When I first heard about this spot I assumed the story was one of those backwoods tall tales that country people so enjoy telling. I assumed that I would discover that these two rivers crossed in some abstract sense due to a trick in verbiage or a mapmaker's convention that couldn't quite be made to describe the actual geography.
So I drove up to Rivervale to check it out for myself, and sure enough these two rivers do cross at right angles right here. You'll never notice it unless somebody points it out to you, but it's a real geographical oddity, all right. I didn't believe it until I saw it for myself. Here it is. Just one more Arkansas feature that sounds like it ought to be impossible.
In the top picture we have Little River, crossed by the highway 135 bridge in the background. That shoal is the trailing edge of a concrete slab top of a culvert. To give you an idea of scale, there are a couple of bowfishermen standing on that slab. For years this was a popular spot where locals would wash their cars, parking them in the shallow water on top of the culvert.
In the bottom picture picture we have the Right Hand Chute of the Buffalo River (Not the Buffalo National River, this is another river with the same name.) which flows under the Little River through a culvert which was built in 1925. The top of this culvert runs under the road to the right, serves as the bed of Little River beyond that, runs under a causeway and road on the far bank of the river, and reemerges on the other side looking like the mirror image of this picture.
This spot is adjacent to the Saint Francis Sunken Lands, which sank some fifty feet in elevation during the New Madrid earthquake. When those lands shifted, all these well-established river courses had to find new ways to the lowest available elevations, so lots of farmland became swamp land as water made its way toward the Sunken Lands. I don't understand fully why this solution was used rather than just have these rivers converge and then run confluently into the Sunken Lands, but I'm told that the engineers decided that the best way to solve the flooding problems of this particular patch of the delta was to route this river underneath that one.
If you want to see this sight for yourself, take highway 135 north out of Lepanto, and on your way you can stop at the Lepanto Museum and let them explain it to you the way it was explained to me.