AN ASSESSMENT OF THE LITHOSTRATIGRAPHY AND ERODIBILITY OF HOLOCENE ALLUVIAL FILLS (DEFOREST FORMATION) IN NORTHEASTERN KANSAS, USA

Streambanks are the primary source of sediment in Midwestern watersheds. In much of this region, deposits of fine-grained Holocene alluvium comprising streambanks have been assigned to a single lithostratigraphic unit: the DeForest Formation. This study examines the stratigraphic relationships and measures the erodibility of the different members of the DeForest Formation in three watersheds in northeastern Kansas.

Distinct differences in erodibility, measured in terms of critical shear stress (τ_c), were observed between the different members of the DeForest Formation. The most erodible member is the Camp Creek Member (average τ_c = 1.0 Pa) while the most resistant is the Gunder Member (average τ_c = 10.4 Pa). Variability in erodibility between and within the members of the DeForest formation is attributed to the magnitude of weathering from pedogenic processes as well as the inherent variability in the different parent materials. Grain-size results indicated a weak positive correlation between clay content and τ_c, that may provide a useful tool for approximating the erodibility of streambanks based on grain-size data. Resistance to erosion by fluid flow was found to be significantly greater where clay contents exceed 28%.

Although the Camp Creek Member was found to be the most erodible, it always occurs, stratigraphically, as the uppermost member. Bankfull stage data indicate that bankfull discharges rarely attain elevations sufficient to erode Camp Creek Member deposits. Therefore, other members of the DeForest Formation are able to exert some control on the rate of bank erosion. The documented complexity in the stratigraphic relationships between the various members of the DeForest Formation has important implications for streambank erodibility and is important for accurately determining likely areas of streambank erosion in Midwestern streams.