An Ecological Assessment of the Louisiana Tensas River Basin Chapter 1 
12 
How Were the Landscape Indicators 
Summarized? 
Usually, a watershed is defined as a catchment area 
that is drained by a single stream or river (i.e., the 
Mississippi watershed consists of all the area drained by 
the Mississippi river system, including all tributaries). 
The dividing lines between watersheds are formed by 
ridges. Water on one side flows into one stream, water 
on the other side flows into a different steam. Thus, 
watersheds are a natural unit defined by the landscape. 
Watersheds can be defined at several different scales. 
The USGS has divided the contiguous U.S. into 2,099 
watershed units known as 8-digit hydrologic accounting 
units (HUCs). The Tensas River Basin is defined as one 
of these 8-digit HUCs and serves as the boundary of our 
study area. 
The Tensas River Basin 8-digit HUC was further divided 
into 11-digit HUCs or subwatersheds (defined by the 
USDA) as a basis for analyzing and summarizing the 
landscape data (Figure 1.9). In many ecological studies, 
especially those which assess water-related concerns, 
subwatersheds are an appropriate unit for summarizing 
data. 
Fig 1.9 
Tensas River Basin divided into subwatersheds. 
Each subwatershed was given a unique number 
(2-9) for this report. 
The next chapter will look at the landscape from a na¬ 
tional perspective. As you read about the national land¬ 
scape, see how the Tensas River Basin compares to 
other watersheds in the United States. 
Subwatershed Hectares 
2 5625 
3 45532 
4 92534 
5 62012 
6 11136 
7 7353 
8 79748 
9 72365 
Total 
372000 (Hectares) 
930000 (Acres) 
i 
