CONTROLS on KNICKPOINT FORMATION, BIG SOUTH FORK RIVER, TENNESSEE

Knickpoints (or rapids) are zones of locally steep channel gradients. Along bedrock rivers rapids form at lithologic contacts, tributary junctions, bedrock outcrops that constrict the channel, and where mass wasting processes impinge on the channel. Field and laboratory studies were conducted to investigate the cause of rapids in the Big South Fork River gorge on the Cumberland Plateau of Tennessee. The Big South Fork is eroded into Pennsylvanian age sandstone and shale. The following variables were collected at rapids: channel width, valley width, channel slope, rock hardness (Schmidt hammer test), rock type, and clast size. Most rapid-forming boulders in the study area consist of well-cemented sandstone derived from mass wasting along valley walls. Sandstone samples were chemically examined to determine if different cementing agents (calcite or silica) were more common in rapid-forming boulders. Results indicate that no one variable can explain the location of rapids. Valley width-to-channel width ratios at each rapid ranged from 4 to 9. However, valley width at rapids showed no significant difference from average valley width. Large, rapid-forming boulders had greater hardness values than smaller boulders in rapids. Also, iron banding was common on many rapid-forming boulders. Large boulders were also found in the channel along pool sections, indicating that mass wasting from valley walls by itself may not always result in the formation of a rapid. Rapids in the study area tend to form at points where large boulders (fallen from valley walls) cover more than half the channel width, enabling the capture of smaller, bedload-carried clasts. The interaction of both fluvial and mass wasting processes appears to be important to rapid formation.