9434 - Laserfiche WebLink

Home Browse Search

continuous reach map. Each line segment (arc) and enclosed polygon was assigned attributes according to what they represent on the map. An eddy in a flow pattern map, for example, is an enclosed polygon consisting of several arc segments. The segments were attributed as eddy fence or shoreline substrate type. The polygon. itself was attributed according flow conditions such as recirculating, stagnant, or downstream flow (Fig. 4). Lengths of arcs and areas of polygons were calculated, and tables containing lengths, areas and attributes were exported into a spreadsheet application for manipulation. Final maps were created and printed using ArcView ,-°-~~ software. C/zannel Cross-Sections Twenty-two channel cross-sections were established in the 4 study reaches. Cross- E~ sections were used to characterize channel geometry, identify bed material, help characterize geomorphic variability within and between reaches, and to establish long-term channel monitoring sites. Five to 6 cross sections were surveyed at about 1.6 km intervals within each of ~ the 4 study reaches. These cross sections were established in 1995 at approximately bankfull discharge using a geodetic total station and depth-recording echo sounder. Cross-sections were located near river mile markers located by Belknap (1994), so as to create a systematic sample of channel types and bed materials. Waves in rapids cause many difficulties in measuring the bed, so cross-sections were not surveyed in rapids. Cross-sections were established in small riffles, zones of strong downstream flow, pools, and eddies. Cross-section endpoints were monumented with either rebar or fence posts which were pounded to depths such that about i 0-cm are exposed above ground surface. Transects were measured by attaching alength-calibrated Kevlaz tag-line to the endpoints. The total station. was used to measure ground-surface elevations under the tag line as well as water-surface elevation. Depth of the bed was measured from a cataraft equipped with apaper-trace echo sounder and outboard motor. The boat operator maintained the boat cazefully under the tag-line while slowly crossing the river, and a second individual measured depth and tag-line position using the echo sounder at the marked locations along the tag-line. At least 4 passes were made under the tag- line, and points in the channel were also surveyed with a total station where wading was feasible. Echo-sounder depths were compared with total station measurements to gage the accuracy of the echo-sounder. Echo-sounder measurements concurred with total station measurements and ~ rarely deviated more than a few centimeters. Coordinate data from the total station was reduced E<w to distance and elevations relative to endpoints. These data were combined with the average depth recorded with the echo sounder at marked points under the tagline to create a complete ~° profile of the transect between the endpoints (Fig. 5) The standard deviation of the depths at -.f each point was calculated and used as values for the determination of error. Errors were highest where the bed had a steep slope toward the thalweg or where boulders were on the bed. These errors are reported. as the error bars on plots ofcross-sections. Photo Matching `-" Archival searches were conducted to locate historical oblique photos taken in Desolation and Gray Canyons which show the river and alluvial valley in past years. Once located, the ,, A-6