RATES AND PROCESSES OF BLUFF RECESSION ALONG THE LAKE-MICHIGAN SHORELINE IN ILLINOIS

We examined bluffs along 30 km of the Lake Michigan shoreline from Wil mette to Waukegan, Illinois, to measure amounts and variation in retre at rates and to determine what factors control rates and processes of retreat. The predominant bluff-retreat process is shallow- to intermed iate-depth translational landsliding triggered by heavy rainfall and w ave erosion at the base of the bluff, rotational slumping and shallow creep and earth flow also are common. Using historical maps and airpho tos, we measured amounts of bluff-top retreat at 300 locations. For tw o time periods, 1872-1937 and 1937-1987, rates of retreat vary from 10 to 75 cm/yr between discrete segments of bluffs (defined by lithology ) and between time periods for a given bluff segment. The average retr eat rates for the entire area, however, do not vary significantly betw een the two time periods and are approximately 20-25 cm/yr. Long-term average and short-term extreme lake levels and precipitation also do n ot vary significantly between the two periods, and thus local temporal variations in retreat rate cannot be attributed to these factors. Sho re protection built to date may have altered the spatial distribution of retreat rates in the area but has had little overall effect on the average regional retreat rates. The temporally constant regional retre at rates and the regular form of the local shoreline indicate that a l ong-term uniform rate of retreat prevails and that local variations in rates balance out through time to produce long-term parallel (in map view) bluff retreat in the area. This parallel bluff retreat probably is controlled primarily by the uniform retreat rate of the lithologica lly homogeneous shoreface in front of the bluff.