Some Thoughts on Isostatic Rebound as a Hypothetical Factor

 in Lake and Strait Characteristics in the Great Lakes

P. M. Eckel

Missouri Botanical Garden

Res Botanica

May 4, 2005

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Some Thoughts on Isostatic Rebound as a Hypothetical Factor

 in Lake and Strait Characteristics in the Great Lakes


P. M. Eckel

Missouri Botanical Garden

P.O. Box 299

St. Louis, MO 63166-0299



My essay: "Some Potential Impacts of Conditions in the Saint Clair River on NYPA Relicensing, Lake Erie, and the Niagara River" has been recently posted on line:


In this essay, I mention glacial rebound, isostatic rebound in the Great Lakes region, as a possible explanation for some of the effects of dredging in the Saint Clair River on upper lake levels. This idea was intriguing enough for me to consider additional thoughts on the rebound issue and how it might play itself out elsewhere in the Great Lakes system, such as downstream of Lake St. Claire.


There are at least two paradigms to bear in mind when viewing water level issues, one is that atmospheric weather patterns are changing, average annual temperatures are rising and that generally things are becoming more arid. Lowered lake levels, according to this scenario, may be attributed to reduced moisture entering the watershed.


Another scenario to account for lowered lake levels is the isostatic rebound paradigm, here used to explain to some extent the increase in erosion in the Saint Clair River. If I am not mistaken, the model is that in the Laurentian region of Ontario/Labrador where the nucleus of the great ice sheet of the last "ice age" originated, around Hudson Bay, the land affected by the ice sheet development, was depressed under the weight of this mass of ice. Softer, hotter, molten rock upon which the upper geologic strata "float", yielded to this weight. With the final melting of this ice sheet, slowly the brittle, cold upper layers are rising in elevation relative to sea level.


The effect on the hydrology of the Great Lakes Watershed is to increase the stream gradient on north to south trending flow patterns through streambeds, such as the Saint Clair and the Detroit rivers. Bodies of water would have their lakebeds tilted, the north shores rising relative to the south shores in these basins: Lake Superior would tip its water mass toward the southern shore, presumably putting hydrostatic pressure on the strait [see note at end of essay] of Saint Mary, or the Saint Mary's River with possible augmented stream flow through that river through the Sault Saint Marie.


In Lakes Michigan and Huron, both basins elongated in a north-south direction, the water mass would press upon the southern parts of the basin: shorelines would tend to become drowned at their southern ends, more exposed at their northern, hence there should be a decrease in hydrostatic pressure through the Straits of Mackinac. This hydrostatic pressure would stimulate erosion regimes through increased gradient as well, in the straits between Lake Huron and Lake Erie.


The south shore of Lake Erie and Ontario would become drowned relative to the more exposed north shore of both east-west trending lake basins.


The strait between Lake Erie and Lake Ontario is the Niagara River. The curious thing is that this strait, also oriented on a north-south axis like the Saint Clair and Detroit rivers upstream, has its flow character reversed relative to those of the Lake Huron and Erie corridor: the current flows north.


If isostatic rebound accounts for increased hydrostatic pressure by shifting of the water mass in Lake Huron (whose levels may also be determined by increased flow through the Saint Mary's strait from the rise of the north shore of Lake Superior) and from increased stream gradient, then the reverse must be true in the Niagara River.


Stream gradient must be reduced in the Niagara River because its northern end at Fort Niagara, N.Y. and Niagara-on-the-Lake, Ont., has risen relative to its foot at Buffalo, N.Y. and Fort Erie, Ont. Volume in the Niagara River might also be decreased according to the isostatic scenario, because hydrostatic pressure by the volume of water in Lake Erie has declined, due to the retreat of this mass to the southern shores of the Lake, where all the U.S. towns are located, like Cleveland, Ohio.


Furthermore, due to the existence of Niagara's handsome cataracts, the backwater that should be swelling the volume of the water in the River north of the cataracts (called the Lower Niagara River) due to augmentation of the southern shore of Lake Ontario is essentially dammed up at the base of the Falls in the plunge pool area - an accumulation of water useless for scenery and for hydroelectric power generation.


Unfortunately, the cataracts themselves would be rising relative to their elevation at Buffalo-Fort Erie. That this process has a long history can be seen in the number of abandoned gorges or ravines that litter the north-facing Niagara Escarpment through New York State and Peninsular Ontario. These old "spillways" are the remnants of north-flowing surface streams that no longer function as streams, their headwaters probably now the source of wetlands south of their outlets, or the victims of stream capture.


The curious development of islands at the mouths of tributary streams flowing east-west into the north-south Niagara River axis may be due to isostatic rebound contributing to a decrease in gradient at the mouths of these streams, forcing the streams to be deflected at their mouths. Isostatic rebound could also be a factor in the characteristics of stream depth relative to the northern and southern shorelines and island boundaries in the Niagara River as they trend in an east-west departure from the north-south axis of most of the river, in the area between the head of Grand Island and the brink of the cataracts.


At Niagara, the water is so shallow at the brinks of the cataracts, especially in the central and north-central Grass Island pool area where the two channels (Chippawa and Tonawanda) rejoin after flowing around Grand Island, N.Y., that any increase in elevation in the northern area of the stream bed must contribute to a deterioration in the hydrostatic force necessary to ensure the kind of water levels most useful to the two hydroelectric power facilities that divert water from the head of Grand Island. The demands of flow for scenic purposes on which the casinos and their government partners rely for their tourist customers will eventually conflict with diversion demands just upstream from the cataracts.


Isostatic rebound happens throughout the Great Lakes Watershed region, contributing to enhanced elevations in two additional watersheds, that of the Mississippi River in the United States where improvements in stream gradient are probably beneficial, but also in the Red River watershed in Canada that channels water into Hudson's Bay where stream gradient would deteriorate.


Stream gradient would also deteriorate in the strait of Saint Lawrence, connecting Lake Ontario with the Atlantic Ocean, whose trend is from the southwest to the northeast.


Water should then be entrained in both Lakes Erie and Ontario, but since the gradient of the stream outlets of both lakes is being reduced by regional isostatic rebound. The water levels of these two lakes actually may be rising because of modifications in elevation of their present outlets.


The far future scenario using this paradigm is that eventually these watersheds will resume their use of the Mississippi spillway system and the area of the watershed for that river will be increased.


In conclusion, use of the isostatic rebound paradigm to explain matters for increased erosion and flow through the Saint Claire and Detroit rivers perhaps must also explain the great urgency to get more water into the Lake Erie and Lake Ontario lake basins because, as erosive forces build in the southern outlet of Lake Huron, the forces of sedimentation are increasing in the Niagara River, the outlet of Lake Erie. Dredging the Saint Clair would have the benefit of perhaps maintaining adequate water volume in the Niagara and Saint Lawrence rivers upstream to generate hydroelectric power (in addition to navigation issues). This scenario may reinforce the suggestion of a link between government activities in both sections of the Great Lakes.


With oil predicted or expected to rise to $100 per barrel in the fourth quarter of this year when the heating season of the northeastern United States and Canada begins, the value of hydroelectric power generated both by the Province of Ontario and the State of New York would also increase.


Currently, the benefits of financial "resettlement" during the relicensing process of the New York Power Authority are expected to benefit the American side of the Niagara River in U.S. communities most directly associated with the Niagara River.


However, a case might be said that the distribution of resettlement money, derived from users of Niagara's hydroelectric bounty, should perhaps be distributed to a broader region of effect to include upstream states and communities in both the United States and Canada, especially if an energy crisis is imminent with resumption of the cold or heating season in October of 2005.


NOTE: The Great Lakes together constitute a great river with a continuous shore- or coast-line with huge embayments (lakes) connected by narrow channels (straits). This huge river system flows eastward to the Atlantic Ocean, its final journey down the strait of the Saint Lawrence, ending in the gulf of the same name. The mixture of fresh water and salt water in this gulf should have a very interesting biology compared with either the Lakes or the Ocean themselves. A strait is a relatively narrow waterway connecting two larger bodies of water and the Great Lakes has quite a few, all locally called "rivers." A river, in contrast is "a stream of water bearing the waste of the land from higher to lower ground, and as a rule to the sea." (Dictionary of Geological Terms, according to the American Geological Institute). One can imagine the chemical and biological distinctions in the waters of the two systems, a strait and a river.


One of the permutations of the Niagara River (strait) is its division into two channels, the Chippewa Channel which occupies the western branch of the river as it divides and flows around Grand Island (N.Y,) and is wholly in the Dominion of Canada, and the Tonawanda Channel, the east branch that flows around the other side of  Grand Island, and is wholly in the territorial United States. What is seldom understood is that the majority of the water volume in the Niagara River is contained in the Canadian part of the River and the sovereign boundary is not in the middle of the Chippewa channel, as one might expect, but almost up onto the beach on Grand Island (NY), maximizing Canada's claim to the hydroelectric potential at the cataracts just downstream. The reason the Canadian Falls is called Canadian, and the American the American Falls is due to the fact that one cataract resides in Canadian territory, the other in the United States. The Canadian cataract (usually referred to as the Horseshoe Falls) has most of the water volume, that of the American Falls is so poorly hydrated that "control structures" must be built just upstream of Goat Island to divert water over the American Falls for visual reasons. If one scrutinizes topographic maps for Goat Island (USA), one can see the Canadian territorial boundary seems to actually cut across Goat Island at Terrapin Point, which famous promontory may actually reside in the Dominion of Canada.


These hydraulic follies arose out of the boundary settlements of the Treaty of Ghent signed in Europe after the ambiguous ending of the War of 1812 where the engineers of the British Army, who had designs on and had studied the hydraulic potential of the cataracts all during the colonial era,  perhaps had the last laugh as they withdraw behind the new frontier, taking the water with them. The U.S. Army Corps of Engineers still must endlessly dredge the Tonawanda Channel, whereas the Chippewa Channel seems to still exist in its aboriginal character.


A small lake could be said to be formed where the confluence of the Chippewa and Tonawanda Channels of the Niagara River meet just above the cataracts, where the river is again divided by an island (Goat Island, N.Y.). This small lake is called the Grass Island Pool. Grass Island, alas, like so many charming little islets that occurred on both shores above the Cataracts, such as Cedar Island and Hogg Island, was joined to the mainland as a result of water diversion that resulted in these islands having no water in their channels any more and, hence, no claim to island status. Presently, as more and more water is diverted, there are now more islands above the brinks in the stream channels than there have ever been in the history of the cataracts - so many that no one bothers to give them names.


The water level in the Grass Island Pool determines, by international treaty, how much water is diverted to the Canadian or the American hydroelectric plants downstream, and how much goes over the brinks to amuse tourists. The diversion structures of both countries are located on the shores of the Grass Island Pool.