LANDSCAPE RESPONSE TO HOLOCENE BIOCLIMATIC CHANGE RECORDED IN THE ALLUVIAL STRATIGRAPHY OF THE LITTLE OSAGE RIVER VALLEY, SOUTHEASTERN KANSAS

Alluvial stratigraphic analysis of the upper reach of the Little Osage River, southeastern Kansas, illustrates a complex response of stream behavior and floodplain vegetation to Holocene climate change. Carbon isotopic analyses performed on the fill sequences estimated the abundance of C₃ and C₄ vegetation associated with each alluvial stratigraphic unit. Radiocarbon ages determined on charcoal provided temporal control for stratigraphic units and their isotopic profiles. These data suggest that early-Holocene aggradation occurred on a floodplain with a mixed plant community (¹³C = -18 to -20‰). During the middle Holocene, the floodplain transitioned to prairie (¹³C = -12.5 to -13.5‰). This transition was a time of extended erosion characterized by lateral migration and incision of the channel. Floodplain erosion persisted until ca. 4370 ¹⁴C yr B.P., when floodplain aggradation resumed and there was a shift to a mixed plant community (¹³C = -17 to -19‰). At least two episodes of lateral channel migration and incision occurred after ca. 4300, and were each followed by aggradation at approximately 2900 ¹⁴C yr B.P. and 620 ¹⁴C yr B.P. The d¹³C values (¹³C = -18 to -20‰) suggest that a mixed plant community occupied the floodplain during the late Holocene.

The geomorphic events recorded in the upper reach of the Little Osage River are similar to events recognized in larger-order stream valleys in the region, although the chronology of events in the Little Osage lags by approximately 1500 years. The timing of floodplain erosion and aggradation in the valleys of low-order streams may not be dictated by local climate and vegetation changes alone, but rather by the transgression of bioclimatic responses in the larger order valleys headward into the tributary valleys.