Measuring Sediment on the Move
Volcanic events fill river channels with sediment
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.
The Toutle River was transporting nearly 100 times more sediment than before the eruption
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 from Toutle River, Mount St. Helens, Washington
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.
Sediment-retention structure, Toutle River, Mount St. Helens, Washington
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.
Tracking suspended sediment--water and sediment discharge measurements
Direct measurements in rivers from bridges and cables
Scientists make direct measurements of water and sediment discharge from either bridges or cable ways spanning a river. The image to the left shows scientists measuring sediment discharge from a cableway over the Toutle River, Washington (Photograph by L. Topinka in 1982). 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.
Continuous record of a river's stage height: gaging stations
Direct discharge measurements made from a bridge or cable way are compared to a continuous record of the river water level, called stage, or gage height. River stage measurements are collected automatically by instruments in a nearby gaging station (shown left). With a continuous record of a river's stage and instantaneous measurements of water and sediment discharge, scientists calculate the total water and suspended-sediment discharge on a monthly or annual basis. By combining a continuous record of a river's stage with frequent measurements of water discharge and suspended sediment concentration, scientists calculate the total water and suspended sediment discharge to produce daily, monthly, and annual totals.
Collecting "bed load" samples from bottom of channel
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.