MN DNR Minerals Education Workshop 2005
Field Trip guide by Dr. Dean Moosavi and Rick Ruhanen
|On the Courtland Boat Landing in the Minnesota river we have an opportunity to examine sediment deposits from rejuvinated tributaries and the effect on the valley carved by the glacial River Warren. Here we have a mature winding river with no exposed bedrock. the silty sediments are accumulations of the past 10,000 years since the retreat of the last glacier. In the photos at the right and below we see deposits left by a small creek as it slows when it meets the Minnesota River. The larger amounts of soil when the water is moving faster with the higher gradient are deposited into the slower moving river. This deposit forms a natural dam that causes upstream pools of deeper and wider water. Simlar dams caused the formation of Lake Pepin on the Mississippi. In the case of Lake Pepin it is the Chippewa River from Wisconsin. The deposit of sediment at the mouth of the Chippewa River causes the Mississippi River above this tributary to be wider froming the "lake".
A similar situation in a smaller scale happens north (upstream) from here on the Minnesota River. Interesting to me is that the name of the river causing the dam is also the Chippewa, this time at a place called Churchhill for the early mission built on the northeast bank of the river. This natural dam has now been largely replaced by one constructed by humans to help control flooding on the Minnesota River but the "lake" caused by the widening of the river still is present. Called Lac-qui-parle which means "Lake that talks", the lake sounds like it is talking in the spring and fall during the Canada Goose migration when thousands of geese rest here while moving between their winter and summer habitats.
|At this site there is a large meander (bend) in the river. The swampy area at the right is the result of a meander cuttoff, creating what is called an oxbow lake (although very shallow here). This is evidence of a mature river environment where most of the erosion is side cutting. The river here marks the boundary between Nicollet and Blue Earth counties. This boundary has changed recently (in the last 40 years). A map made in 1964 shows the river in it's old location before the meander loop was cut off. this change in the river changed the boundary between the counties and presents a good arguement of why old rivers should not be used as political or especially private property boundaries. The location of the river changes, often enough to be easily recoginzed during one lifetime.|