Ecology of the Mississippi River Delta Region
CHARACTERISTICS OF THE MISSISSIPPI RIVER
HOW MUCH WATER DOES THE MISSISSIPPI RIVER HANDLE?
Stream flow is measured in cubic feet per second (cfs). This measure means that one cubic foot of water passes a point each second.
Imagine a creek in your yard that is one foot wide and one foot deep. You place a string across the creek, drop a cork on the surface at the string, and
one second later mark where the cork has reached downstream. Let's say it traveled one foot. During this one second, the amount of water flowing in your
creek past the string was one cubic foot (remember, one foot wide, one foot deep, and it traveled one foot distance), so the flow rate is 1 cfs.
The Mississippi River is much bigger! Its flow rate during the year varies from over 700,000 cfs to around 200,000 cfs. Flow rates are higher in the spring (especially April) when the northern snows are melting and the spring rains abound, and they are lowest in the fall (especially September and October). See the adjacent figure "Average Monthly Flow Rates of the Mississippi River."
Most of us have trouble visualizing 450,000 cfs. I do, too, so I called my friends at the Audubon Zoo and asked how much water an elephant can
snoot into its snoot at one time. They told me the average is about 3 gal. I calculated that 450,000 cf of water is equal to 3,366,000 gal, so it would take
just over 1 million elephants per second to keep the Mississippi River drained.
This, of course, would be impossible since 1 million elephants could never
change places in a second!
Another way to visualize this amount of water is that it would take just 4 min to completely fill the Superdome! (The dome is 273 ft high and covers 9 acres. 9 ac = 391,500 sf, so the volume of the dome is 273 ft x 391,500 sf = 106,879,500 cf. 106,879,500 cf / 450,000cfs = 237.5sec, or, about 4min).
Yet one more way is to see how the flow relates to the size of Louisiana. Our state is 48,523 sq mi, or, 1.35 trillion sf. Using the average flow rate of 450,000 cfs, this expands to 106.434 trillion gal/yr. These numbers tell us that the average flow rate of the Mississippi River could cover our entire state in 10 ft of water in one year!
Look at the adjacent figure "Average Yearly Flow Rates of the Mississippi River." Now we see that although the trend is to year after year have the high water in the spring and the low in the fall, the annual averages from year to year vary enormously. This handout presents data accumulated over a 30 year (1935-1964) study. The dotted line represents the 30 year average flow rate - 450,000 cfs. This means that, over this 30 year study, the average was that each second of the 30 years, 450,000 cfs of water flowed down the river!!!
The lowest recorded flow rate was 85,000 cfs (November 4, 1939). The highest recorded flows in the Mississippi River include:
| 1927 | 2,345,000 cfs |
| 1973 | 2,261,000 cfs |
| 1983 | 2,150,000 cfs |
| 1945 | 2,123,000 cfs |
| 1950 | 2,054,000 cfs |
| 1979 | 2,005,000 cfs |
| 1937 | 1,977,000 cfs |
| 1975 | 1,927,000 cfs |
[These flow rates are based on latitude flow. Latitude flow is a designation used by the U. S. Army Corps of Engineers to designate the combined flows of the Mississippi, Red, Ouachita, and Black rivers. Imagine a line drawn across the Old River Control Structure near Red River Landing at about latitude 30° E58’N. All the water flowing across that line is contained in the measurement, regardless of its source.]
In 1945 and 1950, the year average was 700,000 cfs.
HOW MUCH SEDIMENT IS CARRIED IN THE MISSISSIPPI RIVER?
The amount of soil being carried is termed the river's sediment load. It is divided into two portions:
Though sediment loads in the river had been occasionally studied, the annual data collection and reporting began in 1949 for the Lower Mississippi, and 1951 for the Atchafalaya. The data are compiled on 26 samplings between October and September of each year. It is very important to understand that these data, collected and maintained by the U. S. Army Corps of Engineers, are extrapolations based on 26 collections, coupled with estimates of water flow in the river. They are the best information available, and the methods followed in their accumulation are rigorous. However, one should be aware of the variability that may (probably) exist in a system when several variables are estimated based on occasional samples. This is not stated to focus on a weakness, but instead to caution about the true nature of current information.
Before March 1958, the Mississippi River samples were taken in Baton Rouge. Between March 1958 and June 1963, they were taken at Red River Landing, which is located at mile 302.5 on the Mississippi River. When the Old River was closed in 1963, the sampling site was move to Tarbert Landing (mile 306, at 31E00'N), in order to stay just below the point where the Mississippi splits into the Lower Mississippi and the Atchafalaya. In the Atchafalaya, the measurements are taken at a gauge in Simmsport.
Since 1949, the total sediment load has varied from a combined low of 137,680,000 tons in 1987-88 (lowest for Lower Mississippi: 1987-88 - 79,900,000 tons; lowest for Atchafalaya: 1962-63 - 44,876,000 tons) to a combined high of 604,850,000 tons in 1951-52 (highest for the Lower Mississippi: 1950-51 - 548,330,000 tons; for the Atchafalaya: 1956-57 - 225,474,000 tons). It does, however, fluctuate greatly as shown in the following table.
Sediment Loads in the Mississippi River System
| 1951-52 | 604,840,0000 | 1974-75 | 322,743,0000 | |
| 1952-53 | 447,810,0000 | 1975-76 | 171,547,0000 | |
| 1953-54 | 161,860,0000 | 1976-77 | 138,135,0000 | |
| 1954-55 | 304,850,0000 | 1977-78 | 243,779,0000 | |
| 1955-56 | 228,395,0000 | 1978-79 | 306,686,0000 | |
| 1956-57 | 516,862,0000 | 1979-80 | 211,653,0000 | |
| 1957-58 | 540,164,0000 | 1980-81 | 165,620,0000 | |
| 1958-59 | 313,734,0000 | 1981-82 | 292,113,0000 | |
| 1959-60 | 450,112,0000 | 1982-83 | 300,296,0000 | |
| 1960-61 | 365,126,0000 | 1983-84 | 248,073,0000 | |
| 1961-62 | 415,944,0000 | 1984-85 | 289,481,0000 | |
| 1962-63 | 145,273,0000 | 1985-86 | 245,962,0000 | |
| 1963-64 | 174,288,0000 | 1986-87 | 211,230,0000 | |
| 1964-65 | 312,549,0000 | 1987-88 | 137,680,0000 | |
| 1965-66 | 263,167,0000 | 1988-89 | 148,571,0000 | |
| 1966-67 | 166,910,0000 | 1989-90 | 222,111,0000 | |
| 1967-68 | 276,928,0000 | 1990-91 | 249,510,0000 | |
| 1968-69 | 270,812,0000 | 1991-92 | 188,711,0000 | |
| 1969-70 | 224,005,0000 | 1992-93 | 289,430,0000 | |
| 1970-71 | 254,354,0000 | 1993-94 | 196,881,0000 | |
| 1971-72 | 241,753,0000 | 1994-95 | 174,774,0000 | |
| 1972-73 | 352,042,0000 | 1995-96 | 166,142,0000 | |
| 1973-74 | 340,119,0000 |
Since it is the movement of water (its energy) that supports the load, slower moving water cannot carry as much as faster moving water. Studies in the early 1960s showed that the Mississippi River's sediment load is a minimum of 0.7 tons/day/cfs at low stage in October. The maximum is 2.6 tons/day/cfs at flow rates of 700,000 cfs and above (once the 700,000 threshold was reached, increased flow rate did not result in increased sediment load). The daily average sediment load at the time was about 1.8 tons/day/cfs.
How much sediment per year? 1.8 tons/day/cfs average X 450,000 cfs average flow rate = 810,000 tons/day or about 300,000,000 tons/year.
Play time: Dirt weighs about 3375 lb/cu yd (125 lb/cu ft).
300,000,000 T = 177,777,777 cu yds
It would take 11,111,111 dump trucks (average 16 cu yd variety) to move this soil. These trucks are about 20 ft long, so if we
placed them bumper to bumper, they would form a line 42,087 miles long, or about twice around the world. That's a lot of dirt!!
Studies in 1983 indicated that the sediment load had decreased to 183,000,000 tons per year, or about half that of the 1960s. It is tempting to believe that there is a trend toward a massive decrease in the amount of soil carried by the river. Two data points, however, don't tell us much. They simply indicate that there is half the sediment load now that we measured 20 years earlier, but we don't know if it is part of a cycle that continuously occurs or if it actually is a trend. We don't know what the sediment load is now; we don't know what it was in 1943.
It will be interesting to see what happens with sediment transport during and after the great floods on the upper Mississippi during 1993.