INFLUENCE OF LATE PLEISTOCENE YARDANG FORMATION ON MODERN FLUVIAL DRAINAGE PATTERNS IN NORTHWESTERN ILLINOIS AND EASTERN IOWA, U.S.A

Enhanced LiDAR elevation data reinforces the interpretation that the landscape of northwestern Illinois and eastern Iowa is marked by abundant yardang occurrences. Yardangs are streamlined, often long, ridges bounded by rounded troughs that formed by eolian erosion. Yardangs are oriented parallel to the prevailing wind direction at the time of formation. In these areas, during the Latest Pleistocene, the prominent wind direction was west-northwest to east-southeast. The strong winds eroded the silt and sand from the upland areas and deposited the sediments downwind (mostly as parabolic dunes) in lowlands areas in both Iowa and Illinois. Many of the fluvial drainages in eastern Iowa appear to flow in the same orientation as the yardangs. Study of the LiDAR elevation data indicates that Strahler Order 1 and Strahler Order 2 streams commonly occupy yardang troughs. Strahler Order 3 and higher order streams, however, often occur in valleys with bluff walls that are oriented parallel to the prevailing yardang orientation. In these larger valleys and yardang troughs, the strong winds appear to have modified the pre-existing valley walls (by erosion and infill deposition) to have formed straight-lined bluffs that mimic yardang ridges. Late Pleistocene yardang formation had a profound influence on the drainage pattern of many of the uppermost tributaries in the modern fluvial system, but the primary flow direction had been established earlier and was only modified during this time period of erosion. The widespread occurrence of yardangs features indicate that soil conditions were not stable during this time—temperature was above freezing and plant cover was insufficient to prevent widespread erosion by the strong west-northwest wind.