USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
River Channel Surveys - Cross Section Networks
- Mount St. Helens River Channel Surveys
- River Channel Surveys - Surveying Methods
- Toutle River Drainage
- Lewis River Drainage
Mount St. Helens River Channel Surveys
|
From:
Brantley and Topinka, 1984,
Volcanic Studies at the
U. S. Geological Survey's David A. Johnston Cascades Volcano Observatory,
Vancouver, Washington: Earthquake Information Bulletin, v.16, n.2,
March-April 1984
-
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.
River Channel Surveys - Surveying Methods
|
-
-
MSH81_USGS_hydrologists_measure_erosion_06-26-81.jpg
U.S. Geological Survey hydrologists measured changes in erosion along
the stream channels affected by the May 18, 1980 eruption.
These measurements were 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 kilometers)
southeast of the volcano's base. Rod person (lower left) and
instrument persons (upper right) give scale.
USGS Photograph taken on June 26, 1981, by Lyn Topinka.
[medium size] ...
[large size]
From:
Meyer and Dodge, 1987,
Post-Eruption Changes in Channel Geometry of Streams in the
Toutle River Drainage Basin, 1983-85, Mount St. Helens, Washington:
USGS Open-File Report 87-549
-
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.
From:
Martinson, et.al., 1986,
Channel Geometry and Hydrologic Data for Six Eruption-Affected Tributaries of
the Lewis River, Mount St. Helens, Washington, Water Years 1983-84:
USGS Open-File Report 85-631
-
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.
From:
Meyer and Dodge, 1987,
Post-Eruption Changes in Channel Geometry of Streams in the
Toutle River Drainage Basin, 1983-85, Mount St. Helens, Washington:
USGS Open-File Report 87-549
-
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
From:
Martinson, et.al., 1986,
Channel Geometry and Hydrologic Data for Six Eruption-Affected Tributaries of
the Lewis River, Mount St. Helens, Washington, Water Years 1983-84:
USGS Open-File Report 85-631
-
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
Return to:
[Volcano and Hydrologic Monitoring Menu] ...
[Hydrology and Hydrologic Processes Menu] ...
URL for CVO HomePage is:
<http://vulcan.wr.usgs.gov/home.html>
URL for this page is:
<http://vulcan.wr.usgs.gov/Monitoring/Descriptions/description_river_survey.html>
If you have questions or comments please contact:
<GS-CVO-WEB@usgs.gov>
09/22/04, Lyn Topinka