Watershed Overview

Earth River Watershed. Initially this lake drained to the east via the present-day Cannon River valley. When the front of the glacier receded to the area around Mankato, a new, lower drainage pathway opened via the Minnesota River valley. Drainage of Glacial Lake Minnesota via this route initiated erosion of the present day Minnesota River valley from Mankato to its confluence with the Mississippi.

As ice continued to recede, meltwater streams fed this new drainage and began to etch the course of the Minnesota River valley above Mankato. When glacial ice receded into North Dakota and northern Minnesota, a much larger glacial lake system developed. Now known to geologists as Glacial Lake Agassiz, it grew to twice the size of all five North American Great Lakes (Superior, Huron, Michigan, Erie, and Ontario) combined. This massive lake drained via the Minnesota River valley in at least two distinct phases—from 13,500 to 12,800 years ago, and 11,500 to 10,600 years ago—with peak flow events capable of filling the entire width of the present-day valley. This meltwater-enhanced version of the Minnesota River is known to geologists today as Glacial River Warren, named after G.K. Warren, a 19th century army general and surveyor who first noted the unusually small size of the Minnesota River relative to its valley.

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Le Sueur River Paleochannel

 

Following the last of the major River Warren floods, the Minnesota River valley in the Mankato area began to take its present form—with one major exception. Today the Blue Earth River is the major tributary flowing into the Minnesota River at Land of Memories Park. Two miles above its mouth, the Blue Earth River receives flow from its largest tributary, the Le Sueur River, just west of the intersection of highway 66 and county road 90.

 

The nearly six miles of abandoned Le Sueur valley created by this event looks much the same as it did when river water last flowed through it 11,000 years ago. Geologists from Minnesota State University, Mankato were recently able to determine the age of the capture event by testing sediments deposited by the ancient Le Sueur River using a technique called Optically Stimulated Luminescence (OSL), which measures the age of the last exposure of quartz grains to direct sunlight. Although the capture event occurred relatively early on in the Le Sueur’s history, it gives scientists an opportunity to study how natural erosion rates have changed through time in the watershed, which can inform our understanding of erosion and water quality issues today. The paleochannel (ancient river valley) is also a unique part of our regional geologic heritage, as clear, relatively recent cases of stream capture of this scale are relatively rare.

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The paleochannel meets the Red Jacket Trail and highway 66 at the west edge of Rasmussen Park, and follows them to Indian Lake Road, where it continues south. The paleochannel then briefly follows highway 90 before turning to the west at Copper Mountain Drive, and meeting the present day Le Sueur. The dramatic change in course of the Le Sueur River occurred following a stream capture event. About 11,000 years ago, the Blue Earth River captured the flow of the neighboring river, offering the Le Sueur a shorter, steeper path to the Minnesota River.

 

If you were to visit the Mankato area before about 11,000 years ago, you would have encountered a very different configuration of rivers. Instead of flowing into the Blue Earth River, the Le Sueur River flowed independently into the Minnesota River, near the intersection of Stoltzman Road and Riverfront Drive. Stoltzman Road follows the ancient Le Sueur valley (or paleochannel) to the entrance of Rasmussen Woods Park, where a narrow valley was carved through resistant bedrock. A small accumulation of river-rounded gravels in the soils above the ‘Cliffs’ area of the park is further evidence that a large river once flowed through the park.

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The exact cause of this capture may never be known, but it was likely triggered by relatively rapid erosion rates in the lower Blue Earth River valley, which caused either a ravine or cutbank to erode into and capture flow of the Le Sueur River.