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 are flushed out of Kero Sewers, they end up being flung down the falls. They land on a barrel floating in the river, and ride it to the end, collecting coins along the way.
Participating in a barrel-rolling contest, Mario collects many coins and has to avoid the river's irritable jumping fish. The Toad at the river's end exchanges the coins Mario collected down the falls and river for Frog Coins. When Mario does 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, which is fitting since many Gold Coins can be collected in the river.
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