A couple of years ago in Wat on Earth (Fall, 1999) short-term variations in Great Lakes water levels, mainly resulting from meteorological effects, were described with passing reference to the effect of crustal tilting, which is a slow, long-term process.
Water-level gauges are located various places around the Great Lakes and show that the ground is tilting up to the northeast. For example, measurements at gauges around Lake Ontario show that Kingston is rising relative to Hamilton about 30 cm per century. We know this tilting has been going on for thousands of years because the shorelines of ancient lakes such as glacial Lake Iroquois, which existed in the Ontario basin over 12,000 years ago, has been warped upward to the northeast. The Iroquois beach is at an elevation of 110 m at Hamilton today (Lake Ontario is 75 m above sea level) but north of Kingston is more than 280 m above sea level. That is, what was a horizontal water surface 12000 years ago has been tilted up to the northeast by about 175 m.
If one continues northeast to Ottawa, evidence of even greater uplift can be found because there are old beaches there containing sea shells (the Champlain Sea) now at an elevation of 210 m. We know that at that time sea level was lower than present by about 60 m because of water diverted into the great glaciers of the northern hemisphere, so adding that on we can account for 270 m of uplift of the ground at Ottawa in the past 11000 years. This process of ground tilting is found wherever there were large ice sheets and we believe the weight of the glacier caused the Earths' crust to sink, then as the ice melted and the ice load lessened, the crust rose again in a process we call isostatic rebound.
In the case of the Lake Ontario basin, the outlet (St. Lawrence River) is near Kingston. With the present drainage and water level in the basin controlled by the sill at Kingston, if that area is rising at the fastest rate of anywhere around the lake, then the water level in the lake will rise on the land all around the lake. We can see this affecting landforms. For example, rivers flowing into Lake Ontario have their lower reaches drowned; a good example in the Niagara Peninsula is Jordan Harbour, but there are many others. Although this process is so slow in human terms we need precise instruments to measure it, this is another factor that affects Great Lakes water levels. The Lake Erie basin, with its outlet river (the Niagara) in the northeast end, is in a similar situation to the Ontario basin, and drowned river mouths can be found in its southwest end as well. This process is not limited to the immediate vicinity of the lakes but all the land between as well, in fact over most of Canada. Rapid rise of the land around Hudson Bay is causing the Bay to shrink over time as the east shore has risen more than 300 m in the last 8000 years.
This
tilting
affects
the
gradient
of
streams
too.
Those
that
flow
north
have
their
gradient
lessened
(creating
the
marshes
at
Lake
Simcoe
-
Holland
Marsh
-
and
at
Lake
Scugog),
and
those
that
flow
south
have
their
gradients
increased.
In contrast to lakes Ontario and Erie, Lake Huron has its outlet in its south end (the St. Clair River). In this case the effect on water levels is just the opposite, with the sill at Sarnia left behind by the shoreline to the north rising more rapidly, so the water level in the lake is dropping relative to the land all around the basin.
There's more to come - stay tuned!