THE POTENTIAL FOR REMOBILIZATION OF EOLIAN LANDFORMS WITHIN THE GREEN RIVER LOWLAND, NORTHWEST ILLINOIS

The Green River Lowland (GRL), located in northwest Illinois, contains numerous parabolic and compound parabolic sand dunes. Currently these eolian landforms are stabilized. However, historic aerial photographs indicate that dunes within the Green River Lowland were, at least locally, periodically reactivated during the last 100 years. Vegetation cover appears to be the primary limiting factor inhibiting dune reactivation within the region. Aerial photographs ranging in age from 1939 through 2005 were used to visually compare the extent of vegetation cover over time. Vegetation cover was then evaluated in respect to the Lancaster Dune Mobility Index, and the Palmer Drought Severity Index. Calculated PDSI values for northwestern Illinois from 1932-1990 indicate that the region experienced severe drought during the 1930's, the mid-1950's and the early 1960's, with mild to moderate drought in the late-1980's. During the droughts of the 1930s, 1950s and to a lesser extent the late 1980's vegetation cover was reduced significantly on the eolian landforms. Identifiable windbreaks planted on the lee side of dune crests during the 1930's and 1950's indicate that many crests were active at this time. However dune mobility indices during these same periods never exceeded the threshold value of 50 believed to be necessary for crest reactivation. In some cases, sharp, linear N-S or E-W boundaries marking fence lines separate areas of active sand from vegetated surfaces suggesting that land use practices played a role in dune mobility. However, it seems unlikely that land use was the only causal mechanism as eolian deposits within the GRL remain stable today even under intense row-crop cultivation. These results suggest that dune mobility within the GRL may be linked to factors both extrinsic (climate) and intrinsic (e.g. local disturbance like overgrazing) to the geomorphic system.