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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 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 email: patricia.eckel@mobot.org 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: http://www.mobot.org/plantscience/resbot/Niag/LakeLevels/StClairRiver.htm 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 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. |
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