ONGOING RESEARCH IN GEOMORPHOLOGY AND HYDROGEOLOGY AT THE COOPER CREEK COLLABORATIVE EXPERIMENTAL WATERSHED, CINCINNATI

The 2.9-km² Cooper Creek Collaborative Experimental Watershed in Cincinnati, OH was established in 2018 to assess mitigation strategies needed to restore a relatively natural hydrology to a headwater stream draining a densely developed urban area (48% impervious) in Hamilton County, Ohio. Cooper Creek is a natural tributary of Mill Creek, a strongly impacted urban watershed. The upper Cooper Creek catchment is shallowly underlain by Ordovician Bellevue Limestone. The natural soil of the catchment is developed from Illinoian-stage loess, glacial till and outwash. During individual heavy rain events, Cooper Creek and similar streams regionally exhibit very flashy behavior. The site offers substantial opportunities for both applied and basic research, including ongoing and planned projects in sediment transport, fluvial geomorphology, and hydrogeology.

For example, an emerging question in bedload transport is how transport thresholds change based on particle shape, with implications for using typical metrics of e.g. Shields stress to assess sediment transporting flows. In the study area, Cooper Creek is mostly floored by partly-imbricated 10-50 cm limestone slab cobbles and boulders over in-place bedding planes of limestone. Over 300 PIT-tagged rocks across different size and shape classes have been deployed for particle tracking through the reach. This research directly relates to an ongoing applied study of streambed change and ecological health proxies in response to additions of large wood to experimental reaches of the stream.

Hydrogeologically, upper Cooper Creek is complex, with a mixed substrate of fractured bedrock, floodplain deposits, soil, and urban elements such as sewer lines and construction fill. Hyporheic exchange dynamics are poorly understood in these environments. We discuss first results from near-surface geophysical investigation of the subsurface heterogeneity structure and flow dynamics of the hyporheic zone; specifically seismic refraction tomography, spatial patterns of electrical conductivity, and time-lapse images of subsurface electrical resistivity. This work will further contribute data important for urban water management (e.g., mitigating baseflow problems related to urban karst).