Significant Topographic Changes in the United States
The primary result of the data processing explained in the Methods section is a polygon feature dataset that locates and describes the areas in the conterminous United States that have experienced significant topographic change during the 20th century. The spatial distribution of the 5,263 polygons is shown below.
The individual change polygons range in area from 0.03 to 9.79 square kilometers, with the average size at 0.24 square kilometers. The table below lists the overall summary statistics for the full collection of change polygons.
The next table shows the proportion of the change polygons that fall into major land cover categories. Over two-thirds (67.5 percent) of the polygons represent cut areas (significantly decreased elevation), and the remainder (32.5 percent) represent fill areas (significantly increased elevation).
The total amount of material that has been displaced is 4.33 x 1010 cubic meters. This total was derived by summing the volume attribute for all the polygons, regardless of the type of change (cut or fill). When the volume of a cut is considered to be negative and the volume of a fill is considered to be positive, the total volume for the entire dataset sums to –1.25 x 1010 cubic meters, which is considerably different from the previously calculated total. The discrepancy between these two numbers provides some insight into the nature of the collective surface changes represented by the polygon dataset.
As noted above, the majority of the polygons are cut areas, where elevation is decreased (and the volume moved is recorded as a negative value). If the number of cut areas were equal to the number of fill areas, it could be expected that the negative and positive volumes would balance each other and the overall total volume would equal zero. In other words, the volume of earth material excavated (cut) would equal the volume deposited (fill).
As observed, there is a net loss of material in the system instead. The cuts account for 2.79 x 1010 cubic meters of material removed, whereas the fills account for 1.54 x 1010 cubic meters of material deposited. The difference between these figures is the observed loss to the system of 1.25 x 1010 cubic meters. In direct anthropogenic processes, material must first be excavated before it can be deposited. In this context, nearly 45 percent of the volume of material removed either was not deposited or was not deposited in manner that resulted in features that are detectable through elevation differencing.
Considering that the primary land use represented by the change polygons is mining, it can be assumed that a large portion of the “lost” material is the mineral content that was extracted from the mining sites. In reality, this volume of material was not completely lost from the system but removed from the mining site, distributed, and consumed elsewhere, so there is no detectable deposition of fill material to balance with the cut volume. This is an example of the unbalanced flow of materials described in the literature—much of the material moved in anthropogenic processes is not replaced. In general, much of the material moved by human action is only transported over short distances. While this is certainly true for overburden in surface mining operations, it is not the case for the extracted mineral. As a valuable commodity, it is worthwhile to move the mineral material over longer distances, and its transportation is not hindered by cost constraints.
As described in the Methods section, features were extracted that are characterized by significantly increased elevations coinciding with flat areas in the SRTM data. In many cases these are reservoirs, lakes, or other water bodies that have experienced a water level rise or inundated area expansion. This figure shows the locations of the 364 polygons that fall into 239 counties across the conterminous United States (nearly 8 percent of the total).
Of the change polygons in this class, 15 percent are labeled as “new” where NLCD did not indicate a land cover of water; 78 percent are labeled as “existing” where the polygons share a border with an existing water body; and 7 percent are labeled as “mining” where NLCD indicated the mining land cover class as the majority within the polygon.
Additionally, the 364 special case polygons are contained within 213 hydrologic cataloging units (see below), which represents about 10 percent of the total of these watershed delineations in the conterminous United States that are commonly used for water resources management. The presence of hydrologic alterations as indicated by the change polygons may be important for water resources management within the affected units.