About 12,000 years ago, the Blue Earth River and its tributaries
were already evolving. Glacial Lake Minnesota, created when the melting
glacier kept the water from flowing northeast through the pre-Warren
Jordan lowland, was long since gone (The scenic Cannon River valley was
carved by water flowing out of Lake Minnesota.). While it was there,
millions of tons of glacial debris settled to the bottom. When the
water left, a smooth, gradually sloped plain remained. With a fall of
less than 100 feet between the cities of Blue Earth and Mankato, the
average slope of the land was about two feet per mile and long-range
erosion was negligible. Barring cataclysmic disturbance, the plain
would slowly level out, with the help of periodic flooding, to a slope
of about one foot per mile. But Lake Agassiz was growing.
A mountain of ice the size of Manitoba is a lot of water. That's
what was melting into the Red River Valley; and just as it did in the
case of Lake Minnesota, the glacier was damming the flow, this time
stopping the water from going north to Hudson Bay. Eventually, Lake
Agassiz covered about 200,000 square miles and most of Manitoba. Seven
hundred feet at its deepest, Agassiz was as much as 300 feet deep over
the Red River plain. In comparison, Minnesota covers 84,068 square
miles, Minnesota River drains 16,770 square miles, and Blue Earth River
drains 3,550 square miles.
"During its early stages," states Geology of Minnesota: A Centennial Volume,
"Lake Agassiz had just one outlet, the Glacial River Warren, a high
volume stream that discharged southeastward along the axis of the
Minnesota River lowland where it followed a course previously occupied
by a braided meltwater stream. The highly competent outlet stream
entrenched itself into the landscape and continued to deepen and widen
its valley as Lake Agassiz expanded."
When Lake Agassiz finally found more suitable drainage to the north
and northeast, River Warren was beheaded. By then, however, Warren had
dug a trench, through several layers of glacial till, to bedrock. The
Blue Earth watershed was doomed.
Erosion occurs when flowing water picks up soil and carries it
downstream. The faster the flow, the greater the potential silt load.
The catastrophic Introduction of a 200-ft. drop at the mouth of the
Blue Earth River dramatically increased the overall slope of the
watershed and guaranteed that virtually all soil in the basin will
eventually erode downstream. The drop, which started at the mouth, has
been extending itself upstream toward the headwaters of the Blue Earth
River and its tributaries for thousands of years. The erosion will
continue until that drop is eliminated and the plain is leveled down to
a one-foot-per-mile slope. Unfortunately, the channel created by River
Warren provides an efficient flushing system that negates this leveling
process.
During normal flows, sediments collect in and above the River Warren
channel. When local flood conditions arise, the affected subwatersheds
are quickly flushed of loose sediment, which is added to the collection
building up along the Minnesota River. Silt is always flowing out to
the Mississippi, but a lot gets left behind. When regional floods
occur, water rushes into River Warren from all directions. Minnesota
River banks are quickly overrun and the whole system gets flushed.
Millions of tons of sediment flow to the Mississippi during one of
these floods. Much of it comes from the Blue Earth. In these
conditions, the Warren trench will never be filled, the ground will
continue to flow, and Blue Earth farmers will someday have no place to
plant their crops.