No no guys, niceaft was completely correct in his choice - the muscled (or square top) sail was specifically DESIGNED for use on rivers.
One of the biggest problems riverboat captains complained about when sailing rivers was that all the good wind was way up high because all the vegetation and trees that always grow along the banks of rivers would deflect the wind up and over the top of the river water. The square head sail was designed to catch the good wind that was way up high because the trees deflected all the wind that was near the surface of the water.
The square top sail is still a favorite of river boat captains and in China where sailboats need to sail on rivers to get to all the ports like Halong Bay they still to this day prefer the gaff rigged fully battened square top sail.
The Chinese perfected the square top because captains of heavy loaded boats trying to move cargo up the many rivers in China needed a way to get to the good wind that was up high. Just like on the junk rig (actually, for us European sailors it's a lugsail) you get the best performance with a heavy boat that sails upright like a sampan. Us guys here are all European sailor - we only know the Italian Marconi rig or the square riggers of Britain, Spain, or Portugal , but in the rest of the world where the eastern people live they ply a lot more rivers because they have the Nile, and the Ganges similar waterways so gaffed rigs and lugsails are very popular. Gaffed rigs moved the stones that built the pyramids:
You might say we westerner sailors are "ethno-centric" when it comes to sails. (That's why we say "I like the cut of his sail" - sail choice can reveal your culture).