Characteristics of Columbia River sediment following the eruption of Mount St. Helens on May 18, 1980

OREGON

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GEOLOGICAL SURVEY CIRCULAR 850-J

TABLES

On May 18, 1980, at 8:32 a.m., Mount St. Helens, in southwestern Washington, erupted in a cataclysmic explosion. Fragmented and pulverized rock, blasted from the mountain, devastated the landscape over a fan-shaped, 200 mi2 area to the north. On the mountain, debris accumulated in the channel of the North Fork Toutle River to a thickness of 600 ft, and nearby lakes filled with lesser thicknesses of sediment. Glassy volcanic ash rose 60,000 ft into the air and drifted to the northeast. Ash deposits 10 mm (millimeters) thick occurred as far away as 400 mi. Several cubic kilometers of debris discharged from the mountain, lowering the peak by about 1,300 ft (Findley, 1981; Cummans, 1981).

Melted glacial ice, released ground water, and natural surface flow combined with the enormous quantities of debris to produce mudflows down the valleys of the North and South Forks of the Toutle River that drain to the west and in the Smith and Pine Creeks that drain to the southeast. The most extensive mudflow traveled down the North Fork Toutle River, combined with the mudflow from the South Fork, continued down the Toutle River to the Cowlitz River, and then into the Columbia River at Longview, Wash. Throughout the approximately 70-mi-long course to the Columbia (fig. 1), the mudflow flooded the river valleys and deposited vast quantities of debris in many places across the entire valley floor. At the lower end of the Cowlitz River (fig. 2), the streambed aggraded approximately 15 ft, reducing the channel capacity to roughly 15 percent of normal (Bechly, 1980).

Approximately 36 million ydof sediment, mostly ranging from clay-size particles to /2-in gravel, were deposited in the Columbia River less than 24 hours after the eruption. The 600-ft-wide by 40-ftdeep navigational channel in the Columbia River was reduced to a depth of 14 ft at the mouth of the J2 Cowlitz River, and new deposits were readily measured in a reach 7 mi landward and 2 mi seaward from the confluence (Bechly, 1980; Haeni, 1981).

In addition to the immediate devastation along the Cowlitz River and disruption of navigation in the Columbia River caused by flooding and sediment deposition, serious long-term problems now exist in the lower reaches of the Cowlitz River and in the Columbia river and its estuary from drastic alteration of the sedimentation regimen. Because of (1) increased erosion from extensive areas in the Toutle and Green River drainage basins covered by ash or by unconsolidated and (or) unvegetated surface material, (2) increased bank erosion along the channels, and (3) progressive streambed degradation, sediment loads may be substantially greater

J3 than they were in the past. The increased loads present the potential for increased deposition in the lower Cowlitz and Columbia Rivers. Hence, channel maintenance to reduce flood hazards and sustain navigation very likely will require increased dredging. Additional major mudflows would substantially compound this problem.

Deleterjous consequences in the Columbia River estuary are less obvious, but potentially serious. Mass-balance computations based on radioactivity indicate that prior to the eruption, approximately 30 percent of the sediment finer than 0.062 mm (clays and silts) delivered to the estuary became permanently deposited there (Hubbell and Glenn, 1973). Also, current and bottom-sediment transport patterns suggest that little, if any, sand or coarser sediment leaves the estuary, except possibly during extreme upland-flow or storm events (Hubbell and others, 1971). Continual deposition of unusually large quantities of sediment in the estuary for some time in the future not only will increase the normal dredging burden, but also may have important physical, ecological, and esthetic consequences from significant changes in water clarity and in depositional patterns in nondredged areas.

Because of the variety of potential problems associated with changes in the sedimentation regimen, the magnitude of anticipated changes must be forecast as accurately as possible. Engineering calculations of the competence of the rivers to entrain and transport sediment and of the effectiveness of control measures to perform their design functions, as well as other types of calculations, can be made accurately only if up-to-date information is available on the character of the sediment and its disposition in the rivers. To provide such information, the Oregon District of the Water Resources Division, U.S. Geological Survey, cooperated with the U.S. Army Corps of Engineers, Portland District, in a study of the physical characteristics of sediment deposited in the lower Cowlitz and Columbia Rivers as a result of the Mount St. Helens eruption. The primary intent of the investigation was to ascertain to what extent, if any, the "new" sediment differed from normal Columbia River sediments. In addition, the spatial distribution of suspended sediment in the Columbia River was observed at several locations to ascertain any trends that might exist.

This report presents hydrologic data collected for the investigation and discusses the findings. The information pertains mainly to the particle-size distribution, shape factor, and specific gravity of bottom sediments, although some information is given on the disposition of suspended sediment; the results are based on limited data that were collected over a very short span of time.

Field measurements and samplings for this investigation were made by Oregon District personnel, U.S. Geological Survey. Oregon District personnel also determined the concentration and particle-size distribution of suspended-sediment samples from all locations, except one on the Cowlitz River. Washington District personnel furnished sediment-discharge data for Cowlitz River mile 17.0 and water discharges for the Cowlitz and

J4 Toutle Rivers. Particle-size analyses of bed material by sieving were made by the Materials Laboratory, U.S. Army Corps of Engineers, North ' Pacific Division. Oregon District personnel analyzed the particle size of bed material by the visual-accumulation-tube method, and they determined specific gravities. The writers gratefully acknowledge the contributions of all involved groups and individuals, who worked under a demanding time schedule.

From August 11 to August 20, 1980, suspended sediment was sampled intermittently at locations in the Columbia River in the vicinity of Longview, Wash. The purpose of sampling was to determine the particle-size distribution, concentration, discharge, and spatial distribution of suspended sediment at locations upstream and downstream from the mouth of the Cowlitz River, after a moderate period of restabilization. In addition, some suspended-sediment samples were obtained in the Cowlitz River and in the two forks and main stem of the Toutle River, to define suspended-sediment discharges during the investigative period.

Depth-integrated suspended-sediment samples were collected on several occasions from cross sections at Columbia River miles (CRM) 73.5, 63.8, and 54.0, and at Cowlitz River mile 0.2; one crosssection measurement also was made at Cowlitz River mile 4.2. Except for the measurements made on August 19 and 20, 1980, all sampling in the Columbia was done at five verticals in the cross section by the equal-discharge-increments (EDI) method (Office of Water Data Coordination, 1978) using a U.S. D-77 sampler modified to operate isokinetically as a collapsible-bag sampler (Stevens and others, 1980). Near-equal-volume samples of about 2 L (liters) of water-sediment mixture were obtained at each vertical and were composited for analysis. On August 19 and 20, samples also were collected at five verticals by the EDI method, but a U.S. P-61 suspended-sediment sampler, rather than a D-77 collapsible-bag sampler, was used. Sampling at each vertical was by one-way integration over the full depth. To provide representative samples, individual samples (bottles) were obtained from depth segments not exceeding 30 ft, until the entire depth was sampled. In this process, the sampler was traversed vertically at a selected constant rate that provided a volume of water-sediment mixture from the entire vertical, which was about equal to the volume from each of the other verticals; the near-equal volumes from each vertical were composited for analysis.

Depth-integrated suspended-sediment samples from the Cowlitz River were collected at seven verticals spaced according to the equal-widthincrements (EWI) method (Office of Water Data

J5 Coordination, 1978) using a U.S. P-61 sampler; the quart-size samples were composited for analysis. Toutle River samples were obtained by wading and using a U.S. DH-48 depth-integrating hand sampler. Because of streambed instability, some measurement sections on the Cowlitz and Toutle Rivers could not be waded and sampled across their full widths. Despite this difficulty, the sampling was considered to be in accordance with the EWI method at all sections, and samples were composited for a single analysis at each section and time.

Suspended-sediment discharge-measurement data for the various cross sections and times of sampling are listed in table 1. Sediment in lowconcentration samples was divided by sieving into portions finer than 62 J.Lm (micrometers) and equalto or coarser-than 62 J.Lm, to permit the discharge of silt plus clay to be determined separately from the discharge of sand. Complete size analyses were made of samples from the Cowlitz and Toutle Rivers that contained an adequate amount of sediment. Particle-size distributions defined by the analyses are given in table 2.

Point-integrated suspended-sediment samples were obtained in the Columbia River at CRM 73.5 and 54.0 at three different times, and at CRM 63.8 on two occasions. For each measurement, duplicate quart samples were collected with a U.S. P-61 sampler, generally from five points in the depth, at

J6 a single vertical in midstream. Because of relatively low concentrations, only percentages, by weight, of sediment finer than 62 J.Lm (by sieving) could be accurately defined. These values, with other pertinent information, are shown in table 3.

Samples of bed material (bottom sediment) were collected at six cross sections in the Columbia River and at nine locations along the Cowlitz and Toutle Rivers. Except for samples collected from the Interstate Highway 5 bridge at Vancouver, Wash. (CRM 106.4), on September 17, 1980, all Columbia River sampling was done on August 12 and 13, 1980. Sampling on the Cowlitz and Toutle Rivers was done on August 11, 1980. The primary purposes for examining bed material in the Columbia River were to determine the particle-size distribution of bottom sediments in the reach

s s s s

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where sediment derived from the Mount St. Helens eruption was initially deposited and to learn if the shapes of particles that presently (August 12-20, 1980) make up the bed material in this reach are substantially different from the shapes of normal Columbia River sediments as characterized by bed material at Vancouver. Cowlitz and Toutle River samples were obtained and analyzed to define the size distributions and shapes of bed-material particles that could be transported to the Columbia.

Samples from the Columbia River were collected at CRM 106.4, 73.5, 70.0, 69.2, 67.0, and 63.8 with a U.S. BM-54 sampler. Five or more samples, generally spaced an equidistance apart across the full width, were obtained from each cross section. A midstream sample from river mile 0. 9 on the Cowlitz River also was collected with the U.S. BM-54 sampler. At the other locations on the Cowlitz and Toutle Rivers, samples were collected with scoop-type samplers; because of physical conditions, sampling was limited to the parts of the cross sections that were accessible by wading. Although the sampling on the Cowlitz and Toutle Rivers probably was not completely representative of the entire cross section, samples generally were satisfactory for the purposes of the study.

Many of the bed-material samples collected from the Columbia River near the mouth of the Cowlitz

the basis of sieve separations, roughly 21 percent, by weight, of sample material collected from the north (Washington side) two-thirds of the channel at CRM 73.5, 70.0, and 69.2 consisted of particles coarser than 2.0 mm (2,000 J.lm), whereas only about 7 percent of the material from the south onethird of the channel was of that size. A typical sample having particles coarser than 2.0 mm consisted of well-sorted material between 0.125 and

particles. Samples having a high percentage of particles coarser than 2.0 mm invariably had several very large pumice particles up to about 25 mm in diameter. At CRM 73.5, virtually all particles coarser than 2.0 mm were visually observed to be pumice. At CRM 70.0 and 69.2, roughly one-fourth of the coarser material was pumice; the remainder

J8 was fairly angular rock fragments. During sampling at several of the cross sections, pumice was observed to be floating on or near the surface.

Material collected from the lower Cowlitz River contained fine, clay-size particles. Unlike clay, however, the fine material exhibited little or no cohesion. Typically, in the lower Cowlitz, exposed sandbars were capped with a 1/2-in layer of this fine material (fig. 3).

To facilitate determinations of the shape factor and specific gravity of bed-material particles and to permit more representative comparisons of the particle-size distributions of pre- and post-eruption bed material, detailed size analyses were made only of the material finer, by sieving, than

tively excluded most of the pumice and all the large particles (erratics) from the analyses. In addition to practical reasons, this procedure seemed warranted, because the distribution of visually discernible pumice and other large particles is random and local; and the present (August 1980) disposition of pumice is a temporary condition due to its extreme mobility and susceptibility to fracturing.

Particle-size distributions of all bed-material samples are given in table 4. The size distributions

of the total sample represented by the tabulated size distribution is listed in the third column of table 4. To convert the listed percent finer values to represent the whole original sample, multiply each value by the fraction, P/100.

The portion of each sample finer than 2.0 mm was analyzed by two different methods- the visual-accumulation tube (VA) method (Guy, 1969) and sieving (S). VA analysis expresses the size distribution in terms of fall diameters; the distribution indicates the percentage of particles, by weight, that have fall diameters less than the indicated diameter (size). The fall diameter of a particle is defined as the diameter of a sphere that has a specific gravity of 2.65 and the same standard fall

J9 velocity as the particle. The standard fall velocity of a particle is the average rate of fall that the particle would finally attain, if falling alone in quiescent distilled water of infinite extent, at a t~mper ature of 24 oc (degrees Celsius) (Inter-Agency Committee on Water Resources, 1957). The fall diameter of a particle depends on the size, threedimensional shape, and density of the particle. Sieve analysis expresses the size distribution in terms of a linear dimension; the distribution indicates the percentage of particles, by weight, that can be oriented so they will pass through a square opening having a side dimension equal to the indicated diameter size. The sieve diameter of a particle depends only on the size and cross-sectional shape of the particle. Because sizing is based on different criteria, particle-size distributions defined by the two methods generally are different. The purpose for the dual analyses is discussed in the section on shape factor and specific gravity of post-eruption bed material. For a few samples, sizes finer than 62 #-'m were determined by the pipette method (Guy, 1969). This method also gives sizes in terms of fall diameters.

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Suspended-sediment discharge data presented in table 1 have been arranged in table 5 to facilitate comparisons of suspended-sediment discharges of

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material coarser and finer than 62 J.Lm at various locations in the reach between CRM 73.5 and 54.0. Although the data are sparse and partly estimated, several general conclusions can be made about the disposition of suspended sediment in the Columbia River. All suspended-sediment data suggest that, during the data-collection period, sediment was

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scoured from the streambed immediately downstream from the mouth of the Cowlitz River (CRM 68.0). It is not possible to determine from the data how far downstream the degradation occurred; however, a significant amount of sediment was scoured from the reach between CRM 68.0 and 63.8, as evidenced by the consistently higher suspended-sediment discharges, on all days, at CRM 63.8 than at CRM 68.0 (table 5). Conversely, the general decrease in suspended-sediment discharges from CRM 63.8 downstream to CRM 54.0 during the same period suggests that suspended matter was being deposited in that reach (table 5). Because the decrease in suspended-sediment discharge between CRM 63.8 and 54.0 is roughly half the increase between CRM 68.0 and 63.8, apparently the Columbia is capable of transporting the suspended sediment presently being discharged by the Cowlitz River, but it cannot sustain the seaward transport of the additional burden of the total amount of sediment scoured upstream from CRM 63.8. Although the Columbia River is capable of transporting all the suspended sediment now being supplied by the Cowlitz River, this might not be true if suspended-sediment inflow from the Cowlitz became significantly higher.

The general absence of suspended sand at Cowlitz River mile 0.2 suggests that the quantity of bedload discharged into the Columbia River from the Cowlitz during the data-collection period was relatively small. Also, because hydraulic conditions are not greatly different, it seems unlikely that bedload discharges in the Columbia River upstream from CRM 68.0 would be substantially larger than those at CRM 54.0. For these reasons, the scour and fill sequence suggested by the suspended-sediment discharge data probably can be considered indicative of actual net degradation and aggradation.

Point-integrated suspended-sediment samples were collected at CRM 73.5, 63.8, and 54.0 (table 3) to provide information on whether the vertical distribution of suspended sediment downstream from the mouth of the Cowlitz River is unusual in any way because of the presence of deposited eruption-derived debris. The data were analyzed within the framework of conventional suspended-sediment theory.

J12 Under conditions of moderately steady flow, the vertical distribution of suspended sediment in a narrow size range can be approximately characterized by

where Cy and Ca are the concentrations of suspended

u.

For any height, a, with a given distribution and

total depth, Ca and (-a-)z are constants,

and equation 1 can be written as

The exponent Z in equation 2 can be determined for measure distributions by plotting values of ey, defined from point-integrated samples collected at various heights, y, above the bed, against corresponding values of (d- y)/y on logarithmiccoordinate graph paper; Z is the slope of the straight line defined by the points. Low values of Z result when the concentration of the size range is nearly uniform (constant) throughout the depth, and high values occur when the concentration is much higher near the bottom than it is near the surface.

Data listed in table 3 for material finer than 62 I-'m and coarser than 62 I-'m were used to compute values of Z (table 6). Because of generally low concentrations and tide effects, comparisons among data for different days and different locations are inconclusive. However, by comparing concentrations at CRM 73.5 at all levels throughout the depth with those at CRM 63.8 and 54.0 (table 3), it sediment in the size range at heights of y and a, respectively, above the streambed; is the total depth of flow at the vertical; and is an exponent that theoretically equals VJk u.

is the fall velocity of particles in the size range; is the von Karman coefficient for turbulent exchange; and is the shear velocity. (Hubbell and Glenn, 1973) are plotted in figure 4 to represent pre-eruption conditions. None of the pre-eruption samples from CRM 64 and fewer than 15 percent of the samples from CRM 59 or CRM 54 contained particles coarser than 2.0 mm; hence, the data essentially are comparable with the data in table 7. Arbitrary lines, based on the pre-eruption data, are drawn to delimit the range in which M+ values would be expected to occur along the length of the Columbia between river miles 106.4 and 14.0. Post-eruption data from table 7 also are plotted in figure 4.

Computed average M+ values of material presently in the river at measurement cross sections close to the Cowlitz River mouth generally lie outside the expected range (fig. 4) for pre-eruption conditions. Upstream, at CRM 70.0 and 69.2, the material appears to be significantly coarser (about 0.8 phi unit) than the pre-eruption bed material. Immediately downstream from the confluence, there is an abrupt change in the mean particle size. M+ increases, roughly, from 1.0 to 2.3, which indicates that the mean size of present bed material downstream from the confluence is less than half the mean size of material upstream from the confluence (0.20 mm compared to 0.50 mm). Material downstream from the confluence is roughly 0.5 phi unit finer than pre-eruption material in that reach.

Whereas material in the navigational channel generally is coarser than that on the marginal slopes and adjacent flats, the reduction in mean size at CRM 63.8 from pre-eruption material is fairly consistent over the entire width (see Hubbell and Glenn, 1973, p. L53). The increase in fine material downstream from the confluence probably is due to the continual influx of significant amounts of fines from the Cowlitz River, as well as the presence of remnant material from the mud-

J14 flow. The slight downstream decrease in size between CRM 67.0 and 63.8 suggests that fine material being entrained immediately downstream from CRM 68.0 (see table 5) may be depositing in the vicinity of CRM 63.8 and downstream from that point.

Average o+ values at all cross sections except CRM 63.8 show that the relative range of particle sizes finer than 2.0 mm included in the present bed material is virtually the same as it was in the past, even though M+ values are different. At CRM 63.8, the material in the north half of the channel is composed of an unusually wide range of particle sizes.

Because of the large quantities of debris that were created through violent explosion and pyroclastic activity, the potential existed for much of the sediment delivered by the Cowlitz River to the Columbia to be substantially more angular and of a different specific gravity than normal Columbia River sediments. To ascertain differences between the "old" and "new" bed material, specific gravities were measured in the laboratory, and particleshape factors were estimated from empirical relations between fall and sieve diameters.

Specific gravities of selected samples were determined by the standard pycnometer procedure (American Society for Testing and Materials, 1969). In this procedure, specific gravity (sp gr), is computed from where Ws is dry weight of sediment introduced into

Ww is weight of distilled water at tem-

Material coarser (by sieving) than 2.0 mm was excluded from the specific gravity determinations as it had been from the particle-size analyses. The purpose of rejecting this material was to eliminate large lightweight pumice particles and other erratics. Visual observation of the samples revealed that pumice particles coarser than 2. 0 mm occurred in random quantities, and that many samples contained no pumice particles of that size.

Results of the specific-gravity determinations are given in table 8. With the exception of several samples that visibly contained fine pumice (see "Remarks" in table 8), measured specific gravities of bed material from the Columbia River at anstl below CRM 73.5 averaged 2.63 and had a standard deviation of 0.07. Similar specific gravities were measured for the bed material in the Cowlitz and Toutle Rivers. Interestingly, the two suspended-sediment samples that were analyzed had lower specific gravities of about 2.57 (table 8). On the the pycnometer,

perature, T, required to fill the pycnomete:r:,

at temperature, T, in the filled pycnometer.

J16 basis of these determinations, it seems that the specific gravity of Columbia River bed material finer than 2.0 mm has not changed significantly, and a nominal value of 2.65 can be used to characterize the bottom sediments in the Columbia, lower Cowlitz, and lower Toutle Rivers.

The validity of computing shape factors in this manner is contingent on the accuracy of figure 5 and the extent to which the following conditions are satisfied:

1. The specific gravity of individual particles
2. The "ordered" position of each particle relative

When condition 2 is satisfied, the same particles are in the same relative weight ranges, regardless of the method of analysis. Because of this equivalency within any selected relative weight range, representative fall and sieve diameters for any range can be used with the relations in figure 5 equals 2.65.

to every other particle is exactly the same whether ordering is based on standard fall velocity, which establishes the fall diameter, or on controlling cross-sectional area, which establishes the sieve diameter. That is, the particle having the lowest standard fall velocity (smallest fall diameter) also has the smallest controlling cross-sectional area (smallest sieve diameter), the particle with the second smallest fall diameter has the second smallest sieve diameter, and so forth.

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Fall diameter, in m m

mined from the largest and smallest fall diameters defined from the VA analysis. The two geometricmean diameters for each defined size range were used to obtain a shape factor from figure 5. These shape factors, in turn, were compared with shape factors given in table 7 for the same sample. Because particle sizes represented in the individual size separates did not necessarily coincide closely with sizes represented by the percentile ranges listed in table 7, only nine computed shape factors

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were truly comparable to values in the table. The purpose of the comparisons was to determine if shape factors computed from the entire sample (S.F. -1 values), under the assumption that condition 2 was satisfied, agreed with shape factors computed from size separate data (S.F. -2 values) for which condition 2 was known to exist.

Statistical analysis of seven possible comparisons from the Columbia and Cowlitz River samples indicates a correlation coefficient of 0. 71 notation), is essentially the same as it was prior to the eruption.

Specific-gravity determinations of bed material finer than 2.0 mm that was sampled at and downstream from CRM 73.5 averaged 2.63 and had a standard deviation of 0. 07. From these determinations, it seems that the specific gravity has not changed significantly from pre-eruption conditions and that 2.65 can be taken as a nominal specific-gravity value.

Shape factors determined by an indirect means indicate that variations in the shape of bed-material particles are much greater within a cross section than among different cross sections throughout the reach of the Columbia River from mile 106.4 to mile 63.8. Similarities in shape factors of material from CRM 106.4 and from cross sections downstream of CRM 73.5 imply that there are no large differences between the shape of preand post-eruption surficial bed material. The majority of shape factors are between 0.5 and 0.8.

Reasonable uniformity in the specific gravity of bed material finer than 2.0 mm and in mean shape factors from one cross section to another suggests that, for one-dimensional sediment transport computations and gross dredging or earthwork estimates involving bulk density, no corrections need to be made for unusual bed-material properties.

Sediment-transport and channel conditions observed during the 9-day, mid-August, 1980, data-collection period of this investigation can be expected to change as the Columbia, Cowlitz, and Toutle River systems, aided by man's dredging and river maintenance activities, tend toward a new quasi-equilibrium (sedimentation regime). Additional data, similar to the data in this report, collected periodically could be used to assess the rate of change.

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