Chapter 2: The National Context 
I Before looking in detail at the Tensas River Basin, it is 
helpful to place the Basin within a national perspective. 
This chapter paints a picture of the lower 48 United States, 
showing differences and patterns among watersheds at a 
continental scale. A national context helps us interpret the 
overall condition of the Tensas River Basin, relative to the 
rest of the country. It also helps to determine if conditions 
like those found in the Tensas River Basin are likely to 
exist elsewhere. 
While it would be desirable to look in great detail over the 
entire nation, in practice only a few aspects of environmen¬ 
tal condition can be described in a consistent fashion 
nationwide. The coarse-scale maps in this chapter show 
watershed rankings based on a variety of landscape 
indicators. The rankings portray relative conditions across 
the nation but do not show the absolute values of indicators 
for each watershed. Indicator values are summarized in 
the companion bar charts. 
Data Sources 
Four main data sources were used here. The most 
important was a national map of land cover (Figure 2.1) 
which describes the types of vegetation covering an area, 
whether it is forest, crops or pasture, or covered with water 
or urban areas. Although the resolution (spatial and land 
cover) is fairly coarse (1 square kilometer and 9 of the 
original 160 land cover classes), the familiar national 
pattern is apparent-forests in the East, grasslands and 
crops in the Midwest, and shrublands, deserts, and moun¬ 
tain forests in the West. The Tensas River Basin is typical 
of the alluvial valley of the lower Mississippi River, riverside 
urban areas, agricultural valleys and plains, and forested 
wetlands. The variety of the land cover types in the Tensas 
River Basin, relative to other regions in the United States, 
can make spatial pattern an important ingredient for 
making environmental decisions in this region. 
Some additional information was used to calculate the 
indicators of environmental quality nationwide. Figure 2.2 
shows the maps of roads, streams, and watersheds. 
Clearly, not all the roads and streams are included. These 
maps may be appropriate for a nationwide overview, but 
much more detailed maps are needed for regional assess¬ 
ments such as the Tensas River Basin analysis described 
later. The watershed boundaries (Figure 2.2) identify 2,099 
individual watershed units across the United States. 
figure 2.1 
National land cover map. The U.S. Geological 
Survey produced this map of “Seasonal Land Cover 
Regions of the Conterminous United States. ’’ The map was 
derived from March-October (1990) 1-km Advanced Very High 
Resolution Radiometer (AVHRR) imagery, digital elevation, 
ecoregions, and climate data. The original 160 classes of land cover 
have been grouped into the 9 broad categories shown here. 
Western Forest 
Eastern Forest 
Croplands 
Shrublands 
Grasslands 
Wetlands 
Water 
Barren 
Urban 
