime shifts were robust to reasonable variations in the threshold definition.