The Red River Valley
Tilted Shorelines and
Rebounding Lake beds
By: Don McCollor
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The exceptionally flat terrain of the Red River Valley is a legacy of
glacial Lake Agassiz waxed and waned with the advances and retreating of
lobes of the Laurentide Ice sheet of the last ice age. With massive ice
layers to the north, drainage was through the Minnesota River Valley as
the ice advanced and toward Lake Superior as the ice retreated. By 8500
years ago, the lake had drained away and dried up completely.
Besides the flat floor and deep rich sediment of the lake bed forming
the fertile farmland of the Valley, Lake Agassiz left other traces of its
existence. Lake Winnipeg, Red Lake, and Lake of the Woods are remnants of
the lake. Less impressive, but easily seen are several "beaches"
or shorelines which delimitate the edges of the Valley. These are low ridges
of sand and gravel representing "strandlines" when the level of
the lake was stable. In places they are crossed by deltas extending into
the lake plain marking where ancient rivers entered the lake. The two most
well-developed beaches are the Herman Beach, formed 11,700 years ago and
the Campell Beach, formed 11,200 years ago. Both can be traced for hundreds
of miles through Minnesota and North Dakota. In Grand Forks Country, ND,
the western beaches are in the Emerado area, and near Red Lake Falls in
There is a curious feature about these beaches. The beaches at the northern
end of the Valley near the Canadian border are higher than at the southern
end at the namesake Minnesota towns of Herman and Campbell. Not that the
beach features themselves are piled higher, but that they are higher above
sea level. The slope is very gentle, impossible to observe without accurate
surveying techniques. Even on a topographic map the change is not apparent
until the fine print giving the elevations of the contour lines is scrutinized.
Careful measurements show that the western Herman Beach is 180 feet (55
meters) higher at the Canadian border than at the southern end.
Clearly, something is amiss. The "level" in "water level"
implies just what it says-that all parts of a body of water are at the same
elevation. Of course there are variations around the average water level
due to tides, storms, and seiches. Tides are negligible
in landlocked water bodies smaller than oceans, and wave action due to storm
winds intermittent. To anyone who has experienced the strength and persistence
of the wind in the Valley, the image of the wind keeping the water pushed
to one end of Lake Agassiz is almost in reach of the imagination. However,
given the prevailing northwestern winds, the water and beach lines should
be higher in the southeastern corner. Indeed the eastern Agassiz beaches
show greater working indicating that the prevailing direction of severe
lake storms was from the northwest. No natural force
in our common experience explains the shorelines.
The answer to the tilted shorelines is a phenomenon called isostatic
rebound. Basically, the land sinks when a heavy weight, like a glacier or
lake water and sediments are place on it, and rises again when the weight
is removed. An analog would be a heavy box placed on a vinyl car seat. Removal
of the box after a few hours reveals a box-shaped depression in the seat.
After a few hours the seat "rebounds" and
returns to its original comfy contours. This rising and sinking of large
land areas is a common concept in geology, as in the uplifting of a mountain
range as erosion removes some of the weight. The concept of solid rock being
springy like a car seat is difficult to imagine, especially in a geologically
stable area such as the Valley. In places like California, where the landscape
occasionally (and suddenly) moves up, down, and sideways; it is easier to
grasp. Rock is stiffer, but it does deform and rebound, albeit over hundreds
and thousands of years rather than hours. In the Lake Agassiz plain, the
effect of the rebound is complicated by weight of water and lake sediments
which accumulated as the ice sheet retreated. And although both the northern
and southern ends of the beach line presumably raised, there is no reference
point to measure this by. The only measurement possible is the difference
between heights of the northern and southern beaches, which sets the minimum
value for the rebound of 180 feet (55 meters).
The difference in elevation of the beach lines (and an estimation of
how deformable the underlying rock is) has been used to estimate the minimum
thickness of glacial ice in the Valley. In the Grand Forks area, the ice
layer was probably between 920 feet (280 meters) to 3400 feet (1040 meters)
thick. As the ice melted from south to north, the weight of lake water replaced
some of the ice weight with an estimated 325 feet (100 meters) of water
depth, delaying some of the rebound until the lake eventually drained. However,
during this time up to 150 feet of sediment (46 meters) accumulated in the
lake bed. This remained permanently, weighing down the underlying rock and
preventing the completion of rebound from occurring. It is believed that
rebound has been completed in the valley as far north as Lake Winnipeg,
with the area further north still rebounding.
The phenomena of rebound with a general uplift of the terrain is of modern
interest, especially since it is still occurring in the far north. The Red
River of the North is a gently meandering river with a very low gradient
(drop) of about 0.5 foot per mile (0.1 meter/kilometer) between Grand Forks
and Pembina. The current gradient of the Red is now half what it would have
been had rebound not occurred. (Of course without the glacier ice, Lake
Agassiz and the current Red River water course would not have formed either.)The
continued rebound in the Canadian north where the Red River water eventually
reaches Hudson's Bay may have interesting future consequences. A decrease
in gradient may make the Red (even) more prone to flooding. Or will Lake
Agassiz return? Or perhaps large-scale canal building be needed in a few
centuries to maintain the present drainage.
Ojakangas, Richard W., and Matsch, Charles L., Minnesota's Geology,
University of Minnesota Press, Minneapolis, 1982.
Brevik, Eric C., Isostatic Rebound in the Lake Agassiz Basin Since the Late
Wisconsinan, Masters Thesis, University of North Dakota, 1994.
A seiche (pronounced SAY-sh) is a periodic oscillation of a ocean, lake,
or other body of water apparently caused by storms or changes in atmospheric
pressure. The phenomenon is like the rhythmic rocking of water in a bucket
when it is carried. In appearance, the seiche resembles a tide rather than
a wave, with a change in water level of inches to several feet over a period
of minutes to hours. Unlike a tide, it occurs unpredictably. A seiche can
be particularly treacherous, snatching up shore fishermen and pier strollers,
since the weather effect which causes it may be well out of sight, and the
general rise of the whole water surface not immediately noticed.
Galley, Daniel V., U-505, Paperback Library, 3rd ed., New York, 1971,
Storms on Lake Agassiz must have been awesome, being larger than the
five present Great Lakes combined and with a fetch across hundreds of miles
of open, shallow water for waves to build. Fresh water, lacking the salt
content of ocean water is less dense, producing livelier, quicker waves.
Waves on the Great Lakes are steeper and sharper than their rolling ocean
cousins, leaping and tumbling. Waves thirty feet (9 meters) from crest to
trough have been reported on Lake Superior, driven by seventy-knot winds
(81 mph, 36 m/sec). A bleakly comforting sight during an ordinary Lake storm
are the "Christmas trees"-steep triangular waves silhouetted against
the horizon, appearing like a forest of pine trees. In a severe storm, the
"Christmas trees" disappear as the wave tops are torn off by hurricane-force
Ships Gone Missing, Hemming, Robert J., Contemporary Books, Chicago,
Great Lakes Shipwrecks and Survivals, Ratigan, William, WM. B. Eerdmans
Publishing Co., Grand Rapids, Michigan, 1977.
The sinking and rebound of the earth is a factor to consider when large-scale
engineering projects concentrate large masses, such as dams and associated
reservoirs or removed large quantities of material in the course of open-pit
mining and canal construction. During the building of the Panama canal,
Culebra Cut, besides being noted for landslides (the most notorious being
Cucaracha [the cockroach] Slide, named for its creeping habits) also had
the floor of the Cut heave upward as over 100,000,000 cubic years of overburden
to a depth of 260 feet was removed. The unstable rock of the Cut and the
weight of the sides contrived to heave the bottom up twenty feet (6 meters)
in several places, and thirty(9 meters) in one area. One section of the
cut, complete with stream shovel sitting atop it lifted nine feet (2.7 meters)
in one afternoon.
The Panama Canal, Hammond, Rolt and Lewin, C.j., Fredrick Muller Ltd.,
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