From the Super Mario Wiki
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The River World, as its name may imply, is filled with rivers, the largest of which seems to cut right through the land. Trouters seem to be the only fish that inhabit these rivers. The only portions of land ever seen in the River World are small beaches and woodland areas. The Mouth of the River, a gypsy-like woman who knows everything about the River World, inhabits one of these wooded areas in a caravan.
It seems the status as "King of the River" (presumably the title given to the king of the River World) can be obtained by winning a race down the largest river of the River World; the current King of the River is Mark Twang. Ever since Mark Twang's race against Captain Koopa for title of the King of the River, there has been a gaping hole in the River World's main river, which was caused by a Black Pit Bob-omb utilized by Captain Koopa.