I get it now. The workers are attempting to restore the river banks to a previous location. What I couldn’t capture with the camera was what I saw almost behind me from where I took this shot. During the spring flooding the bank was completely eaten away by some 40 feet (12 meters) so that the bridge abutment on the south end was at risk. One more decent flood and the water would probably cut the bank back far enough to wash out behind the abutment and collapse the road. That would be one hellacious bad mess because all the heavy truck traffic would have to go several miles either side to bypass this bridge.
Thus, you can see in this photo how the huge stones are being piled in the flat riverbed. This will be used to create an artificial bank and the space to the left will be filled back in, probably with sand they recover downstream. That wouldn’t be too hard right now with the water level so low.
Of course, that doesn’t explain the work farther to the north where that enigmatic new stone wall runs alongside the road. I suppose we’ll see soon enough because the workmen are digging what appears to be utilities access in the field beyond the row of yellow equipment visible on the far bank.