Watershed, catchment, and drainage basin are all the same thing: an area of land that drains to a common point. Small watersheds drain into larger watersheds in a hierarchical pattern. For example, the Mississippi River Watershed is comprised of hundreds of other smaller sub-watersheds.
The City of Des Moines is located at the bottom of two large catchments: the Raccoon and the Upper Des Moines River watersheds. These are two of the 42 largest sub-watersheds in the Mississippi River basin. Water draining from more than 5 million acres, from as far away as Jackson, MN, flows through the city. Des Moines Water Works treats water from both rivers to provide Des Moines Area residents with drinking water.
Knowledge of Iowa’s geological history and current land uses are necessary to understand each river and its water quality. Both the Raccoon and Des Moines Rivers flow through and drain the Des Moines Lobe Landform. It was here that a glacier covered the landscape only 12,000 years ago. The weather warmed and the disintegrating ice sheet formed the prairie pothole region that was once home to uncounted millions of waterfowl. After the glacier melted, the landscape was colonized by tallgrass prairie and wetland plants that helped form the region’s tremendously rich soil. The rivers ran clear and cold as rain water was naturally filtered through the perennial vegetation.
The early European settlers found the region nearly uninhabitable due to the swampy landscape and swarms of mosquitoes. A few decades after arrival, they realized that the soils formed under such conditions were ideal for the cultivation of corn. But first, large scale drainage was necessary. Networks of porous drainage pipes (now known as tile) were buried to lower the water table and dry out the soil. These were connected to constructed drainage ditches, essentially forming hundreds of new streams where none previously existed. This enormous project was mostly complete by 1920, although farmers continue to connect into and improve the system in the present day.
Constructed drainage and replacement of perennial vegetation with annual crops turned Iowa into the world’s richest agricultural region. But altering the natural hydrological system also had an impact on water quality in Iowa’s streams, this is will be discussed in the next piece in this series.