128-21 Frankenstein Rivers: Quantifying Glacial Legacy in Post-Glacial Channel Geometry, Sangamon River, Illinois

Abstract:

Rivers in post-glacial landscapes display a wide variety of morphologies reflecting their myriad origins and histories. Some rivers are relict glacial meltwater or sub-glacial channels. Others developed following deglaciation, incising into headlands from existing baselevels, while still others exploited the organization left by glacial flow to rapidly connect initially isolated basins. Meghani and Anders (2025) showed that drainage density in the post-glacial Central Lowlands of the United States likely increased rapidly following deglaciation and hypothesized that this rapid network growth was not achieved purely through headward propagation. Instead, they suggest that post-glacial rivers are akin to Frankenstein's monster: composed of components from multiple histories that together function as a single watershed. To qualitatively assess the relative contributions of different histories to these 'Frankenstein Rivers,' Meghani and Anders (in prep) have produced a geomorphic map of the Sangamon River watershed in central Illinois, USA. This map identifies regions of similar geomorphic texture (likely shared origin), highlights positive and negative relief structures with 'streamlined' orientations, and suggests origins of the modern Sangamon and its tributaries.

A quantitative evaluation is the next step. Channel geometry and discharge are empirically related in fluvial systems, and glacially derived channels, whether glaciofluvial, subglacial, or relict glacial forms, deviate from these relationships. Width-area relationships typically follow a power law in fluvial systems, where width increases downstream as a function of contributing area raised to the power omega, typically 0.1-1.2 (Rhodes, 1978; Turowski et al., 2025). Here I evaluate relationships among valley width, valley slope, modern drainage area, and sinuosity for the Sangamon River and several of its tributaries. Preliminary analysis shows the expected width-area relationship is broadly consistent along the river (omega ~ 0.47), but segments thought to be glaciofluvial in origin deviate substantially (omega < 0.01), consistent with valley forms inherited from glacial rather than fluvial processes. Sinuosity in tributaries may similarly delineate fluvial development from glacial inheritance: high sinuosity occurs in uplands and near tributary junctions with the Sangamon, while low sinuosity may mark candidate knickzones. Together, these metrics move the Frankenstein River concept from qualitative mapping to quantitative measurement, offering a first estimate of how much of the Sangamon's modern form is stitched together from its glacial past versus reworked by fluvial processes.

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