The alphamegalog of rivers lists the largest river in the world for each letter of the alphabet. In this case, ‘largest’ is based on the size of the river, which is not easy to determine consistently, hence subjective measures of greatness come into play. Note that the casual parenthetical location information is neither complete nor consistent as to the balance between general and specific.

Amazon (South America)
Brazos (Texas)
Colorado (Texas)
Danube (Europe)
Euphrates (Middle East)
French Broad (Tennessee)
Guadalupe (Texas)
Hudson (New York)
Iowa (Midwest USA)
Jordan (Middle East)
Kwai (Thailand)
Lackawanna (Pennsylvania)
Mississippi (USA)
Nile (Egypt)
Ohio (Midwest USA)
Pecos (Texas)
Quinn (Nevada)
Red (Texas)
Sabine (Texas)
Tigris (Middle East)
Ural (Russia)
Volga (Russia)
Warta (Poland)
Xi (China)
Yangtze (China)
Zambezi (Africa)