Empirical Evidence of Dynamic Hydrogeomorphic Feature Inundation in a Lowland Floodplain

Floodplains along low-gradient, meandering river systems contain diverse hydrogeomorphic features, ranging from isolated depressions to hydrologically-connected channels. These ephemerally-flooded features inundate prior to river water overtopping all banks, enhancing river-floodplain connectivity during moderately high flow stages. Predicting when and where ecological functions occur in floodplains requires understanding the dynamic hydrologic processes of hydrogeomorphic features, including inundation and exchange. In this study, we examined storm event-scale inundation and exchange dynamics along a lowland, meandering river system in central Illinois (USA). We monitored surface water presence/absence, surface water level, and groundwater level across floodplain hydrogeomorphic feature types (i.e., isolated depression, backwater channel, and flow-through channel). Using these data, we evaluated inundation onset and recession characteristics, drivers of groundwater-surface water interactions, and direction of hydrologic exchange with the river channel. Surface water presence/absence patterns suggested inundation onset timescales were primarily controlled by microtopography and recession timescales were correlated with floodplain elevation. Employing a novel hysteresis approach for characterising groundwater-surface water interactions, we observed distinct patterns indicating differences in water sources across hydrogeomorphic units and event characteristics. Finally, differences in hydraulic head along floodplain channels revealed that channels with multiple inlets/outlets (i.e., flow-through channels) conveyed down-valley flow and channels with single inlets primarily functioned as sinks of river-derived water to the floodplain with short source periods. These results highlight the heterogeneity of hydrologic processes that occur along lowland, meandering river-floodplains, and more specifically, point to the important role hydrogeomorphic features play in controlling dynamic connectivity within the river corridor.