Climate variability and lake-level response in the Upper Laurentian Great Lakes, [approximately]5000 years ago to present

Climate variability drives water level fluctuations in the Laurentian Great Lakes at seasonal, interannual, decadal, and century timescales. Seasonal variability in overlake precipitation, runoff, and overlake evaporation is evaluated during the 20th century and related to formation of anomalous lake levels in Lake Michigan - Huron. Changes in the seasonal water supply to Lake Michigan - Huron include a shift to positive anomalies in autumn overlake precipitation after 1980 and winter runoff post 1965, with a concomitant decrease in spring runoff. Precipitation, air temperature, and stream discharge data from 27 subbasins in the Lake Michigan basin show trends toward warmer and wetter conditions since 1950, favoring earlier melting of fallen snow and winter rain versus snow events. These observations are consistent with patterns of northern hemispheric warming observed during the 20 th century and provide insight in to hydrologic changes during winter in the 21st century resulting from projected climate change. Records of fluctuations in Great Lakes water levels since regression from the Nipissing II phase ∼4,500 years ago can be reconstructed from the elevations of foreshore deposits preserved in individual beach ridges in strandplains adjacent to the Great Lakes, but depend upon accurate and precise dating of strandplain development. Radiocarbon ages for basal organics from swale wetlands are compared against optically stimulated luminescence (OSL) ages on quartz separates from eolian and littoral deposits in beach ridges from 4 strandplains adjacent to Lake Superior and a strandplain adjacent to Lake Michigan. Radiocarbon ages show scattering of 10's to 100's of years for organics from the same or nearby swales and clustering of 14C ages within portions of strandplains. OSL appears to be a credible method for dating Great Lakes strandplains and in many cases is a preferred alternative to 14C methods given a general lack of peat accumulation or preservation in swales at some sites. OSL chronologies at each site consistently identify a decrease in ridge formation rate ∼1400 cal. yr B.P., likely associated with shifts in the rate and magnitude of water-level change and sediment supply induced by the separation of Lake Superior from lakes Huron and Michigan.