Yes, you read that correctly. I briefly touched upon this idea in a post about the Lower Wisconsin River and maybe you caught Eric Carson on PBS talking about it. This post is designed to be a summer "quick hitter" and there is more science and less angling in this post compared to most of my posts...
When you start looking at the evidence, it is one of those "no shit" moments that is surprising it took so long for us to see. It makes me think of something as seemingly simple as plate tectonics and how it took decades for Alfred Wegener's idea to be accepted, at least in North America. Yet if one were to show a child a globe as ask them if it looks like the continents once fit together, they would be very likely to see it. Sometimes we simply lack the technology at the time to provide a better answer, that is part of this story and was part of the plate tectonics story. Of course there are still young Earth creationists who don't just disagree with evolution but geology, chemistry, and physics and flat earthers so maybe no amount of evidence will ever convince some people.
While it is pretty unique that a river reversed direction, that rivers change course is hardly unique. In northern North America, glaciers have been the largest reason rivers have changed their course. Many rivers that are now Great Lakes Tributaries were once blocked from that path by glaciers. You have maybe heard the rather ironic statement that the New River is the oldest river in the world (well, not exactly...but it is old). While the New River has been it its current valley for somewhere around 330 million years, it was once the headwaters of the ancient Teays River. As you can see above, the Teays River cuts through West Virginia, Ohio, Indiana, and Illinois toward the modern day Mississippi River. Of course no such Teays River exists today though parts of more recent rivers flow through this former river valley. The rivers of the Teays River watershed changed course as the ice preventing their northward movement melted. The West Virginia town of Teays Valley sits in the valley the ancient Teays River cut which today is home to much smaller streams that could not have cut this valley - which provided some of the evidence for an ancient river.
One of my favorite concepts in stream ecology is that of stream piracy or stream capture where streams in different watersheds become connected. Stream piracy makes for some interesting biogeography with fishes and other aquatic organisms being in unexpected places.
Like the Wisconsin River, the Amazon River once flowed in the opposite direction. Today the Amazon River flows into the Atlantic Ocean but it once flowed towards the Pacific Ocean before the uplift of the Andes Mountains changed its course.
There are several particularly strong pieces of evidence for the Wyalusing River - the name given to the ancient Wisconsin River - once flowed in the opposite direction.
Eastward slopes of the terraces along the Lower Wisconsin River,
Barbed tributaries of the Lower Wisconsin River,
The curve where the present day Wisconsin River meets the present day Mississippi River is indicative of an inside bend, rather than an outside bend,
And comparative floodplain widths of the Lower Wisconsin and the Mississippi downstream of Wyalusing.
Terrace slopes of the Lower Wisconsin River have lower elevations upstream rather than downstream which is telling of a river that spent the majority of its "lifetime" traveling in the opposite direction. River terraces are created either through the build up for materials from the river (fill terraces) or the cutting through materials (cut terraces). In either case, terraces slope downstream because they are built or cut from upstream to downstream - but the terraces along the Wisconsin River do not follow this pattern.
Image from the Wisconsin Geological & Natural History Survey and the WisContext article, "When the Wisconsin River Flowed East". Features evident in the image are described below.
Second, as seen in the image above, the tributaries of the Lower Wisconsin River are "barbed" meaning that the tributary mouths point upstream. Typically, one of the ways to see the direction of flow of a river is to look at the tributaries because they will point downstream (see image above). Barbed tributaries are evidence that the river - for most of its life - flowed in the opposite direction.
Third, the curve where the Wisconsin River meets the Mississippi River - highlighted by the orange curve above - looks like an inside bend of a river, not an outside bend which it now is. As anglers, you are familiar with the shapes of inside and outside bends of rivers and understand that inside bends tend to be shallow and outside bends are deep. Look at the image above, remove the lower part of the Mississippi River and it very clearly looks like a river that took a hard turn to the East at modern day Wyalusing. The dashed orange line upriver on the Mississippi shows the shape of a typical inside bend - one that looks very similar to the curve at Wyalusing.
Lastly, the Mississippi River valley downstream of the Wisconsin River - depicted by the yellow arrow - is narrower than the Wisconsin River valley is. This is indicative of the fact that the Wisconsin River has been in its valley for longer than that section of the Mississippi River has been in its valley. As rivers flow through their valleys for a longer time, there are three basic patterns - they either cut deeper (i.e. the Grand Canyon), they cut wider valleys, or both.
This is but a brief introduction by someone that really enjoys geology and fluvial geomorphology but is not a geologist. Go read and watch the video (which I can not find a link to embed it) in the links below.
It is a really cool story about Wisconsin's namesake river, one which we only know a bit about. What caused the change is rather unknown but it occurred at least 1.5 million years ago, up to maybe about 2-3 times this long ago (this is an edit after having a chance to ask Eric about the timeline). More difficult yet is where the ancient river traveled east once it left the modern day Wisconsin River valley. Where the ancient Wyalusing River flowed is now buried under huge amounts of glacial drift. Science doesn't know all the answers but it continues to figure out more of the picture as time goes on. As technology and our knowledge increases, we gain a better picture of our past, our present, and maybe our future.