From the Super Mario Wiki
Midas River is a large river that is fed by Midas Falls; it is seen in Super Mario RPG: Legend of the Seven Stars. When Mario and Mallow were flushed out of Kero Sewers, they ended up being flung down the falls. They landed on a barrel floating in the river, and rode it to the end, collecting coins along the way.
Participating in a barrel rolling contest Mario collected dozens of coins and had to avoid the river's irritable fish. The Toad at the rivers end exchanges the coins Mario collected down the falls and river for Frog Coins. When Mario did well enough, he might even get a Nok Nok Shell.
The path from Midas River leads to Tadpole Pond.
The name "Midas" comes from the Greek mythological character, King Midas, who turned everything he touched into gold and the fact of collecting Gold Coins.
 Names in other languages