This fact sheet shows the location and ranking of the 20

largest rivers in the United States. It is common knowledge that the Mississippi is the largest U .S . river, but what is the rank of other major U

.S . rivers? Rivers areconsidered large on the basis of one or more of three characteristics: total length from source to mouth, area of basin (watershed) drained by thestream, and average rate of flow (discharge) at the mouth. The alphabetical list on the back of this sheet shows these characteristics of 32rivers so as to include the 20 largest rivers in each of the three categories. Among the 32 rivers, 16 are tributary to other rivers on the list; the remaining rivers discharge directly into oceans, seas, gulfs, or bays. As dynamic parts of our environment, rivers and their characteristics vary in space and time in response to climatic changes and to man's activities. The causes include seasonal and annual changes in precipitation and temperature, cycles ' of erosionanddeposition (especially during floods), diversions of water (for irrigation, power, and other purposes), and the construction of public works-dams, levees, locks, and canals. For example, combinations of these effects, but principally diversions, have reduced the averageflow of the Colorado River near its mouthfromabout 22,000 cubic feet per second (ftl /s) for the period 1903-34to less than 4,000ft'/s during the period 1951-80 . However, the annual flowin 1984 averaged 17,500 ftl /s, a consequence of record-breaking precipitation on the river basin. Aflow

of 1,000 ft l /s is equal to 646 million gallons per day, 724,000 acre-feet per year, or 28.3 cubic meters per second. (One acre-foot is thevolume ofwater that would cover I acre to a depth of I foot .) River lengths or river-length data are affected not only by some of the natural andartificial causes noted in the preceding paragraph, but also by the precision of various techniques of measurement, by the scale of available maps or aerial photographs, and by somewhat arbitrarydecisions. For example, the length may be considered to be the distance from the mouthto the most distant headwater source (irrespective of streamname) or fromthe mouth to the headwaters of the streamcommonly identified as the source stream . The names of some rivers, such as the Mississippi River and the RioGrande, are unchangedfrom source to mouth. In contrast, the name of the source of the Mobile River-Tickanetley Creek-changes five times before becoming Mobile River 45 miles north of Mobile Bay. The lengths of meandering rivers, such as the Mississippi River south of Cairo, Ill ., undergo significant changes in length from time to time becauseof a natural or excavated cutoff (a channel severing a narrowstripof land, thus bypassing a large bend in a river) that reduces river length and therefore navigation time. For example, between 1766and 1885, the length of the Mississippi River fromCairo, Ill ., to New Orleans, La., was reduced by 218 miles because of 18 cutoffs (Elliott, 1932, page 59) . Reference cited-Elliott, D

.O . (U.S. Mississippi River Commission), 1932, The improvement of the lower Mississippi River for flood control and navigation: Vicksburg, Miss., U .S. Waterways Experiment Station, U

.S. Army Corps of Engineers, 345 pages. For additional information write to: U .S Geological Survey Water Resources Division MS 419, National Center Reston, Virginia 22092of the 32 rivers listed here the 20 largest in three categories-discharge, drainage basin, and length---are ranked from 1 to 20 these ranks are shown in parentheses . Abbreviations : ft 3/s=cubic feet per second; mi l =square miles All data have been rounded to no more than three significant figures . Sources of data : Stream discharge and drainage area-mainly U .S Geological Survey reports and files; length-publications and files of U.S. Geological Survey, U.S. Army Corps of Engineers, U.S

. Environmental Protection Agency, and the Tennessee Valley Authority ; data for the St . Lawrence River from "Facts from Canadian Maps," Canada Department of Energy, Mines and Resources, 1972. Period of record for most rivers is 1951-80 Some data are provisional, and subject to revision . Compiled b Y J .C. Kammerer, U .S Geological Survey !VOTE Rank from 1 to 20 in each category is shown in parentheses .