POSTGLACIAL STREAM DEVELOPMENT RATES: CLUES FROM A DRAINAGE CAPTURE RECORDED IN ALLUVIAL SOILS

Drainage network expansion in low-relief hummocky landscapes exposed after continental glaciation often requires breaching divides between closed upland basins and advancing channel heads. However, the rate and timing of network expansion following deglaciation in these settings is often poorly known. In addition, the process of divide breaching – whether it proceeds by advance of the channel head (bottom-up) or fill-and-spill from closed basins (top-down) – remains unclear.

Here we describe the geomorphology and alluvial stratigraphy of the Dry Creek watershed, a small first-order creek (drainage area 7.9 km²) in central Iowa that developed in Late Wisconsinan glacial deposits of the Des Moines Lobe. A barbed stream geometry and truncated alluvial valley suggest that Dry Creek captured a portion of the adjacent watershed at some point in its postglacial evolution. We describe observations of alluvial stratigraphy in Dry Creek and the adjacent truncated valley indicating that this capture likely occurred no earlier than the late Holocene. Basin morphology suggests that capture was likely enabled by headward (bottom-up) growth rather than top-down. Numerical modeling of long profile evolution before and after capture provides additional constraints on the timing of capture. Taken together, these observations suggest that approximately 13,000-15,000 years were required for postglacial channel head advance to integrate the captured uplands into Dry Creek.