Yes I do happen to know that rivers are unidirectional and flow in the same direction not the opposite.
The rivers flow because of some kind of slop in the mountain or the valley or the plains.
Now,Rivers flow because there is a slope and this slope creates pathway for water to flow.
And there is no slop towards the opp side of the river to flow up the mountain thus,it is unidirectional!
Keep asking,gudday :)