BEDLOAD AND BANK PARTICLE SIZE TRENDS ALONG STREAMS OF THE CUMBERLAND PLATEAU, TENNESSEE

In general, bedload particle size decreases downstream due to either 1) sorting during flood stages; or 2) particle abrasion during bedload transport. Where it has been measured, bank sediment particle sizes also decreases in the downstream direction, although the mechanisms that cause this trend are less clear than in the case of bedload. The way in which bedload and bank sediment sizes are related is not well understood. For example, what are the effects of tributary junctions on both bedload particle size and bank particle size? A field study was conducted along mixed bedrock- and gravel-bed streams on the Cumberland Plateau in Tennessee to investigate particle size trends in both bank and bedload sediments. The diameter of gravel, cobble, and boulder size clasts making up in-channel bars was measured in the field using a gravelometer. Sandy bank deposits were collected and returned to the lab for settling tube analysis. Samples were collected at points ranging in drainage area from 25 to 800 km². Results indicate that median bedload particle diameters decreases from an average of 230 mm in the upper reaches to 52 mm downstream. However, tributary inputs cause abrupt increases in the particle size of bedload. However, particle size begins to decrease again immediately downstream from tributaries. More work is needed to determine how overbank sediment fining is related to changes in bedload fining. Also, this research points out the need to understand how events external to the channel, such as mass wasting, affect bed particle size. Results from this study fill an important data void of bedload size measurements on remote streams in the Cumberland Plateau.