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|Title:||Characterization of the suspended-sediment regime and bed-material gradation of the Mississippi River Basin, Volume 1|
|Authors:||United States. Army. Corps of Engineers. New Orleans District.|
United States. Army. Corps of Engineers. Lower Mississippi Valley Division.
Keown, Malcolm P. (Malcolm Price)
Dardeau, Elba A.
Causey, Etta M.
Mississippi River Basin
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Report (Potamology Program--P-1 (U.S.)); 1.|
Abstract: The Mississippi River drains an area in excess of one million square miles, covering part or all of 31 states and 2 Canadian provinces. Even prior to settlement the main stem was probably a heavy sediment carrier due to the character of the climate and soils in the basin. If the stream had been left to itself, the Mississippi would probably be building a new main-stem delta in what is now the Atchafalaya drainage. Several cultural impacts over the past two centuries have shaped the current character of the Mississippi main-stem suspended-sediment regime: (a.) Much of the basin that once had been primarily forest and grasslands was turned to agricultural activities. (b.) The Old River Control Structures became operational in 1963 preventing unregulated flow from the Mississippi to the Red-Atchafalaya system. (c.) Sediment-retention structures were constructed (1953-1967) and channel improvement features were placed on the Missouri River and its tributaries. (d.) Sediment-retention structures were constructed (1963-1970) and channel improvement features were placed on the Arkansas River and its tributaries. In addition, improved land-use management practices and the placement of numerous streambank protection works and sediment-retention structures on high-order streams throughout the Mississippi River Basin have undoubtedly reduced main-stem suspended-sediment loads, although these impacts are difficult to quantitatively assess. The U.S. Army Engineer District, New Orleans, regards the suspended-sediment sample collection station at Tarbert Landing, Miss., as monitoring the load of the Mississippi main stem that is available for transport to the Gulf of Mexico; the station at Simmesport, La., reflects the current best estimate of the suspended-sediment load that is passed into the Gulf from the Atchafalaya River. Thus, the sum of the loads at these two stations (Tarbert Landing and Simmesport) can provide an approximate value for the sediment yield of the entire Mississippi River Basin, although not accounting for the bed load passing these two stations. The estimated annual yield of the basin is nearly 900 million tons. This estimate was made on an areal basis and does not reflect measures that have been taken to minimize sediment loads; thus, 900 million tons more properly represents the potential areal yield, rather than the actual yield, of the basin. Prior to 1963, the sum of the loads measured at Tarbert Landing and Simmesport was 434 million tons per year; however, the value has currently declined to 255 million tons per year. This reduction reflects the positive benefits of numerous upstream channel improvements made by the U.S. Army Corps of Engineers, as well as improved land-use management techniques. In the reach of the Upper Mississippi River main stem upstream from its confluence with the Missouri River, commercial dredge operators report that the bed materials have retained essentially the same gradation over the past 50 years, consisting of more than 90 percent sand. Of the total fraction less than 5 percent is silt; however, 75 percent will usually pass a U.S. Standard No. 16 sieve (1.19 mm). Thus , the major component of the bed material above the confluence is fine to medium sands. The reach of the Upper Mississippi downstream from the confluence is greatly influenced by the coarse bed-material input of the Missouri River (approximately 70 per cent of this material is medium to coarse sand). Based on information furnished by commercial dredgers, the current bed-material gradation of the Lower Mississippi River immediately downstream from Cairo, Ill., can best be described in terms of the coarse to fine sand input from the Upper Mississippi and the coarse sand input from the Ohio River. Bed-material samples collected in 1932/1934 by the Mississippi River Commission (MRC) indicate that the gravel/sand/silt fractions below the confluence were 20, 72, and 8 percent, respectively, thus supporting the theory advanced by dredgers that gravel once was present in the bed material near the confluence, but has since declined to quantities that are not economically recoverable. Long-term trends in the bed-material gradation of the Lower Mississippi downstream from the mouth of the Arkansas River can be evaluated by comparing the 1932/1934 data developed from samples collected by MRC with post-1965 data provided by the U.S. Army Engineer Districts, Vicksburg and New Orleans. Examination of these data indicate that the gravel/total sand/silt fractions have remained relatively constant from Arkansas City, Ark. , downstream to Tarbert Landing, Miss.; however, from Donaldsonville, La., to the Gulf of Mexico there has been a pronounced shift from the sand to silt fraction. In 1932/1934, 92 percent of the bed material near the Gulf was sand, whereas currently the material is one-third sand and two-thirds silt. The range between the D84 and D16 grain sizes has decreased from Arkansas City downstream to Tarbert Landing, indicating that the bed material has tended to become more uniformly graded through this reach; the D50 grain size has remained relatively constant. From Donaldsonville downstream the D84, D50, and D16 grain sizes have all decreased between 1932/1934 and the present, further supporting the conclusion that the bed material through this reach is becoming finer. NOTE: This file is very large. Allow your browser several minutes to download the file.
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