Modeled Postglacial Landscape Evolution at the Southern Margin of the Laurentide Ice Sheet: Hydrological Connection of Uplands Controls the Pace and Style of Fluvial Network Expansion
Landscapes of the US Central Lowland were repeatedly affected by the Laurentide Ice Sheet. Glacial processes diminished relief and disrupted drainage networks. Deep valleys carved by glacial meltwater were disconnected from the surrounding uplands. The upland area lacking surface water connection to the drainage network is referred to as non-contributing area (NCA). Decreasing fractions of NCA on older surfaces suggests that NCA becomes drained over time. We propose that the integration could occur via: 1) capture of NCA as channels propagate into the upland or, 2) subsurface or intermittent surface connection of NCA to external drainage networks providing increased discharge to promote channel incision. We refer the two cases as “disconnected” and “connected” since the crucial difference between them is the hydrological connection of the upland to external drainage. We investigate the differences in evolution and morphology of channel networks in low relief landscapes under disconnected and connected regimes using the LandLab landscape evolution modeling platform. We differentiate between connected and disconnected upland cases by considering two different flow routing schemes across the land surface. The disconnected case is modeled by the existing FlowRouter module in LandLab, and it assumes that all the closed depressions are isolated from the river channel network. Alternatively, in the connected case we assume that all the closed depressions are always filled with water to their spill points and precipitation falling on the upland flows through these lakes and across flat land surfaces into the river channel network. We created and implemented two additional modules to represent the filling and spilling of isolated depressions. The PitFiller module fills the depressions up to their spillover points. The FlowRouterOverFlat module calculates the flow direction across topography with flats created by the PitFiller module. It creates a gradient away from higher edges of depressions as well as a gradient towards lower edges of depressions, resulting in a convergent flow field without changing the actual topography. We observe substantially faster rates of erosion and integration of the channel network in the connected case. The connected case also creates longer, more sinuous channels than the disconnected case. Sensitivity tests indicate that hillslope diffusivity has little influence on the evolution and morphology. The fluvial erosion coefficient has significant impact on the rate of evolution, and it influences the morphology to a lesser extent. Our results and a qualitative comparison with landscapes of the glaciated US Central Lowland suggest that connection of NCAs is a potential control on the evolution and morphology of post-glacial landscapes.