The invention of the wheel by the Sumerians revolutionized the transportation. Wagons could be used to carry heavy loads. The Tigris and Euphrates rivers made transport of goods easy and economical. Riverboats were used to transport goods for trade. Strong currents moved the boats downstream, but because of the current they could travel in one direction only. The boats had to be dismantled after the trip downstream. The Mesopotamians used three types of boats: wooden boats with a triangular sail, the turnip or Guffa boat which was shaped like a tub, made of reeds and covered with skin, and the kalakku which was a raft of timbers supported by inflated animal skins.