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Second-level reach characterization

A second-level reach characterization also is conducted at all fixed sites. This is a detailed reach characterization and is designed to provide additional quantitative data on geomorphic and hydraulic properties that are critical to the evaluation of temporal changes in the environmental setting and stream habitat. The second-level reach characterization consists of an analysis of hydraulic properties and channel geometry plus additional components tailored to enhance an understanding of temporal changes. The analysis of channel geometry consists of longitudinal profiles of the water surface, flood plain, and channel bed; cross-sectional surveys with levels; a map of the reach; and a quantitative analysis of bed and bank materials. Additional suggest ed components of the second-level reach characterization include permanent plot vegetation analysis and detailed quantitative mapping of habitat features throughout the reach. Study unit personnel are responsible for developing an appropriate form for re cording the second-level reach characterization.

The longitudinal profile of the channel bed is conducted along the thalweg (or the approximate center of the channel if a thalweg is not apparent) on the basis of channel-bed elevations recorded at intervals equal to one channel width. This distance is generally sufficient to determine the mean slope of the reach (Emmett, 1975). The water-surface profile can be determined simultaneously by having the rodman record the water depth at each location and add this value to the channel-bed elevation. Profiles of the flood plain along both banks also are conducted. In nonwadeable reaches, longitudinal profiles of the channel bed are determined using a hydroacoustic depth meter, and water-surface elevations are determined along one bank or both banks.

At a minimum of three locations (both reach boundaries and a location that includes a prominent geomorphic feature), leveled cross-sectional surveys are conducted from left flood plain to right flood plain. Each cross-sectional survey is plotted, with elevation recorded on the ordinate axis and distance in meters along the abscissa. All surveys are conducted in relation to the reference location. A map of the reach is constructed, indicating the locations of the longitudinal profiles and the cross-sect ional surveys. Cross-sectional surveys of nonwadeable reaches include as much information as can possibly be recorded.

In addition to an analysis of channel geometry, a quantitative analysis of channel substrate particle size is conducted. Pebble counts (Wolman, 1954) are conducted to determine bed material particle-size distribution in wadeable reaches. At the three surveyed cross sections, a pebble-count transect is established, and the pebble count is conducted in the following method:

(1) Begin the count at each transect at bankfull elevation on the left bank and proceed to bankfull elevation on the right bank.

(2) Proceed one step at a time, with each step constituting a sampling point.

(3) At each step, reach down to the tip of your boot and, with your finger extended, pick up the first pebble-size particle touched by the extended finger.

(4) To reduce sampling bias, look across and not down at the channel bottom when taking steps or retrieving bed material.

(5) As you retrieve each particle, measure the intermediate axis. If the intermediate axis cannot be determined easily, measure the long diameter and the short diameter of the particle, and determine the average of the two numbers.