Are bluffs along the Great Lakes eroding faster now than ever before? A case study of modern and historical rates and drivers of bluff retreat along Lake Michigan in Western Michigan, U.S.

Abstract:

Numerous studies have documented bluff recession along the Lake Michigan shoreline; yet, few have explored how bluff retreat rates vary through time and how rates are linked to natural and anthropogenic drivers of change over large spatial scales. In particular, studies lack how changing lake levels, storm patterns, and human disturbance alter bluff retreat rates. A study from the 1970s compared bluff crest locations along the Lake Michigan coast to those from a General Land Office Survey from the mid-1800s. In our study, transects from the 1970s study were resurveyed to update bluff retreat rates, determine if rates have changed over time, and discern factors controlling bluff retreat on various timescales. High risk erosion areas defined in the 1970s study were re-surveyed using RTK-GPS. The location of the bluff crest will be measured and compared to the 1970s data to document the rate and magnitude of bluff retreat at each site. Aerial photography from each decade since the 1970s and recent airborne LIDAR data will be utilized to further document how the rate and magnitude of bluff erosion has changed over time. As multiple periods of high and low lake levels have occurred since the 1970s, the role of water level fluctuations in facilitating bluff retreat will be examined. The role of storms and human disturbance (i.e. shoreline modification) on bluff retreat will also be evaluated. The ultimate goals of this study are to generate data and knowledge about rates, processes, and patterns of bluff retreat at high-risk erosion sites that can assist coastal communities with proactive land management.

Preliminary data, collected in November of 2019 at a bluff in Manistee, Michigan, suggests that bluff retreat is increasing in magnitude since the 1970s study. Between 1837 and 1977 the bluff crest at this site had retreated 76 meters with an average annual retreat rate of 0.6 meters per year. When resurveyed in 2019, the bluff crest had retreated an additional 30 meters with an average annual retreat of 0.73 meters per year between 1977 and 2019. These results suggest that the rate of bluff retreat has increased in recent decades, however, more research must be completed to identify when the rate increased, how the rate varies during different lake level phases, and whether this pattern holds up along the rest of the eastern Lake Michigan coast.