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Reaches are sections of channel that are expected to behave differently over time. The expected differences are most often defined by the following four variables: 1) Hydrology source. For example, if the majority of a project is comprised of a 4th order stream, but a significant linear footage of a 2nd or 3rd tributary also falls within the project boundary, then the tributary would be considered a different reach. 2) Priority Approach. In this case, the word priority is not very informative. Regardless, the word priority refers to whether or not the basic channel design approach involves elevating the channel bed elevation by installing rock seals to allow the channel to regain access to its natural flood plain (P1), or a basic channel design approach where the existing channel elevation is not changed and a new "flood plain" is excavated (P2), or an approach where a minimum amount of flood plain area is excavated (P3). For the purpose of discussions within the monitoring group, we sometimes refer to projects as a "low grad P2", where utilities, roadways, greenways, houses, bad designs, etc. substantially limit the amount of flood plain creation. In this case, the end result is a channel that almost qualifies as a P3 and isn't really given a new flood plain, only more overall volume space through which to flow. As you might imagine, we don't get much credit for a approve may P3 projects and low grade P2's are relatively problematic because they are typically urban, sometime involve subsoil exposure, are confined to the point of not having room for vegetation community establishment, etc. If we discover a correlation between failing veg plots vs. any particular Reach category, it will most likely involve P2. A "high grade P2" is a project with plenty of flood plain area and topsoil in which a targeted vegetation community can be installed over reasonably fertile soil. 3) Mitigation Type. The three mitigation types are Restoration, Enhancement (E1, E2, and E3) and Preservation. It is becoming increasingly common for our stream restoration projects to involve sections of channel that are only approved for Enhancement credit. By definition, E1, E2, and E3 approaches do not involve changes to the more than two of the following channel parameters: pattern, dimension and profile. In other words, the enhancement approach basically involves minor reshaping of stream banks and installation of trees. We also have an increasing number of stream restoration projects involving sections of Preservation where large trees and stable banks already exist, or where riparian wetlands in relatively good shape already exist. 4) Channel Type. The publication that has served as the primary guide for stream restoration designers is "Applied River Morphology" by Dave Rosgen. This book assigns a significant amount of terminology to stream channels and outlines methods for measuring and analyzing some of the more meaningful morphological variables. There has been much debate about the extent to which the principles outlined in this book can be applied, particularly in developing watersheds with increasing hydrologic highs and decreasing hydrologic lows. When we experience project failures, they can sometimes be explained in a quantitative manner using the basic principles outlined in this book. It's a quick read, and if you don't have anything else to do, you may want to scan it. Chapter 4 is titled "Geomorphic Characterization" and outlines the basis for distinguishing between channel types Aa+ through G, which vary with regard to pattern, dimension, profile, slope, etc. In this chapter, Rosgen has basically pioneered channel classification and has attempted to establish basic channel taxonomy that can be applied to moving water occurring over a range of conditions from the slopes of mountains to the oceans. If you imagine a stream restoration project with sections of confined fast moving water flowing over artificial rock structures designed to dissipate energy vertically followed by sections of slow moving meandering water designed to dissipate energy horizontally onto the outside meanders of stream banks, you can see how a given project can incorporate different channel types.
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