By association

Parshat Lech L'cha

In 15:18 at the conclusion of the Brit Bein Ha'B'sarim, Hashem promises to Avram his children would be given "From the river of Egypt until the great river the river of P'Rat".

Rashi says that the P'rat river was seemingly the smallest and most minor of the 4 rivers as it is the fourth and last river mentioned (and had no description ascribed to it). Yet it still calls it here "the great river, the river of P'Rat".

Rashi explains because it is connected to the Land of Israel it deserves the title "The great river". Just by association.

Associate with good people and ideas and you will be thought of and considered to be good. Associate with evil and you will be thought of that way as well...