GEOMORPHIC VARIABILITY IN URBAN AND RURAL STREAMS IN THE PIEDMONT REGION OF SOUTH CAROLINA

The land transformation associated with urban sprawl generally includes deforestation, increased impervious surface cover, and a decrease in the riparian vegetated corridor. These changes lead to increased storm water runoff and stream flashiness. Increased storm discharge is geomorphically expressed as increased cross-sectional dimensions, increased incision and entrenchment, and alteration of sediment grain size distribution.

The objective of this research was to understand the effects of urbanization on channel morphology and sediment texture in three watersheds in the Piedmont region of South Carolina. Urban land cover in the watersheds studied ranged from 2% in the rural watershed to 76 % in the most urbanized watershed whereas the actual percent imperviousness ranged from 4% in the rural watershed to 39% in the most urbanized watershed. Prior to urbanization, this area had a history of intensive cotton farming, which led to intense soil and stream channel erosion. Channel cross-section geometry, streambed grain size, and riparian vegetation buffer width were all measured to determine if urbanization had an effect on stream geomorphology.

For all three watersheds, bankfull width increased downstream, and most channels were incised and moderately to highly entrenched. There were no significant differences in bankfull width or incision among the three watersheds (P=0.96 and P=0.10, respectively), but entrenchment was marginally significantly higher in the urban streams (P=0.07). On average, the sediments were poorly sorted, coarse sands with no significant differences in texture and no significant correlation with urban cover. Overall, the lack of significant relationships between land cover and stream geomorphology in this study are consistent with previous studies of stream geomorphology in this region. These results may result from past land use. Until the 1950s, intensive cotton farming was a predominant land use in the piedmont of South Carolina. Decades of erosion and stream channel alteration due to farming activities followed by changes induced by urbanization have mixed signatures that are currently difficult to separate. All streams in this region are likely still evolving, adjusting to reach a new equilibrium condition in accord with current land cover conditions.