Controls on Whitewater Rapid Formation Big South Fork, TN

Maner, James

ABSTRACT: Field and laboratory studies were conducted to determine a geologic reasoning for the positioning of whitewater rapids on the Big South Fork River. The hypothesis is rapids are located at high gradient sections of a river where constrictions exist. Measurements of channel width and river gradient at each rapid were the main variables. Also measured was rock hardness, clast size, chemical variations in boulders and bedrock per rapid, tributary influences, mass-movement evidence, rapid length, rapid width, and vertical drop. Analysis of the data accumulated from field excursions shows whitewater rapids are not limited to steep gradients and constrictions in the channel. A positive correlation between clast size and rock hardness exhibits a linear trend almost parallel to the rock size axis. Rock hardness, in this case may be controlled by a third variable, possibly a mineral coating on the boulders. Chemical testing was halted due to equipment failure. Some sections of the channel were almost entirely filled with large boulders, yet there were no rapids. Sections of the river that had large clasts (clast size being greater than 2 meters) were accompanied by smaller boulders armoring the slope prior to the whitewater rapid. In conclusion, there were no two variables that when compared to one another could prove why a whitewater rapid occurs where it does. At least three different situations causing rapids occurred on the Big South Fork river: 1) Accumulation of boulders in the river bed allowed for a significant vertical drop on the downriver side of the accumulation causing a rapid, 2) channel constrictions funneled the river through a narrower section of river causing a greater degree of incision on the down river side of the constriction producing a whitewater rapid, and 3) bedrock outcrops at river level produced ledges allowing whitewater rapid formation.