By Jeff Kart
February 10, 2011
The Earth is moving underneath our feet in the Great Lakes region. It’s just difficult to notice.
The lakes do notice, however, and GIA, or Glacial Isostatic Adjustment, has changed the region’s apparent water levels over time. The levels are still changing, and that’s one consideration in drafting a new regulation plan as part of the International Upper Great Lakes Study, or IUGLS.
“With time, the water is gradually getting deeper on the U.S. side of Lake Superior, and shallower on the Canadian side for the same lake level,” explains Chuck Southam, a water resources engineer participating in the IUGLS.
GIA is a slow geologic process whereby the Earth’s crust and underlying mantle recovers from its deformation caused by the weight of the glaciers during the last glacial era. The Great Lakes were formed more than 10,000 years ago when the last glaciers retreated. The phenomenon of GIA has been known since the mid-1800s, but its impact on water levels is not as well understood.
Two types of effects on water levels have been identified in the Great Lakes from GIA, also known as post-glacial rebound.
The apparent effect refers to a change in the land-to-water relationship around each of the lakes due to the tilting of the Earth’s crust. At the shoreline, water levels can appear to be falling or rising compared to local frames of reference like piers and large rocks, even though the absolute elevation of the water level relative to sea level may not have changed.
The physical effect refers to the impact of the crustal movement on the absolute water levels of the lakes. That is, their actual surface elevation at any given time relative to mean sea level.
A St. Clair River Report done as part of the IUGLS has identified that the apparent effect of GIA accounted for 4-5 centimeters (up to 2 inches) of a 23 centimeter (9 inch) decline in the head difference between Lakes Michigan-Huron and Erie between 1963 and 2006.
This apparent impact exists because the 23 centimeter (9 inch) decline was determined based on the difference between water levels recorded at Harbor Beach on Lake Huron and Cleveland on Lake Erie, instead of gauges located at each lake’s outlet.
Because of limitations about the impact of the GIA on the absolute water levels of each lake, the report was unable to determine the physical effect of GIA on the head difference between Lake Michigan-Huron and Lake Erie over time.
During the last ice age, regions beneath glaciers were depressed and part of the underlying mantle was squeezed out into regions on the perimeter, causing them to bulge upward. When the glaciers melted, the crust underneath began to rebound, while the perimeter began to subside.
The extent of the change can be directly measured by the Global Positioning System (GPS), which can detect motions as small as 1 millimeter (0.04 inches) per year, according to a study conducted by Dr. Michael Craymer, with Natural Resources Canada (NRCan), and U.S. researchers at Northwestern University, University of Miami and Louisiana State University.
Craymer and his team at NRCan, in collaboration with the National Oceanic and Atmospheric Administration in the U.S., have been continuously monitoring and improving their estimates of the effects of GIA by installing permanent GPS monitoring sites around the Great Lakes.
As a partial result of GIA, locations like Parry Sound, Ontario on the northeastern side of Lake Huron are rising about 24 centimeters (9.4 inches) every 100 years, relative to Lake Huron’s outlet near Sarnia-Port Huron.
“A century seems like a long time,” Southam said, “but someone living on Parry Sound since 1960 would have seen a fall in local water levels in addition to the lake’s ups and downs as a whole. That’s because the Parry Sound area has risen by about 12 centimeters (4.7 inches) relative to the lake’s outlet and its water surface since then.”
In contrast, water levels in Holland, Michigan, on the southeastern shore of Lake Michigan, appear to be rising gradually over time because the land there falls by about 8 centimeters (3.1 inches) per century relative to the Sarnia-Port Huron outlet.
For more information on GIA, see Chapter 6 of the St. Clair River Report.
The lakes do notice, however, and GIA, or Glacial Isostatic Adjustment, has changed the region’s apparent water levels over time. The levels are still changing, and that’s one consideration in drafting a new regulation plan as part of the International Upper Great Lakes Study, or IUGLS.
“With time, the water is gradually getting deeper on the U.S. side of Lake Superior, and shallower on the Canadian side for the same lake level,” explains Chuck Southam, a water resources engineer participating in the IUGLS.
GIA is a slow geologic process whereby the Earth’s crust and underlying mantle recovers from its deformation caused by the weight of the glaciers during the last glacial era. The Great Lakes were formed more than 10,000 years ago when the last glaciers retreated. The phenomenon of GIA has been known since the mid-1800s, but its impact on water levels is not as well understood.
Two types of effects on water levels have been identified in the Great Lakes from GIA, also known as post-glacial rebound.
The apparent effect refers to a change in the land-to-water relationship around each of the lakes due to the tilting of the Earth’s crust. At the shoreline, water levels can appear to be falling or rising compared to local frames of reference like piers and large rocks, even though the absolute elevation of the water level relative to sea level may not have changed.
The physical effect refers to the impact of the crustal movement on the absolute water levels of the lakes. That is, their actual surface elevation at any given time relative to mean sea level.
A St. Clair River Report done as part of the IUGLS has identified that the apparent effect of GIA accounted for 4-5 centimeters (up to 2 inches) of a 23 centimeter (9 inch) decline in the head difference between Lakes Michigan-Huron and Erie between 1963 and 2006.
This apparent impact exists because the 23 centimeter (9 inch) decline was determined based on the difference between water levels recorded at Harbor Beach on Lake Huron and Cleveland on Lake Erie, instead of gauges located at each lake’s outlet.
Because of limitations about the impact of the GIA on the absolute water levels of each lake, the report was unable to determine the physical effect of GIA on the head difference between Lake Michigan-Huron and Lake Erie over time.
During the last ice age, regions beneath glaciers were depressed and part of the underlying mantle was squeezed out into regions on the perimeter, causing them to bulge upward. When the glaciers melted, the crust underneath began to rebound, while the perimeter began to subside.
The extent of the change can be directly measured by the Global Positioning System (GPS), which can detect motions as small as 1 millimeter (0.04 inches) per year, according to a study conducted by Dr. Michael Craymer, with Natural Resources Canada (NRCan), and U.S. researchers at Northwestern University, University of Miami and Louisiana State University.
Craymer and his team at NRCan, in collaboration with the National Oceanic and Atmospheric Administration in the U.S., have been continuously monitoring and improving their estimates of the effects of GIA by installing permanent GPS monitoring sites around the Great Lakes.
As a partial result of GIA, locations like Parry Sound, Ontario on the northeastern side of Lake Huron are rising about 24 centimeters (9.4 inches) every 100 years, relative to Lake Huron’s outlet near Sarnia-Port Huron.
“A century seems like a long time,” Southam said, “but someone living on Parry Sound since 1960 would have seen a fall in local water levels in addition to the lake’s ups and downs as a whole. That’s because the Parry Sound area has risen by about 12 centimeters (4.7 inches) relative to the lake’s outlet and its water surface since then.”
In contrast, water levels in Holland, Michigan, on the southeastern shore of Lake Michigan, appear to be rising gradually over time because the land there falls by about 8 centimeters (3.1 inches) per century relative to the Sarnia-Port Huron outlet.
For more information on GIA, see Chapter 6 of the St. Clair River Report.