Table of Contents

Figure 1. -- Average concentration of suspended sediment in rivers and average discharge of suspended sediment at the mouths of selected rivers of the conterminous United States. Map was simplified from Rainwater, 1962. See Table 1 for the ranking of rivers.

Figure 1. -- Average concentration of suspended sediment in rivers and average discharge of suspended sediment at the mouths of selected rivers of the conterminous United States.

Figure 2. -- Average concentration of suspended sediment in rivers and average discharge of suspended sediment at the mouths of selected large rivers of Alaska. (Source: Compiled by R.H. Meade from U.S. Geological Survey data, including reports by Burrows and Harrold, 1983; Knott and Lipscomb, 1983; and Scott, 1982.)

Figure 2. -- Average concentration of suspended sediment in rivers and average discharge of suspended sediment at the mouths   of selected large rivers of Alaska.

SPATIAL PATTERNS OF SEDIMENT CONCENTRATION IN THE UNITED STATES

In the conterminous United States (fig. 1), the patterns of suspended-sediment concentration reflect such influencing factors as climate (especially rainfall) and the properties if the rocks and soils that are exposed to erosion. In the Eastern and Northwestern States, suspended-sediment concentrations generally are low, except in two areas: parts of western Mississippi, western and central Tennessee, Illinois, and Iowa that are underlain by loess (easily erodible windblown silt deposits) and southwestern Washington, where the recent eruption of Mount St. Helens has added large quantities of sediment to the lower Columbia River stream system.

On the High Plains of South Dakota, Colorado, Oklahoma, Texas, and New Mexico, consistently large concentrations of suspended sediment are the result of a combination of easily eroded sedimentary rock and relatively little protective vegetation. Although intense rainfall events on the High Plains are frequent enough to cause significant erosion, the total amount of precipitation is too small to allow the development of the kind of vegetation that would protect the soil from erosion (Langbein and Schumm, 1958). Similar combinations of erodible soils and sporadic, but intense, rainfall also account for most of the large concentrations of suspended sediment in rivers in the Southwestern States.

Mean annual suspended-sediment loads discharged to the oceans, in millions of tons per year, are portrayed in figure 1 by half-circles at the mouths of selected rivers. These sediment loads, which are averages as of 1980, reflect a number of artificial influences, not the least of which is the interruption if the down-river flow if sediment by dams and reservoirs. The dominance of the Mississippi River, as a mover of sediment is readily apparent. In spite of the large dams that have been built across its major tributaries, the Mississippi River still ranks sixth or seventh in the world in suspended sediment discharge to the oceans (Milliman and Meade, 1983, p.2). Next in rank in the conterminous States is the Columbia River which is shown in figure 1 with two different values of suspended sediment discharge: 10 million tons per year (ton.yr), the average load transported before the 1980 eruption of Mount St. Helens, and 40 million ton/yr, the estimated annual suspended sediment load transported after the eruption. Although over 140 million tons of suspended sediment from Mount St. Helens was discharged by the Cowlitz River into the Columbia River in the first 4 months after the eruption, this discharge has decreased considerably in the last few years. The additional sediment attributed to Mount St. Helens has declined to about 30 million ton/yr.

Less information is available on the concentration and discharge of suspended sediment in the rivers of Alaska (fig. 2). It is reasonably certain, however, that suspended sediment concentration are low in the rivers of northern and western Alaska. Sediment concentrations are larger in south-central Alaska, where glaciers erode the mountain slopes and glacial melt-waters carry large sediment loads, but these concentrations still are not as large as those in the arid and semiarid parts of the western conterminous United States. The present-day sediment discharges if three rivers that drain the glaciated peaks if the Alaska Range, the Copper, the Yukon, and Susitna rank, respectively, second, third, and fourth, among the rivers of the United States.

Ten rivers of the United States that are important by virtue of their large sediment discharges or their large drainage areas are listed in table 1. The sediment discharges of the Mississippi River, the Rio Grande, and the Colorado River have diminished by dams and reservoirs. Although it discharges three-quarters as much water to the ocean as the Mississippi River, the St. Lawrence River carries relatively little sediment because the Great Lakes act as natural sediment traps.

Suspended-sediment discharge of major rivers


Table 1. Discharge of suspended sediment to the coastal zone by
10 major rivers of the United States, about 1980 (tons/yr = tons per year) 
(Meade and Parker, 1984).

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Rivers                                                     Average annual      
                                                          sediment discharge  
                                                        (million tons/yr)
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Rivers that discharge the largest sediment loads:          ---                 

     Mississippi (includes Atchafalaya River)              230                 

     Copper                                                80                  

     Yukon                                                 65                  

     Susitna                                               25                  

     Eel                                                   15                  

     Brazos                                                11                  

     Columbia:                                             ---                 

          Before Mount St. Helens eruption                 10                  

          (Since Mount St. Helens eruption -- approximate  40                  

Rivers with large drainage areas:                          ---                 

     St. Lawrence                                          1.5                 

     Rio Grande                                            0.8                 

     Colorado                                              0.1                 

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