"Glaciers Left Their Mark on the Mississippi River"
By Pat Middleton, Ruth Nissen, Wisconsin DNR and other Contributors.
Additional Links at the bottom of the page.
The Mississippi River, like all rivers, is in a constant state of change. Various forms
of the Mississippi River have flowed through our area for more than a million years, but
the Upper Mississippi River Valley as we know it was primarily shaped during the most recent glacial stage of the Great Ice Age, The Wisconsin period.
The Wisconsin glaciation period started about 75, 000 years ago and ended about 12,000
years ago when the North American climate began to warm. It was a world hardly
recognizable today. Minnesota and Wisconsin were populated by a variety of very large
animals called megafauna which lived in the cold climate on the margins of the glacial
ice. Because larger animals lost body heat more slowly, bigger was indeed better.
Giant-sized animals including mastodons, mammoths and caribou roamed
this area. Because of the north-south orientation of the river, many of our ancient fishes
had merely moved south as the waters grew too cold, and then expanded back into the
northern portion of the river as the glaciers retreated. Ancient fish that predate the
glaciers and still live in the waters of the Upper Mississippi River include long-nosed
and short-nosed gar, sturgeon and paddlefish.
The melting of those enormous ice sheets that, at their maximum, were 5,000 to 10,000
feet thick and covered hundreds of thousands of square miles, released tremendous amounts
of water, forming huge glacial lakes. The largest of the glacial lakes, Lake Agassiz,
covered northwest Minnesota, parts of North Dakota and the Canadian provinces of Manitoba,
Saskatchewan and Ontario. The southern discharge outlet to this lake was called Glacial
River Warren, which eventually excavated the valley now occupied by the Minnesota River.
(The Mississippi River flows into the valley carved by the River Warren at St. Anthony
Falls in Minneapolis.)
The St. Croix River which drained Lake Duluth, a glacial lake that covered the western
Lake Superior basin, joined the River Warren about 30 miles downstream from the present
confluence of the Mississippi and Minnesota Rivers. In Wisconsin, another glacial lake
drained first into what is now the Black River and later the Wisconsin River, both of
which emptied into the River Warren. During the 3,000 years that River Warren carried
water from these and other smaller glacial lakes, the Mississippi River valley was carved
bluff to bluff and the resulting valley was approximately 250 feet deeper than it is
However, there is also
evidence that the main valley is much older than the River Warren and underwent several
cycles of cutting and filling during the Great Ice Age. Even though the Coulee country
escaped the land-leveling effect of the ice mass, the glaciers left their mark by carving
the spectacular Mississippi River that cuts through the heart of this area with the
torrents of water draining from the melting ice mass.
With the demise of the glacial lakes, the tremendous flow of water was diminished and
the Mississippi River became the most important stream in the area. The many tributaries
continued to bring in sand and gravel (sediment) into the wide deep valley of the
Mississippi that the river could no longer carry away. Consequently, the sediment began to
fill in the recently scoured river valley. At each point where major tributaries joined
the Mississippi River, natural sediment dams eventually built up to pond the upstream
portion of the river, forming a natural river lake. The largest
is Lake Pepin which at one
time extended all the way up to St. Paul.
Read more about
the effects of the Ice Age on the Mississippi River and the people who live
along it today! The New Madrid Earthquake, the effects of flooding since the
construction of Locks and Dams, stats on the floods of 1993 and others... and
more... are included in DISCOVER! America's Great River Road, an indispensable
4-Volume guide to "life along the Mississippi River."
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