Describing the environment and habitat

 
In EDT we distinguish the environment in a stream from the habitat of a focal species in that environment. Habitat is a derivative of the environment relating to the productivity and capacity of the focal species.

EDT starts with a detailed description of the stream environment. The stream is divided into sections that are the individual pixels that comprise the picture being created. These may be stream reaches or watershed sections such as HUC units. For each stream section, the size of the environment is described in terms of eight Stream Unit Types. The quality of the environment in each section is described by the 35 Environmental Attributes (also referred to as Ecological Attributes or Correlates), which are factors such as temperature, flows, sediment, and so on. Some of these, like flow, temperature and channel width, are shaped by month within a year, but most are single descriptive values. Stream Unit Types, Environmental Attributes, and general Geographic Descriptors form the basic inputs for EDT.

The next step in the EDT process is to rate quality and quantity of the habitat with regard to the needs of a focal species. To rate the quality of the habitat, EDT employs a set of biological rules that relates the Environmental Attributes to the survival of one or more life stages of the focal species. Various combinations of the Environmental Attributes are collected to form 17 Habitat Survival Factors. For example, the attribute of habitat diversity is formed by bringing together the survival relationships for a life stage (e.g., juvenile rearing over the summer) with the Environmental Attributes gradient, natural and artificial channel confinement, riparian function, and woody debris. Thus, survival of the juvenile summer rearing life stage is related to habitat diversity, which is a function of gradient, confinement, riparian function, and woody debris.

To rate the quantity of habitat, the amount of different stream unit types in a reach and summed up and weighted according to their potential value to a life stage. For example, small cobble riffles are weighted heavily for the spawning life stage, whereas primary pools receive zero weight for this life stage.