Long-Term Change in Regime of the New River Indicated by Vertical Variation in Extent and Weathering Intensity of Alluvium

Abstract

Investigations of stream history in Appalachian valleys have, for the most part, been confined to deposits within a few meters of the valley floor where radiocarbon dating is possible. Detailed study of New River alluvium in the Radford North and Eggleston 7.5-minute quadrangles of southwest Virginia shows that meaningful information can be extracted from high-level, unpaired terrace deposits, even where solution of carbonate bedrock has obscured terrace form. A statistical analysis of terrace elevations, performed by superimposing a 100-m grid over terrace maps and then determining the elevation of each grid point above modern river level (AMRL), shows that alluvial deposits are more abundant at 0-12 m (0-40 ft) and 30-49 m (100-160 ft), while the 12-30 m (40-100 ft) interval has relatively few deposits. Munsell color, percent sand, silt, and clay, and point counts of the heavy-mineral and light-mineral sand fractions were made in the B horizon at 35 sites ranging in elevation from 1.1 to 85.4 m AMRL. These properties indicate a dramatic increase in weathering intensity between about 20 and 30 m AMRL, corresponding to the break between lower and higher terraces. It is hypothesized that the concentration of alluvial surfaces at 30-49 m AMRL represents a time of relative stability during which broad floodplains and terraces were formed. The gap in terrace-elevation frequency between 12 and 30 m corresponds to a subsequent interval of downcutting during which only a few narrow terraces were formed. The method described can increase understanding of long-term stream history in areas where other methods cannot be used.