There is no "minimum length" to be classified as a river. However, width, and to a lesser extent depth do figure into whether or not it is called a river. If one can toss a rock across it, and it is...
it is joined by the river Saône at Lyon, before going south. Along the Rhône Valley, it is joined on the right bank by Cévennes rivers Eyrieux, Ardèche, Cèze and Gardon or Gard, on the left Alps...
From its source to its mouth. .............................................................................. If you didn't find that helpful, which most of you should not, the longest river is the...