Midas River is a large river that leads down to Tadpole Pond. There are also numerous of fish that leap from the river during the barrel-jumping part.
After defeating Belome, Mario (and Mallow) gets washed out of Kero Sewers and lands in Midas River. He goes down the large waterfall collecting flowers and coins along the way and eventually reaches a river. He jumps on barrels once again collecting coins and flowers and he eventually reaches the end. Mario can now take Mallow back home to Tadpole Pond.