The Great Lakes contain 20 per cent of the world’s freshwater supply and act as a major transportation route from the North American heartland to global markets.
However, low water levels over the past ten years have resulted in hundreds of kilometres of damage to coastal ecosystems and beaches as well as major losses for the billion-dollar shipping industry.
"These [Great Lakes] are some of the largest lakes in the world, a major freshwater resource and an important trading route for more than 30 million people in two nations,” says John Johnston, an assistant professor and sedimentologist in the Department of Earth and Environmental Sciences at the University of Waterloo.
Johnston is using the sediment record to reconstruct water levels in the Great Lakes to identify natural patterns and trends and help predict future possible scenarios. In the 1980s, for example, the region experienced record high water levels that caused flooding and extensive coastal damage.
His work has been part of a larger study by the US Geological Survey, but now specifically addresses gaps identified by the International Upper Great Lakes Study commissioned by the International Joint Commission to evaluate outdated water level regulation plans.
Even though the Great Lakes have one of the most extensive water level tracking systems in the world, we have data going back only 150 years. Water levels in the Great Lakes, however, rise and fall naturally on three different timescales: decadal, centennial and millennial.
To understand these long-term natural patterns, Johnston is reconstructing water levels going back five thousand years but with a higher multi-decadal resolution. This framework is helping scientists better understand modern water level records and evaluate the human influence on this complex natural system.
Johnston and his colleagues studied 800 cores from beach ridges at 15 key sites around the upper Great Lakes, mostly on the US side of lakes Superior, Michigan, and Huron. As the water level rises and falls, it creates a series of sandy ridges along the shoreline, similar to bathtub rings, which act as water level signatures. Johnston has migrated away from traditional carbon dating techniques to employ a new approach called optically simulated luminescence to determine the age of the shorelines.
The researchers triangulated their data around the lake basin to get the most accurate assessment of age and water level elevations to date. These data were then combined into a paleohydrograph – a graph of relative lake level elevations going back in time.
Preliminary results suggest the upper Great Lakes are currently experiencing a decadal low riding upon both a centennial high and a millennial low, meaning the recent lower lake levels are part of a complex natural pattern.
Still additional data is needed to accurately resolve water level oscillations near lake-basin outlets. Johnston hopes to expand his sampling work next year to key sites on the Canadian side of the upper Great Lakes.
Prof. Johnston’s work was recently featured in a special publication of the Lyell Collection by the Geological Society of London. His research was funded by the USGS Global Climate Change program.