Significant Topographic Changes in the United States
Regional Differences in the Effects of Surface Mining
Because the topographic change inventory has near-national coverage, comparisons can be made among broad regions. This figure shows a set of five Level III ecoregions that were used as a basis for regional comparisons of topographic change.
Specifically, the cumulative effects of surface mining were compared across the following ecoregions: Central Basin and Range, Wyoming Basin, Northwestern Great Plains, Northern Lakes and Forests, and Central Appalachians. These five ecoregions were selected because they contain significant numbers of mining sites, and collectively they represent a wide variety of the types of surface mining in the United States.
Examples of mining for precious metals are found in the Central Basin and Range; mining typical of the western U.S. coalfields is found in the Wyoming Basin and Northwestern Great Plains; open pit iron ore mining is prevalent in the Northern Lakes and Forests; and mountaintop and contour mining for coal is concentrated in the Central Appalachians. The subset of topographic change polygons labeled according to the NLCD as quarries/strip mines/gravel pits was extracted for each of the five ecoregions and treated as a separate dataset. As documented earlier, surface mining is the primary direct anthropogenic process responsible for significant topographic changes, so it is useful to further examine its overall effects on the landscape.
As a result of viewing the topographic change polygon dataset overlaid on the NED and SRTM data, it was recognized that the expressions of landscape transformations due to surface mining differed across the study area. In particular, the physical shape of mined lands differed markedly between the western U.S. and Appalachian coalfields. Of course, some of the contrast is due to the difference in the type of natural landforms present before mining began, but the basic terrain parameters of elevation and relief changed in different ways for the two regions.
The figure below shows a scatterplot of change in mean elevation versus change in relief for the mining polygons in the Northwestern Great Plains, which contain the major coal producing Powder River basin, and the Central Appalachians, which contain the mountaintop coal mining areas in eastern Kentucky and southern West Virginia. The scatterplot shows fairly distinct clusters of points indicating that the changes in elevation and relief conditions due to mining are different for the two regions.
In general, relief is decreased by mountaintop mining in Appalachia as the there is an overall flattening of the landscape. Ridges are leveled and adjacent valleys are filled to about the same level. In contrast, in the western coalfields where the natural relief is lower, the surface mining generally increases relief as pits are excavated and overburden is deposited on adjacent land. The differing character of relief changes as noted in the plot supports the observed conditions (see figure below). Another contrast between these two ecoregions in terms of the effects of surface coal mining is the potential for further surface modifications. In the mountaintop mining areas the land cannot be returned to its original shape as part of the reclamation process, whereas in the western U.S. coalfields there is a much greater potential for the land to be returned to its premining form, and it generally is restored after mining operations have ceased.
The following three figures compare changes in mean elevation, relief, and mean slope, respectively, for mining polygons in the five focus ecoregions. Changes in mean elevation follow a similar pattern for all five ecoregions. However, for changes in relief and mean slope, the Central Appalachians ecoregion stands alone compared to the other four ecoregions. Changes in relief and mean slope are negative, which is the opposite of the other four ecoregions. This is especially true for changes in mean slope (the last of the three figures below) where the mining polygons in the Central Appalachians show an overall decrease in slope at least twice the magnitude of the increase seen for the other ecoregions. These significant decreases in relief and slope for the Central Appalachians reflect the general leveling of the land surface in mountaintop mining areas. Such leveling stands in contrast to increased relief and slope for surface mining areas in other regions.
As described above in the section on hydrologic effects, surface mining operations can have significant effects on streams. To compare the coincidence of streams and topographic change areas across ecoregions, the change polygons were intersected with the EDNA national stream network. A specific interest here was to determine if there was a difference between cuts and fills in terms of their locations relative to streams.
The figure below shows the total length of streams covered by change polygons for each of the five focus ecoregions. The fill polygons in the Central Basin and Range and Northern Lakes and Forest ecoregions impact the most total stream length, with each exceeding 120 kilometers of affected streams.
The next figure compares the proportions of affected total stream length covered by cuts and fills for each ecoregion. Once again, the Central Appalachians ecoregion stands in contrast to the other four ecoregions. Nearly 100 percent of the streams affected by topographic changes fall within fill polygons. It is likely than many of the occurrences are at locations of valley fills, one of the primary environmental issues associated with mountaintop mining.