Setting Up an Aquatic AIM Sample Reach

This reach set up video covers what an Aquatic AIM sample reach is, how a sample reach’s length is determined, and how to set up a sample reach. You can find reach set up information in the 1735-2 Field Protocol on pages 18-24. Each year, hundreds of Aquatic AIM sites are sampled throughout the western United States. All of these data are collected within a standardized unit referred to as a sample reach. A sample reach is the total length of a stream where data are collected. Each sample reach is located by a set of GPS points, called the point coordinates, that are the center of the sample reach. Data are collected throughout the reach at 11 main transects and 10 intermediate transects evenly spaced throughout the reach An Aquatic AIM sample reach is designed to capture the variety of habitat complexity that exists within each stream and river system. For example, small streams that have small bankfull widths contain instream habitat, such as pools and riffles, which may vary dramatically over a relatively short distance. In large streams with larger bankfull widths, a pool or riffle may extend for many meters. Therefore, to capture the complexity for each system, the length of an Aquatic AIM sample reach is directly based on a stream or river’s average bankfull width. This means that reach length sampled in this protocol scales commensurately with stream width. The average bankfull width is multiplied by 20 to calculate the reach length. However, regardless of the streams size, reaches must be a minimum of 150 m long to adequately capture habitat diversity. Therefore, the minimum reach length is 150 meters long. The the maximum reach length is 4 kilometers long because this allows for the capture of habitat complexity, but ensures that a site can still be sampled in a reasonable amount of time. For small and medium sized streams with an average bankfull width of 7.5 meters or less, reach length will always be 150 m. For larger streams where average bankfull width is greater than 7.5 meters and less than 200 meters, reach length is 20 times the average bankfull width of the reach. For large rivers where average bankfull is greater than 200 meters, the reach length will always be the maximum reach length which is 4 km. The first step to setting up a sample reach is to walk up and down the stream to examine the stream reach for indicators such as bankfull, floodplain, scour line, and thalweg. Once these features are confidently identified, the bankfull width, or distance between bankfull on the left side and bankfull on the right side of the stream, is measured at five locations. These five measurements will determine the length of the reach. These locations should be areas that have a typical width and are not places where the stream is uncharacteristically wide or narrow. It is also best practice to take these measurements on straight sections of the stream and not on meander bends. Once the five typical bankfull width locations are identified and then measured, they are averaged to determine the reach length. For example, the average bankfull at this reach is 2.03 meters. 2.03 meters times 20 is 40.6 meters, which is less than the minimum reach length of 150 m. If the average bankfull is less than 7.5 meters, the reach length will be 150 meters. In contrast if the average bankfull width is greater than 7.5 meters, let’s say 10.02 meters like on this stream, the reach length will be 20 times 10.02, or 200.4 meters. Once reach length is determined, the reach can be set up. A sample reach contains 11 main transects and 10 intermediate transects equally spaced throughout the reach. Main transects are labeled A through K with A being the most downstream transect and K being the most upstream transect. A and K are the barriers of the reach and define the top and bottom of the sampling area. The original point coordinates correspond to the middle transect, transect F. This point remains the center of the sampled section of river or stream. Main transect spacing is calculated by taking the total reach length and dividing by ten. For example, in a small stream with a reach length of 150 meters, main transects will be 15 meters apart. In a larger stream that has a total reach length of 400 meters, main transects will be 40 meters apart. Intermediate transects are located ½ way between main transects To locate the intermediate transects, divide the transect spacing by half and place them between each main transect. For example, a stream with 40 meter spacing between main transects would have intermediate transects located every 20 meters. If thalweg is being collected, intermediate transects can also be located during thalweg collection. Starting from F and moving up or down stream, transects are measured along the thalweg and using a measuring tape. The tape is allowed to follow the thalweg and is not held taught. This may require allowing the tape to rest in the stream flow or is woven through vegetation. Each transect is then marked with two flags labeled with their corresponding transect letter, one on each bank. This process is continued up or down the reach until the top or bottom half of the reach is completely set up. After returning to the F transect, the same process is followed, but this time in the opposite direction from the center of the reach. When a transect is setup, it is important that each set of transect flags is placed so that an imaginary line connecting them perpendicularly intersects the thalweg. Where should the left bank flag in this photo be placed so that the transect is perpendicular to the thalweg? Option A is the correct answer. Once the reach is set up and the transect flags are placed, Aquatic AIM sampling can begin.

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