Based on USGS data, showing the two HUC 6 subwatersheds that comprise the Connecticut River watershed (itself a HUC 4). Hydrologic units are hierarchical, Smaller units (larger HUC values) correspond to units with less total area, and hydrologic units at each level are nested within the level above. The two HUC 6 watersheds in the Connecticut River watershed divide the watershed into northern and southern units and were used to ensure an even distribution of aquatic and terrestrial core areas across the watershed. This was particularly important in the CT River watershed that tends to be heavily forested in the northern components of the watershed and less so in the southern portion. Scaling by the entire CT River watershed (HUC 4) would have led to a greater concentration of cores in the northern portion of the watershed. The result of scaling is that IEI scores and other measures are relevant only to others within the particular HUC 6 watershed.
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Read the Technical Abstract for this data layer (huc6Ctr.shp).