You are on the lower walkway in Pillsbury Park. This picture shows an unused man-made tunnel that I suspect may have used to divert the river at one point.

Minneapolis started out as the city of St.Anthony, on the east bank of St.Anthony Falls. The falls are nearby, and were the best source of power for flour and lumber mills in the area. After mills crowded both shores, The river was diverted to power all of them from underground, via tunnels. I don't understand exactly how this was done, but I suspect this tunnel may have been somehow involved in that project. It has a creekbed coming out of it, but no water. The next pictures, of the Pillsbury "A" mill, might demonstrate the function of the tunnels better.