Brazil's Rio Negro, one of the Amazon's main tributaries, is truly black.
So black, in fact, that light penetrating to a depth of 1 ft. is only one-tenth as bright as light on its surface. At 2 ft., it is only one-hundredth as bright; at 6 ft., there is no light at all. Reason: unlike the Amazon's clear-water tributaries, the river does not originate primarily in mountains and course through relatively narrow channels, but flows sluggishly across flatland. jungle and swamp areas. Each year at flood stage the Rio Negro overflows its banks, while draining some 253,000 sq. mi....
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