The Wyalusing River:
when the Wisconsin River flowed east.


Script - The Wyalusing River

Welcome to , where we explore the history, stories, geology, geography, people and communities of the Driftless region .

Today’s episode is about the Wyalusing river, when the Wisconsin river ran east.

The Wisconsin River is familiar to everyone in the state. It starts at Lac Vieux Desert on the Michigan border and flows into the Mississippi at Wyalusing. But that’s only a small part of the story.

Did the ancestor of the Mississippi River once take a turn at Wyalusing and run east up the Lower Wisconsin River valley to empty into the Great Lakes and then Atlantic Ocean via the St. Lawrence River? Such a claim requires extraordinary evidence.

Thanks to Eric Carson of the Wisconsin Geological and Natural History Survey and other geologists, that evidence has been assembled. They have published their findings and even made a PBS show about it. Let’s look at the science.

Some of the evidence is evident once you look at the landscape, even to non-geologists like me. Where the Wisconsin enters the Mississippi, it should angle downstream as it enters the Mississippi, but it doesn't. Instead the bluff at wyalusing curves upstream. No matter where the river runs today, the bluffs record where it flowed thousands of years ago.

Tributaries entering the Wisconsin do so from valleys that tend to the east, indicating that the river flowed east when the valleys were formed. This is known as a barbed tributary and is a sign that the river has changed in some major way. Again, the current waterways have adapted to the westerly flow, but the bluffs tell an older story.

As one drives down the river valley from Spring Green to Wyalusing, the valley slowly narrows and the bluffs that define it get higher and steeper. Near Spring Green the distance between the bluffs is 3-4 miles, far enough so its not always clear you’re in a valley. By Boscobel, the valley has narrowed to 2 miles. Just west of Wyalusing, its under 1 ½ miles. This is the opposite of how rivers usually work.

Military Ridge, a resistant formation topped with harder rock, runs across the southern dirftless, but stops at the Mississippi River. But the same formation starts on the other side of the Mississippi at Pike's Peak, Iowa. Did this once function as a continental divide, separating the waters flowing to the Gulf from those headed for the Atlantic?

Similarly, the distance between the bluffs on the Mississiipi testifies to the change. At the Marquette - prairie du Chien bridge, the valley is 2.4 miles wide. South of Wyalusing, it is a mile narrower. This indicates that the gap there is more recent and not subject to the same forces that carved the river north of there.

Some evidence isn’t self-evident, but the result of years of work by scientists, work that could have disproved the hypothesis, but instead verifies it.

As the river erodes into the bedrock, the sections of the former river beds are sometimes left stranded above the new river. These are called terraces and are clues to the past. The terraces are higher than the current river bed, but noticeably lower than the bluffs.

Geologists were able to identify three such terraces, all on the north side of the river. They are at bridgeport, Wauzeka and between Muscoda and Blue River. The bedrock they are on tilts to the east, indicating that the river once flowed that way. The difference is slight, but modern equipment can measure that precisely.

If the Wyalusing river existed, it would have carved a valley running to Green Bay and Lake Michigan, showing the course of the river. This valley would have later been buried under glacial debris and could only be detected by mapping the underlying bedrock. Sure enough, geologists discovered just such a valley. It deepens as it trends to the east, showing the direction of the flow. Since the debris is sometimes hundreds of feet deep, they used data from 60,000 wells to map the buried valley.

Finally, Records of sediment in the Gulf of Mexico indicate that the continental divide was south of Wisconsin at the time in question. The unique rocks of Wisconsin, Minnesota and North Dakota do not appear in sedimentary layers from that time. Instead they were carried into the Great Lakes and then out the St. Lawrence

Just how the river reversed is not fully understood, but this is the mechanism that the geologists propose: The Wyalusing was flowing east, but an advancing ice sheet blocked its flow, probably somewhere near the baraboo range. This led to the valley flooding, with the water rising up to the very tops of the bluffs. The water found a way, as water will always do, especially in limestone and karst geology. Once a small gap was established, the melt water from the ice sheet widened it and the river’s flow was reserved.

As we do here at, the link in the notes will take you to the complete research papers, maps and videos used and discussed. They go into much greater depth than I can in a short video. Thanks for watching.