An Experimental Study of Drainage Network Development by Surface and Subsurface Flow in Low-Gradient Landscapes

Abstract

Rivers form elaborate drainage networks with morphologies that express the unique environments in which they developed. My research seeks to understand how drainage networks evolve in low-gradient, poorly-drained landscapes by integrating internally-drained areas, referred to as non-contributing area (NCA), through time. I assessed the erosional processes of channel growth and NCA integration by conducting experiments in a small-scale drainage basin. My experiments developed channels by overland flow or seepage erosion to varying extents depending on the substrate composition, rainfall rate, and relief of the drainage basin. These factors mediated whether the drainage basin primarily routed precipitation to channels by the surface or subsurface, causing more channelization by overland flow or seepage erosion, respectively. Overland flow channels formed at the onset of experiments and expanded over a majority of the basin area by forming broad dendritic networks. Large surface water contributing areas to overland flow channels supported numerous first-order channels that integrated more NCA than seepage erosion. When overland flow was the dominant process, channels integrated NCA at a similar, consistent rate under all experimental conditions. Seepage erosion began later in experiments after channels had incised enough for exfiltrating groundwater to initiate mass wasting of headwalls. Periodic mass wasting of channel heads caused them to assume an amphitheater-shaped morphology. Since groundwater drove seepage erosion, seepage-driven channel heads had smaller surface water contributing areas that captured less NCA than overland flow. The experimental results provide insight into drainage networks that formed in glacial sediment throughout the Central Lowland province (USA) and other areas affected by Pleistocene continental glaciation. The region has broad areas of subtle topography comprised of glacial sediments in which rivers are creating drainage networks. The geologic diversity, coupled with changing climate, vegetation, and other characteristics of the landscape, may have driven different processes of channel development through time. Consequently, drainage networks throughout the region represent distinct, complex histories of channel development by different processes that resulted in varying degrees of network integration.