An Ecological Assessment of the Louisiana Tensas River Basin Chapter 3 
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backswamps then become the vegetation filtering areas 
for pollutants and nutrients. These landforms create a 
diverse physical and ecological region. Bayous, channels, 
streams, and rivers direct the flow of water across the 
landscape and are dominant features in the Tensas River 
Basin (Figure 3.2). In the previous chapter we looked at 
the Tensas River Basin in the context of a watershed 
within the lower 48 contiguous states. In this chapter, we 
further divide the watershed into topographically relevant 
subwatersheds or zones and examine landscape indica¬ 
tors based on these subwatersheds. These 
subwatersheds, known as 11-digit hydrological accounting 
zones, were defined by combining the USGS 8-digit HUC 
boundary with the NRCS 11-digit boundaries and are 
shown in Figure 3.3. 
Figure 3.2 
1991/92 NALC Image 
60-meter False Color Composite 
Note that Zones 2, 7, and 9 were not defined as 11-digit 
hydrologic accounting units by NRCS but do fall within the 
boundary of the 8-digit USGS HUC boundary. 
Subwatersheds 2 and 7 may be parts of other 
subwatersheds or contain most likely, bayous in which 
water could flow in many directions. 
Therefore, these areas are included in the combined 11- 
digit boundary but may not actually be totally hydrologi- 
cally linked to the Basin. Subwatershed 9 appears to be 
linked to the Basin through a major tributary. The indica¬ 
tor values calculated for zones 2 and 7 are probably not 
as reliable as the values for those zones in which an 
entire subwatershed was in the assessment area. 
Combination of USGS 8-digit HUC boundaries with 
NRCS 11-digit boundaries. Zones 2,7, and 9 are shown in red. 
USGS 8-Digit 
Accounting Units 
NRCS 11-Digit 
Accounting Units 
Combined 8 and 11 Digit 
Accounting Unit Zones 
Figure 3.3 
