INFLUENCE OF LAKE LEVEL ON RECESSION OF HIGH AND LOW SHORE BLUFFS ON WISCONSIN'S LAKE MICHIGAN SHORELINE

At two 4-to-5 mile long reaches in Ozaukee and Manitowoc Counties, recession rates are determined from digital orthophotos constructed using historical aerial photographs taken over intervals ranging between 6 and 17 years from the 1940s to present. Rates for the toe and crest of the site with high (30–45 m) bluffs and for the crest, but not the toe at the site with low (9–11 m) bluffs. Both the toe of the high bluffs and the crest of the low bluffs show temporal recession-rate patterns that closely match the changes in the average lake-water level and correlative wave-impact height (WIH, height on the bluff that the waves reach). The annual average of the peak monthly WIH shows a dramatic rise during 1964-75, a period during which the maximum toe recession rate is measured for the high-bluff site. A similar sharp rise inWIH occurs near the beginning of the 1967-75 period, during which the maximum crest-recession rate is measured for the low-bluff site. This supports the notion that because low bluffs fail mostly by shallow slides and flows, the crest responds rapidly to wave erosion at the base. The crest of the high bluffs recedes at a rate that is relatively insensitive to lake level changes. This appears to be because the high bluffs are protected by large slump blocks that are eroded by waves until they are removed (perhaps 50 or 100 years). At that time a large slump occurs, causing the bluff crest to be isolated from wave impact. Thus, lake level and WIH are relatively good predictors of bluff top retreat for low bluffs, but not for high bluffs.