Iowa's Western Boundary

When Iowa became a state in 1846, most of the western boundary between Iowa and Nebraska was defined as "the middle of the main channel of the Missouri River." North of Sioux City, Iowa’s border ends at the Big Sioux River, which flows between Iowa and South Dakota.

Locating "the middle of the main channel" of the Missouri River has been a big problem. This is because the "Mighty Mo," as some people call it, has changed its course many times.

The Ever-Changing "Mighty Mo"

The Missouri River was once made of many small streams woven into and out of the main channel, much like braided hair. When spring arrived, ice would block some parts of the river. Unfrozen free streams poured water on top of this ice. Because the river banks could not hold all the extra water, it spilled over the banks, flooding towns and farms. Other times chunks of ice blocked the channel and forced the river to cut a new channel.

Sometimes, huge pieces of land were cut off by sudden changes in direction of the powerful river current. Only the river moved, of course. The land stayed put. Some of these pieces of land became islands. If the river moved a great distance, land near the river could become part of the opposite state. This is what happened to the land on which the small Iowa town of Carter Lake is located.

An Iowa Town in Nebraska?

Carter Lake, a town of nearly 3,500 people, is very unusual— it is the only Iowa town which sits entirely on the Nebraska side of the Missouri River. There is no way to get to Carter Lake, Iowa, without first going into Nebraska! This was not always true. Carter Lake had been on the east side of the Missouri, just like Council Bluffs is today. It was clearly inside the Iowa boundary. In the late 1800s, there was a quick change in the course of the river. The Missouri River channel shifted 12 miles eastward. This left Carter Lake on the west side of the river—the Nebraska side. Both Iowa and Nebraska claimed this land.

In 1892 the United States Supreme Court ruled that the area was still part of Iowa. Even so, there has been much debate over which state should govern Carter Lake. As late as 1979 Carter Lake residents had a Nebraska zip code. They also picked up their mail at a nearby Omaha post office, and they were not even listed in Iowa phone books!

"Mighty Mo" Unchanging at Last

Around 1935 projects were begun to straighten and to stabilize the wandering river. The United States Army Corps of Engineers constructed dams, dikes and levees on the river. These structures would prevent flooding and help keep the river channel from changing. The "middle of the main channel of the Missouri River" should finally stay the same and be easy to find.


  • Jeffrey Madsen, “The Western Boundary,” The Goldfinch 4, no. 3 (February 1983): 6-7.