The combination of geomorphic channel units, channel width, and minimum and maximum length criteria must be considered when the reach is selected. For example, if the presence of two examples of each of two types of geomorphic channel units results in a wadeable reach length less than 150 m, then the minimum length criteria of 150 m is established. However, if possible, the reach should incorporate whole geomorphic units. Therefore, unless the 150-m boundary of the reach is located at a transition between geomorphic channel units, then the reach length should extend to the next transition between geomorphic channel units. If more than one geomorphic channel unit is present but the length of two geomorphic channel units exceeds the recommended maximum length criteria, then the reach should be located so that nearly equal parts of the geomorphic channel units are included.
Once a boundary of the reach has been determined, a semipermanent marker is installed on a surface not subject to frequent inundation. The marker may consist of a capped iron pipe or concrete reinforcing bar driven about 60 cm into the ground. The part extending out of the ground is painted an easily seen color to facilitate location at a later date. If conditions do not permit the use of a marker driven into the ground, a hole can be drilled in an adjacent rock or tree and a standard carriage bolt inserted and painted as the marker. The length of the reach is then determined and a semipermanent marker is installed at the opposite reach boundary. Upon delineation of reach boundaries, a reach characterization of stream habitat is conducted. The proce dures used to conduct the reach characterization depend upon the type of site.
Fixed sites usually number less than 15 per study unit and are sampled continuously over time. Therefore, approaches to the collection of habitat data that allow for characterization of the status of and trends in habitat are necessary at fixed sites. To accomplish a spatial and temporal characterization of habitat, two levels of reach characterization are considered: a first-level reach characterization that provides for relatively rapid, consistent data collection within and among study units, and a second-level reach characterization that provides a balance between detailed descriptions of stream habitat and study unit flexibility in data-collection efforts.
Once the reach has been selected, the first-level reach characterization is conducted using a form such as that presented in figure 5. The instructions for completing the reach characterization form are detailed below, with the numbers corresponding to the items as presented in figure 5.