ADSactly Education - Mississippi River Geology and Flow

The Mississippi River is by far, the mightiest river in America. The Mississippi River and it’s tributaries drain an area from the Rockies to the Allegheny Mountains, roughly 40% of the United States.

The river itself is entirely inside the US borders and runs 2320 mi (3730 km) from Lake Itasca in Minnesota to the Gulf of Mexico.

The Course of the Mississippi

The River is commonly divided into three parts. The upper, middle and lower. The upper River flows out of Lake Itasca in Minnesota at a very modest average flow of 6 Cubic Feet Per Second (CFS). It runs generally south and east through Minnesota to Minneapolis where a series of dams and locks make it a navigable waterway. The upper river continues on to St. Louis, Missouri where the Missouri joins it (and adds 45% of the combined flow). Total length of the upper river is 1157 miles, over half the total length.

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The Middle River runs 190 miles from the confluence of the Missouri to the confluence with the Ohio in Cairo, Illinois. The Ohio River adds right at 50% of the flow of the combined river. The middle stretch of the river is relatively flat and free flowing with only two other rivers joining it.

The Lower Mississippi runs 1000 miles from Cairo, Illinois to the Gulf of Mexico, but not all in one channel. The main channel is split in Louisiana and roughly 30% of the River’s flow is diverted to the Atchafalaya River channel while the rest follows the current natural river channel past Baton Rouge and New Orleans, Louisiana and on to the Gulf. Average flow for the Mississippi River at the mouth (s) is 593,000 CFS.

Geology

There has been a river flowing north to south in the center of North America for a very long time. The biggest impact on the water course and the flow of the rivers in the last 15,000 years was the last ice age.

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The river officially starts with Lake Itasca in north eastern Minnesota, an area that has an immense layer of glacially carried silt resulting in mostly flat plains. When the ice sheet receded the resulting water needed places to go. The flows were just tremendous which leaves the Mississippi and it’s tributaries with channels that are much larger than they need today. Another consequence of the ice retreating north is that many of the rivers that originally fed the Mississippi found their way north and empty into Hudson’s Bay.

The river has a total drop of 1475 feet (450m) from it’s headwaters to the Gulf of Mexico and more than half of that occurs in the upper river. Consequently the only real canyon of the Mississippi is on the upper river, but it’s pretty spectacular with near vertical walls. The reason for that is that the top stratum is silt laid down by the glaciers. Next is a relatively hard layer of rock with a layer of limestone and a layer of sandstone at the bottom. The sandstone is worn away by the flow of the water and it cracks and breaks straight up through the limestone and he hard cap.

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For it’s first 300 miles (480 km) the Mississippi is a shallow, rocky river. After it passes through the Twin Cities, Minnesota it runs through a series of Limestone Bluffs which were formed by runoff water at the end of the last ice age. It also meanders through an area of steep valleys and hills with the canyon carved by the river itself.

The middle section of the river is characterized by high water flows and not much elevation drop where the river tends to be flat and wide. There is a tremendous amount of silt carried from the Missouri and some of the other tributaries on the middle river so the Mississippi in this stretch is brown. The Platte River, a tributary of the Missouri was described by settlers as “Too thick to drink and too thin to plow.” That silt contributes to the status of the lower river and the Delta.

The Mississippi wanders the last 1000 miles (1600 km) through swamps and forests that were all built up from the silt. Many of the swamps are former river channels that are filling up and drying out in the geologic scale of time. In a way, it’s all part of the river because it all has been part of the river in the last 15,000 years.

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The lower river historically changes it’s course about every 500 years. Silt will pile up in one place and the river will simply find a way around the impoundment. The delta is a true ‘fan’ delta where the river often had no main channel, only a series of smaller outlets to the ocean. Today’s river is manipulated and maintained by man, so the channels are quite well designated. The delta continues to fan out and deposit huge quantities of silt at the merge with the ocean.

There is a confusing difference between the Mississippi Delta and the Delta of the Mississippi River. The Mississippi Delta is the area between the Mississippi River and the Yazoo River in the state of Mississippi. The Delta of the Mississippi River is the transition area between the freshwater flow of the Mississippi River and the saltwater of the Gulf of Mexico.

Almost the entire Delta of the Mississippi is in present day Louisiana, and the actual merge with the Ocean changes from year to year. Certainly the deepest part of the Mississippi is near New Orleans where the depth is right at 200 feet (67m). Most of the lower Mississippi is over 50 feet (17m) deep.

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The Mississippi literally cuts right through the heart of America. Historically, it was the western boundary of the United States and was the dividing line between what was considered French territory and Spanish. Today it is considered to be the dividing line between the Eastern and Western United States.

We will explore the cultural history of the Mississippi in the next part of this series.

Unsourced Photos are used courtesy of the author.

While the words and ideas in this post are strictly those of the author these sources were referred to by me to insure numerical and historical accuracy.
Wikepedia: Mississippi River
Mississippi Valley Traveler: Mississippi River Geology