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Photograph by L. Topinka
on 24 April 1981 |
An effective way to track the amount and location of erosion and
deposition in a river valley is to regularly survey a series of
cross sections across the valley floor. Because most sediment
is eroded and transported during periods of heavy rain, the surveys
should be done after significant storms. More than 150 cross-sections
were established and surveyed regularly after the eruption of Mount
St. Helens in 1980 for about 10 years, and a few are still surveyed
periodically.
During a survey, a scientist carries a surveying rod across a river valley along a pre-established cross section. At each significant change in elevation along the cross section, the scientist holds the rod on the ground until a colleague surveys and records the location. Note the "rod person" in the circle on the photograph. This image is looking across Smith Creek valley, about 10 km downstream from Mount St. Helens. |
 Photograph by L. Topinka on
26 June 1981
|
Peering into a deep canyon carved by water, a scientist uses surveying instruments to accurately measure the river channel's cross section. The person at bottom of channel is holding a surveying rod and moves across the channel to mark sites to be surveyed. Benchmarks are placed high on the river banks or valley walls so that the cross section can be surveyed again in exactly the same spot. This channel was carved by the upper Muddy River within two years after the 1980 eruption of Mount St. Helens. The cross section is located less than 2 km from the volcano's east flank. |
 Photograph by L. Topinka in
1982
|
One of the most dramatic examples of rapid channel incision and widening after a volcanic eruption occurred at Mount St. Helens in the North Fork Toutle River. In only two years, this channel was carved into the 1980 landslide deposit. The landslide filled the valley to an average depth of 50 meters for a distance of 25 km from the volcano. Here, the channel is about 350 m wide and 40 m deep. Note helicopter in upper right-hand corner of photograph. |
|
| Surveys of this cross section about 15 km downstream from Mount St. Helens record the swift erosion of material from the landslide deposit. In the first year after the eruption, flowing water rapidly cut a deep channel into the deposit. In the second year, however, most of the erosion occurred as flowing water undercut the left bank, thereby widening the river channel. |
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