Surveying River Channels
Tracking erosion and deposition
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.
How we survey a river channel
Photograph by L. Topinka on 26 June 1981
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.
Regular surveys across river valleys help to track areas of erosion and sedimentation
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.
