THE POTENTIAL FOR REMOBILIZATION OF EOLIAN LANDFORMS WITHIN THE GREEN RIVER LOWLAND, NORTHWEST ILLINOIS: POTENTIAL RESPONSES TO GLOBAL CLIMATE CHANGE

Parabolic and compound parabolic dunes within the Green River Lowland (GRL) formed during a basin-wide period of dune formation from ca. 18,000 -12,000 cal yr B.P., followed by a long period of dune stability and soil formation. This period of stability was periodically interrupted during the late Holocene when localized sand sheets migrated up the western and northwestern flanks of pre-existing dunes. The later periods of eolian activity were local in extent and many dunes remained stable throughout the Holocene. Eolian landforms within the GRL remain stable under current climatic conditions. Wind data from six regional climate stations indicate that the winds within the region are strong enough to entrain sand. The degree of vegetation cover on eolian sands is in part a function of the balance between precipitation and evapotranspiration, and it appears that the dunes within the GRL remain stable because of relatively high P:PE ratios. The potential impact of increased aridity on eolian stability within the GRL was evaluated by computing P:PE ratios and Lancaster dune mobility indices for the 71 year period running from 1932–2003. Regional historic aerial photographs ranging in age from 1939 through 1988 were used to visually compare the extent of vegetation cover over time. Vegetation cover was then evaluated in respect to the dune mobility index (W), and the Palmer Drought Severity Index (PDSI). 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-1980s's. During these same periods the Lancaster dune mobility index ranged between 50-100 suggesting that dune crests could have been active at those times. During the droughts of the 1930s, 1950s and to a lesser extent the late 1980's vegetation cover was reduced significantly on eolian landforms. Identifiable windbreaks planted on the lee side of dune crests during the 1930's and 1950's indicate that indeed some, but not all dune crests were active at this time. The fact that not all dunes were reactivated during these periods indicates that dune mobility within the GRL may be linked to factors both extrinsic (climate) and intrinsic (i.e. local disturbance) to the system.