<P> 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 . </P> <P> 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 ." </P> <P> 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 . 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". </P> <Table> <Tr> <Th_colspan="6"> Key </Th> </Tr> <Tr> <Td> River is not entirely within the United States . </Td> </Tr> <Tr> <Td> Watershed is not entirely within the United States . </Td> </Tr> </Table>

What is the largest river in united states