THE POST-GLACIAL GEOMORPHIC HISTORY OF A PORTION OF THE MANISTEE RIVER

Due to the glacial history of Michigan, the focus of geomorphic research has traditionally been glacial processes. Recent investigations have sought to learn more about the post-glacial development of Michigan landscapes, including the history of the many stream systems that have largely been ignored. An excellent example of a prominent stream that has not been geomorphically studied is the Manistee River, which has a 2219 km2 drainage basin in northwestern Lower Michigan. This paper presents early findings, specifically basic geomorphology and associated radiocarbon dates, from a study conducted on a seven-mile long reach of the stream that lies between the Hodenpyl and Tippy dams. Preliminary results indicate that there are seven fluvial surfaces in the study area, with the highest (T-7) lying ~ 30 m above the modern stream. Surfaces ranging from T-7 to T-2 are underlain by gravelly outwash that unconformably lies on pink (5R/5/6) till. The lowermost suite of surfaces (T-1 complex) is underlain by fine loamy sand. Five radiocarbon dates were obtained from basal peat in filled oxbows on the terraces and indicate that: 1) the T-5 terrace was abandoned ~ 9290 ± 40 cal. yrs B.P. at one site, 2) the T-3 terrace formed between ~ 4970 ± 60 and 3100 ± 40 cal yrs B.P., and 3) the T-2 terrace was abandoned at ~ 1560±70 B.P. at one site. These results suggest that the terraces are largely strath and that downcutting has been the dominant Holocene process in this portion of the Manistee River. This downcutting may be related to complex post-glacial stream responses, baselevel adjustments in Lake Michigan, or both.