Changes in land use change and water quality in the Yadkin River Basin, NC 1951-1990: A time series/GIS analysis

In large river basins, the impact of land use and land cover change on water quality is not easily tested. This study's objective was to evaluate changes in land use, soil erosion, and sedimentation in the Yadkin River basin, a 5896 km{dollar}sp2{dollar} watershed located in the Blue Ridge and Piedmont provinces of western North Carolina. A 40-year daily suspended sediment database, collected by the US Geological Survey from 1951 to 1990 at Yadkin College, NC, provides an unusual opportunity to evaluate the relationship of land use change to sedimentation trends at the river-basin scale. Statistical and time-series analyses were used to create suspended sediment concentration models and to model water quality trends using daily and storm aggregated data. Spatial analyses of land use and land cover (LULC) change and rural gross soil erosion rates were performed using a geographic information system (GIS) within a stratified random sample framework. The GIS results were compared to historic, county-based US Department of Commerce Agriculture Census data. Results of the time series analyses indicate slight, although significant decreases in suspended sediment concentrations over time. This decrease is attributed to declining agricultural land use and soil conservation measures. Storm-aggregated analyses indicate sediment yields are increasing during extreme storm events, attributed to higher storm flows and urban development. GIS LULC analyses estimate a 39% decline in cultivated area (1222 to 749 km{dollar}sp2{dollar}) and a 75% increase in urban area (317 to 554 km{dollar}sp2{dollar}) in the basin between 1955 and 1988. These results correspond well with agriculture census data. A stratified gross erosion analysis indicated that gross soil erosion rates from rural areas of the basin have decreased between 17% (best management practices excluded) and 42% (best management practices included) during this period. Decreases in total gross erosion are attributed primarily to the decreases in cultivated area and only recently to improved soil management. Urban and suburban development and construction activities, although short term disturbances, are considered as potentially major sources of new sediment in the river system. Resuspension of previously eroded material may also be affecting suspended sediment concentrations and yields. Inputs from these other sources may be offsetting water quality improvements attributed to soil conservation efforts and the declining agricultural landbase. Managed control over new sources of sediment and stormwater flow is considered critical for continued improvement of river water quality.