A 40-YEAR LOW STAND IN GEORGIAN BAY, LAKE HURON DURING THE LATTER HALF OF THE 17TH CENTURY

A well-dated submerged white pine stump in Georgian Bay documents a late17th century low lake-stand. After correcting elevations for glacioisostatic adjustment, the water level reconstruction is for at least 1.9-m below the historic record low stand for Lake Huron that persisted for at least 40 years. An examination of recently drowned trees in the same area grown during a more recent low between 1999–2013 (Y2K) along the Lake Huron shores indicates only a few years lag time for colonization to take place on the exposed lakebed.

Durations of low water levels in the Great Lakes basin since 1860 CE are usually about a decade in length. This most recent Y2K low stand is the longest, thus the 17th century low stand was at least three-times longer in duration and furthermore it occurred during the Little Ice Age when conditions were generally drier. Lake level low stands result from changes in precipitation and evaporation and some combination of decreased precipitation at the time of the low may be explained by restricted southerly maritime air mass being replaced by continental western air masses. Evaporation is more difficult to reconstruct beyond the obvious higher wind speeds, because cooling can increase duration and extent of overwinter ice lowering the effect of evaporation during wintertime.

Preliminary tree-ring based reconstruction of Lake Huron water levels back to 1600 CE indicates an interval of about two decades of low stand before an abrupt rise about 1700 CE when the tree was inundated and killed by rising water levels. This tree-ring reconstruction is for the cold months (January–March) and is within the general Little Ice Age when the polar front would be south of its present margin enhancing the westerly, cold-season, dry, more continental air mass passing over the basin and perhaps precluding the intrusion of the more moisture-laden maritime air masses. The discovery and dating of the stump provides a snapshot of past lake levels that can serve to ground-truth tree-ring based lake level reconstruction efforts and potentially provide additional insights into the seasonality of the changes that drive lake levels as well as past forcings.