Five physical, structural, and or biological components or attributes characterize especially sensitive or unique habitats. These are:

• Support or are used by State or Federally-listed rare, threatened, or endangered species, or species of concern throughout their entire life cycle or portions of their life cycle; or
• Critical or unique to a particular species throughout its entire life cycle or portions of its life cycle; or
• Rare or occur infrequently; or
• Becoming increasingly isolated, and are declining in quality or extent, or are in danger of elimination due to human activities unrelated to contamination; or
• Possess some special characteristics, i.e., are preferentially inhabited and used by complex assemblages of vascular or other forms of vegetation, aquatic invertebrates, fish, aquatic and semi-aquatic birds, and mammals; are particularly productive or highly valuable because they serve as important reproduction and spawning areas, nurseries, feeding areas, or migration routes, some of which may be restricted or localized; and serve as pathways for PCBs to bioaccumulate up the food chain in trust resources.

A list of Upper Hudson River habitat types and the characteristics qualifying them as especially sensitive or unique habitats is provided below:

• Nearshore, Shallow, and Vegetated Areas. Alone or in combination with other habitats, nearshore, shallow, vegetated areas provide services to multiple species, and are the foci of the Upper Hudson River food web. These areas vary in size and may consist of relatively narrow strips along the shore, more extensive beds within the photic zone, or constitute large backwater areas. These areas are refugia and foraging areas for early life stages of fish, turtles, frogs, aquatic and semi-aquatic salamanders, mammals such as muskrat (Ondatra zibethica) and mink (Mustela vison), and dabbling waterfowl such as mallards (Anas platyrhynchos), black duck (Anas rubripes), and wood duck (Aix sponsa). Largemouth bass (Micropterus salmoides) spawning and nursery areas occur in two backwater areas within the 40-mile stretch of the Upper Hudson, the west side of Griffin Island (ESUH 8) and cove at Coveville (ESUH 32).
• Mouths of and Deltas Formed by Tributaries. Such habitats serve as spawning and feeding areas for warm water fish and foraging areas for wildlife. An example is the mouth of the Moses Kill (ESUH 11).
• Riparian Forested Habitats. These habitats immediately adjacent to the river, including forested floodplains,  provide roosting and breeding habitat for a large number of bird species including Federal and State protected species and/or a linkage between high value aquatic and terrestrial habitats. These areas are important in the Upper Hudson River and are declining in abundance and quality due to human activities, such as land development. An example is the forested area adjoining the river at the Saratoga National Historic Park (ESUH 40).
• Riparian Wetland Habitats. Such habitats have limited acreage, provide multiple ecological services, are especially sensitive, and provide linkages between aquatic and terrestrial systems. These complexes are important in the Upper Hudson River and are declining in abundance and quality due to human activities such as land development. Examples include wetlands associated with Hot Spot 38 in Mechanicsville (ESUH 48), Hot Spot 39 at Lock 2 (ESUH 50) and  Hot Spot 40 at Quack Island (ESUH 51).
• Riparian/Transitional Zones. These habitats are transitional areas that link habitat types such as aquatic and terrestrial habitat. Ecotones are more dynamic, can have higher species richness and greater nutrient cycling, and function as migratory/access corridors. Examples include the narrow bands of wild rice between shoreline and open water habitat (ESUH 30), and narrow bands of emergent plants between upland and submerged aquatic vegetation (SAV) beds (ESUH 12).