While snorkeling with family in Lake Huron near Parry Sound, Tim Fisher came upon a curious find: a tree stump firmly rooted in bedrock under about 5 feet of water.
The University of Toledo geology professor knew the serendipitous discovery meant that at some point in history, the water level of Lake Huron, and in turn Lake Erie because it is downstream, had to have been at least that low for the tree to grow. In fact, his subsequent research determined that the lake levels at the time — about three and a half centuries ago — were considerably lower than any time in recorded history.
Before he left the lake that day, Fisher chipped a small sliver of the 1-foot-tall stump for radiocarbon dating, and many years later he returned to the lake to find the stump again, saw a small disk from it and conduct further research with funding from Ohio Sea Grant.
“It’s like a bowling ball on a memory-foam mattress. When you remove the bowling ball, the mattress slowly rises up. The Earth’s crust is still coming up.”
Tim Fisher, University of Toledo geology professor
Through radiocarbon dating and analysis of the tree rings, he calculated that the tree started growing approximately 350 years ago after a major drought and died, presumably by drowning as the water level rose again, about four decades later.
Next, Fisher wanted to determine what the tree stump could tell him about lake levels at that time.
“I’ve been looking at changing lake levels across the Great Lakes for the last 20 years, so I’m fairly familiar with what’s going on,” Fisher said. “Part of the reason for lower lake levels is that the land is rising because the Laurentide Ice Sheet during the last ice age depressed the Earth’s crust, and as it melted away, the crust rebounded. It’s like a bowling ball on a memory-foam mattress. When you remove the bowling ball, the mattress slowly rises up. The Earth’s crust is still coming up.”
That factor, along with analysis of historic lake level records and climate records, helped him calculate the low lake level at the time the tree trunk was rooted and then died.
“While the results are tentative because additional stumps were not located, it appears that in the last half of the 17th century, 1660-1700, Lake Huron, and presumably Lake Erie, experienced very low water levels that were about 6.5 feet below the previously recorded lowest level,” Fisher said. “This low level lasted for at least 42 years, which is the number of tree rings in the stump.”
The previously recorded lowest monthly average lake level — 576.02 feet — was in January 2013, according to the Indiana Department of Natural Resources.
Fisher aims to continue the research by sending divers to locate additional stumps for further dating and water level analysis. In addition, the oxygen and carbon isotopes in the tree rings’ cellulose could give indications about climate data during that time period.
Verification of such an exceptionally low lake stand will provide guidance for the Great Lakes-St. Lawrence Seaway infrastructure planning. It also raises a consequential question: How low will the Great Lakes go in the future, and how can we mitigate the expected devastating economic impact?
Extreme high or low lake levels within any particular Great Lake basin have significant societal and economic impact, Fisher noted, for coastal communities, societal infrastructure, recreation users and residences.
Such an extreme drop in lake level would be catastrophic for marinas, shipping and navigation throughout the seaway, particularly considering future climate projections for increased frequency and intensity of extreme lake levels.
Marine cargo and vessel activity in the Great Lakes-St. Lawrence Seaway system generated $35 billion in economic activity in the U.S. and Canada in 2017, according to a Martin Associates report.
The Connecting Channels between the Great Lakes are maintained at depths of 30 feet for the ocean-going ships which carry ore, coal and other cargos between domestic and international ports, according to the U.S. Army Corps of Engineers. A Great Lakes Shipping Study issued by the U.S. Department of Homeland Security reports that a loss of 1 inch of water for a 1,000-foot vessel reduces transport capacity by 270 tons and increases transport costs by 8% to 9% per ton.
“Imagine what the economic loss would be,” Fisher said, “for a water level drop many feet lower.”