AGE ISN’T EVERYTHING; GLACIAL MELTWATER FLOW PATHS INFLUENCE POST-GLACIAL DRAINAGE DENSITY

Continental glaciation profoundly impacts river networks. We are working in the US Central Lowlands to observe the recovery of post-glacial river networks. We expect drainage density to increase with time since glaciation as rivers grow back into the landscape. But is this the only major control? We mapped pre-settlement drainage density by modifying NHD river flowline data to remove artificial flow paths. In doing this, we deduce a more accurate representation of the true relationship between landscape development and age free from anthropogenic influences on drainage density. To quantify variability in space without imposing structure we constructed a fishnet of 5km by 5km squares and calculated drainage density in these boxes. We find significantly higher drainage density along major river corridors than in surrounding areas of the same age. We hypothesized that these river corridors, which were incised by glacial meltwater, increase local relief enough to promote the development of tributaries. At its southern margin, the Laurentide Ice Sheet was largely depositional, and, therefore, obliterated previous low order streams. In the case of high-order streams, the major river corridors were incised by meltwater and therefore maintained or developed during the glacier’s existence. This mechanism emphasizes the importance of bottom-up control in fluvial network development. Our findings show that the dependence of drainage density on age is complicated by the inherited river geometry in the Central Lowlands.