Significant Topographic Changes in the United States
Because the shape of the land exerts strong control over the collection and flow of surface water, changes to the topography can have a significant effect on local drainage conditions. The figure below demonstrates how the local surface drainage features have been altered as a result of surface mining operations.
Standard hydrologic analysis tools were used to delineate watersheds and flow paths (stream channels) from the NED and SRTM data for four adjacent watersheds covering a mountaintop mine area in Perry County in eastern Kentucky. Note how the removal of the local drainage divides and the filling of local stream valleys have significantly altered the surface hydrology of the four watersheds draining the disturbed area. Some upslope areas have changed watersheds and now drain through a different stream network than in the original condition. In mountaintop mining, the valley fills are placed in the heads of hollows, which are the first order drainage basins. When the ridge tops that form the drainage divides between these headwater basins are removed, the result is altered drainage patterns as seen in the figure above.
The next figure shows another example area where the topographic changes classified as cuts coincide with the local drainage divides. The basins and stream lines are from the USGS Elevation Derivatives for National Applications (EDNA) dataset, which was derived from the NED. The basins outlined in yellow are the EDNA reach catchments, with a nominal drainage area of 4.5 square kilometers. The effects of surface coal mining on local hydrologic conditions have been widely studied, including the impacts on base flow, peak and storm flows, and stream bed geomorphology. Thus, the topographic change inventory can be useful for indicating locations where these hydrologic processes may be affected.
Of particular concern in mountaintop mining areas is the length of streams affected by the valley fills. Starting in 1999, the U.S. Environmental Protection Agency conducted an extensive series of studies of mountaintop mining in preparation for an environmental impact statement (EIS). The draft EIS was released in 2003, followed by a public comment period, and the final EIS was released in late 2005. The EIS reports that about 1,930 kilometers of headwater streams (2 percent of the streams in the study area) were directly impacted by mining between 1992 and 2002. Also, an estimated 1,165 kilometers of streams (1.2 percent of the streams in the study area) were buried by valley fills from 1985 to 2001.
A comparison of the topographic change polygons with a hydrography dataset can be useful for describing the impacts of land surface alterations. Such a comparison was done by overlaying the change polygons derived in this study with the EDNA streams layer. The first order EDNA streams are delineated based on a drainage area of at least 4.5 square kilometers. A total of over 2 million unique stream reaches are contained in the EDNA stream network over the conterminous United States. The figure below shows an example of the streams contained within the topographic change polygons for an area in Perry County in eastern Kentucky. As expected for this region, the streams fall within the areas depicted as valley fills. The table below presents the summary statistics for the number and total length of streams contained within topographic change polygons in the conterminous United States. Nearly three-fourths of the polygons of positive topographic change (fills) contain stream segments, whereas less than one-third of the negative topographic change polygons (cuts) contain streams.