The ultimate health of a stream depends not only on the functioning micro-habitats (small physical, biological and temporal scale), but also on what is occurring within the stream's watershed (large physical, biological and temporal scale). And the structure and function of stream's fish and macroinvertebrate communities is strongly associated with instream physiological conditions (e.g. substrate, channel morphology, woody debris) as well as factors operating at the landscape scale (Richards & Host 1994, Lammert & Allan 1999, Williams et al. 2002). So when undertaking stream ecosystem restoration a thorough knowledge of the natural processes that allow restoration projects to reach a sustainable state of dynamic equilibrium is necessary. This knowledge encompasses both spatial and temporal scales and requires a great deal of knowledge and skill to implement. Defining the process of designing a restoration project is critical not only for the completion of the project, but more importantly for increasing the probability that the stream itself will, in time, bring itself back into use attainment.
The integrated network of a stream is simultaneously physical and biological and these characteristics are themselves enveloped within a temporal context. As J. David Allan illustrates in his book 'Stream Ecology Structure and Function of Running Waters', the spatial and temporal characteristics of a stream can be categorized in units varying greatly in magnitude. Spatial characteristics vary from units smaller than 1 mm to 100's of kilometers and temporal characteristics vary from units of less than a day to longer than 10,000 years.
This variation in unit size can also be extended to the diversity of biological characteristics existing symbiotically within a stream. Within a stream, at the micro-scale, there are microscopic processes assimilating elements, breaking down organic matter and forming the basic building blocks of the food chain. At the macro-scale,there are entire networks of riparian corridors that serve as a ready source of food and water and act as migration routes for mammals and birds. The ultimate health of a stream depends not only on the functioning micro-habitats (small physical, biological and temporal scale), but also on what is occurring within the stream's watershed (large physical, biological and temporal scale).
Between these two extremes, however, exist other categories of scale and it is within this range that the 'application' of stream ecosystem restoration techniques can be implemented and it is to these categories of scale that additional assessment and analysis techniques should be tailored. It is within these middle scales that that stream ecosystem restoration design components can be identified, designed, specified and ultimately applied in the field and it is from these components that the realization of macro-scale restoration efforts will be realized.
The 'scale' at which we most often direct our efforts of stream ecosystem restoration is what is known as the 'reach' of a stream - typically in the range of several 1000 feet. "Landscape scale is an important concept in habitat evaluation, protection, and restoration. Restoration efforts should focus on a reach scale rather than on single sites because the influence of habitat accrues at a watershed or subwatershed level. Previous analyses by Rankin (1989, 1995) have demonstrated this phenomenon." (Association Between Nutrients, Habitat, and the Aquatic Biota in Ohio Rivers and Streams; Ohio EPA Technical Bulletin MAS/1999-1-1. DSW//MAS 1999-1-1 Aquatic Biota, Nutrients & Habitat in Ohio Rivers & Streams January 7, 1999)
Natural Channel Design principles have emerged as a cohesive set of tools for designing successful restoration projects., Unfortunately, natural channel design principles do not necessarily take into account micro-habitat and habitat elements found at the 'reach' scale. Serving as the foundation for stream ecosystem restoration, the process of applying natural channel design principles repeatedly at the reach-scale will start to encompass larger stream scales termed stream segments, which in turn, can have a significant effect on the stream ecosystems at the watershed scale.