The Mississippi drainage basin includes the Missouri and the Mississippi rivers, the two longest main-stem rivers in the United States, as well as 18 more of the rivers on this list. The Mississippi main stem is highlighted in dark blue.
The main stems of 38 rivers in the United States are at least 500 miles (800 km) long. The main stem is "the primary downstream segment of a river, as contrasted to its tributaries". The United States Geological Survey (USGS) defines a main-stem segment by listing coordinates for its two end points, called the source and the mouth. Well-known rivers like the Atchafalaya,Willamette, and Susquehanna are not included in this list because their main stems are shorter than 500 miles.
Seven rivers in this list cross or form international boundaries. Two—the Yukon and Columbia rivers—begin in Canada and flow into the United States. Three—the Milk and Saint Lawrence rivers and the Red River of the North—begin in the United States and flow into Canada. Of these, only the Milk River crosses the international border twice, leaving and then re-entering the United States. Two rivers, the Colorado and the Rio Grande, begin in the United States and flow into or form a border with Mexico. In addition, the drainage basins of the Mississippi and Missouri rivers extend into Canada, and the basin of the Gila River extends into Mexico.
Sources report hydrological quantities with varied precision. Biologist and author Ruth Patrick, describing a table of high-discharge U.S. rivers, wrote that data on discharge, drainage area, and length varied widely among authors whose works she consulted. "It seems," she said, "that the wisest course is to regard data tables such as the present one as showing the general ranks of rivers, and not to place too much importance on minor (10–20%) differences in figures."
The primary source for watershed and discharge data in the table below is Rivers of North America. Conflicting data from other sources, if the difference is greater than 10 percent, is reported in the notes. Discharge refers to the flow at the mouth.[n 1] In the "States, provinces, and image" column, the superscripts "s" and "m" indicate "source" and "mouth". Non-U.S. states appear in italics. Except in the "States, provinces, and image" column, abbreviations are as follows: "km" for "kilometer", "mi" for "mile", "s" for "second", "m" for "meter", and "ft" for "foot".
River is not entirely within the United States.
Watershed is not entirely within the United States.
^Dams, diversions for agriculture, and other human alterations to rivers have greatly affected the discharge of some rivers over time. For example, the virgin discharge of the Colorado River is estimated to have been 20,000 ft3/s (566 m3/s) compared to 1,400 ft3/s (~40 m3/s) in 2005.
^Of the total, 10,700 mi2 (28,000 km2), about two percent of the basin, is in Canada.
^Kammerer: 2,340 mi (3,770 km).The Atlas of Canada: 2,348 mi (3,779 km).
^Of the total, 10,700 mi2 (27,800 km2), about two percent of the basin, is in Canada.
^Of this total, 714 mi (1,149 km) are in Canada. This amounts to about 36 percent of the main-stem length.
^The Atlas of Canada also lists the total basin size at 324,000 mi2 (839,200 km2), split between 125,000 mi2 (323,800 km2), about 39 percent, in Canada and 199,000 mi2 (515,400 km2), about 61 percent, in the United States.
^Kammerer: 1,900 mi (3,100 km). University of Texas (UT): 1,799 mi (2,895 km). The river forms the U.S.–Mexico border for 1.251 mi (2,013 km) (about 70 percent of its main-stem length) from El Paso to the Gulf of Mexico.
^Rivers of North America says that of this total only about 170,000 mi2 (450,000 km2) of the basin contribute water to the river. The University of Texas (UT) says, "The river collects rain, snowmelt and spring water from an area [of] about 215,338 km2 [557,722 km2] including closed basins." It says that 87,020 mi2 (225,380 km2) of the basin (about 48 percent), not counting closed basins, are in Mexico, while 93,821 mi2 (242,994 km2) (about 52 percent) are in the United States. Kammerer cites a total basin size of 336,000 mi2 (870,000 km2).
^According to the Utah Division of Water Resources, 75 mi (121 km) of the river are in Mexico. This amounts to about 5 percent of the main-stem length. Of this, 17 mi (27 km) form the border between Mexico and the United States.
^The United Nations Environment Programme cites a total basin size of 246,000 mi2 (637,000 km2), split between 2,000 mi2 (5,200 km2) (about 1 percent) in Mexico and 244,000 mi2 (632,000 km2) (about 99 percent) in the United States.
^About 498 mi (801 km) are in Canada. This amounts to about 40 percent of the main-stem length.
^The Atlas of Canada lists the total watershed at 259,200 mi2 (671,300 km2), split between 39,700 mi2 (102,800 km2) (about 15 percent) in Canada and 219,500 mi2 (568,500 km2) (about 85 percent) in the United States. Kammerer's figure for the total watershed is 265,000 mi2 (690,000 km2).
^Also known as the Dakota River or Jim River and not to be confused with the James River of Virginia.
^This is the average discharge for the years 1982–94, derived by adding the discharge for each of these years and dividing by 13.
^According to the Milk River Watershed Council, a 110-mi (180 km) stretch of the river flows through Canada. This amounts to about 18 percent of the main-stem length.
^The Atlas of Canada: 23,600 mi2 (61,200 km2) split between 8,300 mi2 (21,600 km2) (about 35 percent) in Canada and 15,300 mi2 (39,600 km2) (about 65 percent) in the United States.
^According to the U.S. Army Corps of Engineers, a 115-mi (185 km) stretch of the river forms part of the U.S.–Canada border. This amounts to about 19 percent of the main-stem length.
^Kammerer: 396,000 mi2 (1,030,000 km2).The Atlas of Canada: 519,000 mi2 (1,344,200 km2), of which 324,000 mi2 (839,200 km2) (about 62 percent) is in Canada and 195,000 mi2 (505,000 km2) (about 38 percent) is in the United States.
^The main-stem length is calculated by subtracting the length of the Nabesna River from Kammerer's total of 659 mi (1,061 km). The Nabesna River is roughly 75 mi (121 km) long, calculated by adding the 60 mi (97 km) from the Nabesna mouth to Camp Creek to the distance, 15 mi (24 km), from Camp Creek to Nabesna Glacier, Kammerer's most remote source for the Tanana.