Throughout the 19th century, and largely different from today, people approached the Falls from the south side. Upstream on the road–now Minnehaha Avenue–there was indeed a bridge over the creek, but the roadhouses and hotels and such were south of the creek, and the railroad depot (when the railroads came in) was put where the people were, on the south side. It was closer to the Fort, after all, and the Fort was the only legal settlement in the earliest years. Minnehaha Falls were within the military reservation at the beginning of European settlement in Minnesota.
Someone, some time in those early years, built a bridge to allow people to cross the creek below the Falls.
Few reasons come to mind to provide an improvement at the Falls in this exact and picturesque location, unless one expected to make money off of it. That’s hardly the activity of the United States Army at Fort Snelling.
However, pioneer and strategist Franklin Steele managed to buy Fort Snelling out from under the government in August of 1857, and Minnehaha Falls along with it. And that would have been the opportunity to improve the area: to build a hotel, and put that bridge across the creek. No documents have come up to determine when this bridge was built. But having privatized the area, using it to create more income makes sense. This is the leading explanation.
Joel Whitney took this picture, and marked it as coming from his studio. And that gives us a historic date range of 1851 to 1867. So 1857 is plausible. This picture cannot date from as late as 1866 or later. We know that this bridge was in terrible disrepair in 1866.
The likely date range for this picture is spring of 1858 to spring of 1866. Most probably it was taken around 1863, when the first hotel at the Falls was built.