THE ROLE OF LAKE-LEVEL FLUCTUATIONS IN DEVELOPING GREAT LAKES COASTAL WETLANDS

Wetlands define the shoreline of many coastal areas of the Great Lakes; their immediate locations are often in geomorphic settings protected from the erosional forces of the lakes. In turn, lake-level fluctuations play an important role in development and maintenance of hydrogeomorphic settings. Drowned river-mouth wetlands occur along the channels of rivers and streams that once flowed downhill to the lakes but have since been back-flooded due to differential glacial isostatic adjustment with respect to the outlet. Their hydrology is largely affected by both lake and fluvial systems. Wetlands protected behind barrier beaches and sand spits and in ridge/swale complexes are dependent on sediment supply and lake-level fluctuations to build and maintain beach ridges. Individual ridges form during the final stages of lake-level highs. In some settings, new beach ridges may form during subsequent highs to create a strandplain, but in other settings, amalgamated ridges form a barrier beach that encloses a lagoonal wetland. If a channel connects the lagoon to the lake, hydrology is lake-driven; if there is no channel, groundwater plays a greater role, with the slope of the water table, and thus water depth, affected by lake level. Lake level controls hydrology in both wetland types. Lacustrine wetlands can occur in open embayments and along open shorelines with offshore sand bars or rock to dissipate wave energy. The sand bars are dependent on sediment supply and lake levels for transport; hydrology is directly tied to lake level. Protected embayments formed by sediment- and lake-level-dependent sand spits may also harbor wetlands. Development of wetlands in many of these Great Lakes settings originated during lake-level events dating to the mid-Holocene. Paleo lake-level studies on the upper lakes have identified quasi-periodic behavior, with post-Nipissing fluctuations of about 160 years and 33 years, as well as some longer-term changes. Ecological processes that maintain modern wetland habitats are highly dependent on the shorter fluctuations. Periodic high lake levels kill invading upland plants and canopy-dominating wetland emergent species; succeeding lows elicit a response from the seed bank. The upland and canopy-dominating plants eventually return, but the cycle is then repeated.