DESCRIPTION:
Sediment Sampling

During the water years 1981-1983, the average annual suspended-sediment yield measured at Toutle River at Tower Road was 74,000 tons/sq.mi. That amount was greater than the sediment yield from any other nonglaciated drainage basin of similar size in North America, including the high sediment-yield streams of northern California. The sediment has been largely derived from materials that were deposited on approximately one-third of the 512 sq.mi drainage basin by the May 18, 1980 eruption of Mount St. Helens.

The Lewis River basin had erodible sediments emplaced by the same eruption. Sediment yields for the Muddy River, Clearwater Creek, Pine Creek, and other Lewis River tributaries have increased from pre-eruption levels. ...

Sediment data were collected on a weekly or biweekly schedule by U.S.Geological Survey personnel at gaging stations during normal river flow. Storm runoff usually required a more frequent sampling schedule to adequately define the change of suspended-sediment concentration with time, so samples were generally collected at intervals of one hour or less during many storm periods. The field techniques used for collection of sediment data are standard Geological Survey procedures as described by Guy and Norman (1970).

Types of Sediment Data Collected

The types of sediment data that were routinely collected are listed below.

1. Cross-sections of suspended-sediment samples collected by the EDI (equal-discharge increment) method or by the EWI (equal-width increment) method;

2. Single-vertical samples collected by Geological Survey personnel;

3. Single-vertical samples collected by observers (private citizens instructed by Geological Survey personnel in collection of sediment samples);

4. Fixed-point samples collected by automatic pumping samplers;

5. Bed material samples collected at several verticals (usually determined by the EDI method) in conjunction with suspended-sediment measurements.

Methods of Sediment Analysis

Concentration analyses were performed by the Tacoma and Vancouver sediment laboratories, and particle size analyses were performed by the Sacramento, Tacoma, and Vancouver laboratories. Concentration and particle-size analyses were performed according to methods described by Guy (1969).

Sand-break Analysis:

Sand-break analyses of suspended-sediment samples give the percent of sediment by weight that is finer than 0.062 mm. Sand breaks were performed occasionally for samples collected during October through December 1980. Beginning in 1981, sand breaks were performed for all Mount St. helens samples where feasible. Sand breaks were usually determined by wet sieving.

Particle-size Analysis:

Particle-size analyses give the percent of sediment by weight that is finer than a given size class. Pipet analysis was used for the suspended-sediment fraction finer than 0.062 mm. Wet sieving or visual-accumulation tubes were used for the fraction coarser than 0.062 mm. Samples were analyzed in distilled water and were chemically dispersed.

Cross-section samples from suspended-sediment measurements were generally separated into two representative sets for a specific time. In most cases, one set was analyzed for particle size while the other was analyzed for sand break. There are some instances where single-vertical samples were analyzed for particle size.

Bed material samples were usually analyzed by dry sieving the material, although some wet sieve analyses were performed on bed material samples in 1981.

The high concentration sediment samples from the March 19-20, 1982 lahar-runout flow were analyzed in the same manner as suspended-sediment samples. Observers' samples and automatic pumping samples were usually analyzed for sediment concentration only (no particle size or sand break).

Return to:
[Volcano and Hydrologic Monitoring Menu] ...
[Sediment and Erosion Menu] ...
[Hydrology and Hydrologic Processes Menu] ...