Visitors from drier climates are sometimes shocked to see how huge the rivers can get in the South, how wide and voluminous as they saunter to the sea, their seemingly slow speed belying the massive power they contain. South Carolina ’s big alluvial, or sediment-bearing, rivers originate in the region of the Appalachian mountain chain.
The blackwater river is a particularly interesting Southern phenomenon, duplicated elsewhere only in South America and one example each in New York and Michigan. While alluvial rivers generally originate in highlands and carry with them a large amount of sediment, blackwater rivers—the Edisto being the chief example—tend to originate in low-lying areas and move slowly toward the sea, carrying with them very little sediment.
Rather, their dark tea color comes from the tannic acid of decaying vegetation all along their banks, washed out by the slow, inexorable movement of the river toward the sea. While I don’t necessarily recommend drinking it, despite its dirty color blackwater is for the most part remarkably clean and hygienic.