DESCRIPTION:
River Channel Surveys - Cross Section Networks

The debris avalanche that triggered the (Mount St. Helens) eruption slid north into Spirit Lake and west 25 kilometers down the North Fork Toutle River valley, covering the valley floor with unconsolidated debris to an average depth of 45 meters and as much as 180 meters in some places. Rapid erosion resulting from the breaching of numerous ponds and lakes on the deposit and surface runoff have produced a new drainage system on the avalanche. Streams following the initial drainage pattern quickly eroded narrow channels because of the generally steep slopes and the readily erodible character of the avalanche deposit. Channels more than 300 meters wide and 45 meters deep have been carved by the new North Fork Toutle River. Nearly 4 years after the devastating eruption, erosion rates remain high, and the channels display complex, alternating scour-and-fill sequences. ...

More than 150 cross-sections of river channels are surveyed regularly to determine areas of erosion and deposition along rivers draining Mount St. Helens. These repetitive surveys measure bank and channel erosion and channel deposition at specific locations. Repeated aerial photographs also are used to identify sediment sources and sinks. In many places since the 1980 eruptions, channel modifications have been equal to or greater than those resulting directly from the damaging lahars on May 18. Generally, erosion and sediment transport by channel widening and downcutting dominate the upper reaches of the drainage basins, and aggradation and sediment transport dominate the lower reaches.

[Image,264K,GIF]
U. S. Geological Survey hydrologists measure changes in erosion along the stream channels affected by the May 18, 1980 eruption. These measurements are used to study the erosion processes and to estimate the severity of the sedimentation problem. The view here is along the Muddy river drainage, approximately one mile (1.5 km) southeast of the volcano's base. Rod person (lower left) and instrument persons (upper right) give scale.
-- USGS Photo by Lyn Topinka, June 26, 1981

Cross sections usually span the valley floor, and are located by establishing a line of sight perpendicular to the channel, or perpendicular to the valley floor where the channel is braided. Because of severe channel shifts, cross sections may not remain perpendicular to the channel. Endpoints are monumented with metal fenceposts or sections of steel reinforcing bar. Channel cross sections are usually surveyed using electronic distance measuring (EDM) equipment and theodolite. The instruments are set up on the line with the monuments, which serve as reference marks for horizontal and vertical control. Deep gullies or vertical banks occasionally preclude precise surveying measurements within the available time, and a few points on the cross section are measured with hand level and tape. In cases where the channels cannot be waded on line with the cross section, estimates of maximum depth of the channel are made. During high flows of winter and spring, channels along much of the Toutle River and the lower North Fork Toutle River are surveyed using an air boat. The above-water portions of the cross section, and portions of the section that can be waded are surveyed as described above. Measurements of horizontal distance to the boat from reference monuments are made using either an EDM, or a tag line. Depths are sounded manually from the boat. Cross sections that are located at bridges are often measured by sounding from the bridge. Commonly, this entails use of a bridgeboard, A-reel, and 35 or 45 kg sounding weights.

Field data are entered into a computer, converted to relative horizontal distance and elevation, and referenced to monuments with fixed station distance and elevation. Elevation is referred to National Geodetic Vertical Datum (NGVD) of 1929. Surveys are plotted using digital plotters, and superimposed on earlier surveys of the same cross section.

Accuracy and precision of surveys are dependent on the nature of the channel, weather at the time of the survey, and surveying techniques. Surveys of wide channels are not as precise as surveys of narrow channels. Selection of surveyed points introduces a large degree of uncertainty in reproducing rough surfaces that have not changed. Inadequate visibility and atmospheric temperature and pressure fluctuations affect the accuracy of EDM measurements, and wind and saturated ground occasionally cause difficulties in precise leveling. When at least two monuments remain from previous surveys to establish the cross-section bearing, and points that are common to previous surveys are measured consistently, both horizontal and vertical precision is generally within 0.05 meters, regardless of adverse weather conditions.

Cross sections usually spanned the valley floor and were located by establishing a line of sight perpendicular to the channel or parallel to the valley strike. The endpoints of the cross sections were monumented with metal fenceposts or sections of steel reinforcing bar. The direction of each cross section with respect to magnetic north was measured. Channel cross sections were surveyed using electronic distance measuring equipment and a theodolite. The instruments were set up on line with the monuments, which served as reference marks for horizontal and vertical control. Where deep gullies or vertical banks occasionally precluded precise surveying, a few points on the cross section were measured with hand level and tape. Maximum depth of the channel was estimated where channels were too deep to be waded.

The theodolites used are accurate to about 6 seconds of angular measurement. The precision of electronic distance measurements is generally about 0.05 meters per kilometer and is affected by factors such as atmospheric conditions, ground moisture and stability at the instrument station, rod placement, length of shots, and steepness of terrain.

[Graphic,14K,GIF]
North Fork Toutle River and Loowit Confluence, Cross-Section NF100: comparison of November 1982 survey with July 1990 survey.

Cross sections along the North Fork Toutle, South Fork Toutle, Green, and Toutle Rivers were initially monumented and surveyed during the summer and fall of 1980. Cross section sites were selected using several criteria. Within the Toutle River drainage basin, many cross sections were located at bridge sites or other locations where pre-eruption survey data are available. One or more cross sections are usually located at or near gaging stations. Many cross sections were located to coincide with cross sections measured photogrammetrically from imagery taken during June and July 1980. Where either pre-eruption or photogrammetric cross section locations were reoccupied, or where cross sections were located at gaging stations, additional sections were added to form clusters of cross sections with a spacing of two to three sections per kilometer. Clusters of sections were used to monitor changes in pattern and slope (as well as cross-section shape) within specific reaches, and to sample different environments within the reaches.

MORE - Toutle Drainage Menu

A total of 88 cross sections was established during the summers of 1980-81 in clusters of two or more per stream reach on Muddy River, Smith Creek, Bean Creek, Clearwater Creek, Pine Creek, and Swift Creek. Clusters of two to six cross sections were established in a stream reach to provide data on a range of channel environments, such as river bends, straight reaches, pools, and riffles. Cross sections were established in clusters, rahter than in a random distribution. The clusters define study reaches where adjustments in riparian conditions, channel slope, channel pattern, and streambed material could be studied in conjunction with changes in cross-section geometry. Cross sections also were located above and below the confluence of major tributaries. Usually, one or more cross sections were located at or near pre-eruption stream gaging stations and at bridge sites, where pre-eruption survey data are available.

MORE - Lewis River Drainage

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