You're looking for canal boats. Back in Ye Olden Days, waterways were the preferred method of transporting cargo because land movement took much more effort (and also frequently involved hills and such). If a large industry cropped up in a place where it wasn't convenient or possible to recreate it elsewhere, and the landscape allowed it, they'd build a canal to allow easier shipping of larger amounts of material. They'd also sometimes build canals next to rivers, so that the water would be slower and it'd be easier for a boat to move both up- and down-stream. Think of places like the B&O Canal or the Erie Canal, or the Intercoastal Waterway.

It's possible for someone to barge-pole a boat around a canal, but a more efficient method was to have a horse or mule harnessed to the boat, walking right next to the canal. They'd walk the canal-side towpath, pulling the boat, while the humans then only had to steer the mule and the boat.

What's likely happening in this picture is that there's a cliff or some other hazard that prevents the towpath from continuing on the right side of the canal. A bridge would be very convenient there anyway, to move the mule to the other side of the river, and if you're building a custom bridge anyway, why not build it so that you don't need to really disturb the team anyway, they can just keep going? It's a really cool idea!