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.