Monitoring: | Gas
| Ground
Deformation
| Hydrologic
| Remote
Sensing
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|
Rivers are like giant conveyor belts that move sediment from
volcanically disturbed landscapes and valleys to areas tens of
kilometers downstream. This sediment can fill in river channels
with excess sand and gravel, which promotes the lateral migration
of an active channel across valley floors and causes frequent
flooding during rainstorms. Sometimes, local communities and national
governments take action to either remove sediment from river channels
or build structures designed to trap the sediment as close to the
volcano as possible. Such decisions are based in part on long-term
systematic measurements of water and sediment discharges in the
affected rivers. |
|
 Photograph from U.S. Army
Corps of Engineers, 1982
|
Gray with suspended silt and sand, the Toutle River merges with the Cowlitz River about 70 km downstream from Mount St. Helens. The 1980 eruption deposited about 2.5 km3 of rock debris in the upper reaches of the Toutle River. For several years after the eruption, the Toutle River transported nearly 100 times more sediment than it did before. A significant amount of this sediment was deposited in both the Toutle and Cowlitz Rivers. |
 Dredging sediment
Sediment-retention
structure
|
To prevent flooding along the Cowlitz River, the U.S. Army Corps
of Engineers dredged sediment from the Toutle, Cowlitz, and Columbia rivers
between 1980 and 1986.
As a long-term strategy, a sediment-retention structure was built across the Toutle River in 1987 to trap sediment before it could be carried to the Cowlitz and Columbia rivers. |
Photograph by L. Topinka in
1982
|
Scientists make direct measurements of water and sediment discharge from either bridges or cable ways (shown here) spanning a river. At regular intervals along the cable way, a flow meter and a sediment sampler are lowered into the river to measure the velocity of the flow and to collect a water and sediment sample at different depths beneath the river's surface. Based on the cross-sectional area of the river, the speed at which the water is flowing through different parts of the cross section, and the amount of sediment suspended in the river, scientists can calculate the total water and sediment discharge for the time the measurement was made. These instantaneous measurements are most important during periods of high water during and after periods of heavy rain. This is when most sediment is transported downstream. |
 Gaging station along
Toutle River
|
Direct discharge measurements made from a bridge or cable way are compared to a continuous record of the river water level, called stage height. The river stage height is collected automatically by instruments in a nearby gaging station. With a continuous record of a river's stage height and instantaneous measurements of water and sediment discharge, scientists calculate the total water and suspended-sediment budget on a monthly or annual basis. These data are used to track the long-term history of sediment transport in a specific watershed and to estimate future sediment yields. |
Photograph by S.R. Brantley in 1988
|
An important volume of sediment is also transported along the bottom
of a river channel as bed load. In 1983, scientists estimated the bed
load discharge in the Toutle River accounted for about 10-20%
of the total sediment discharge. The collection and analysis of bed-load
sediment larger than gravel size, however, are difficult and costly to make.
In this image, a scientist dumps a mixture of sand and gravel collected from along the bottom of the Toutle River channel (background). The sediment was collected with a hand-held sampler. |
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