Alternating paths in Niagara
The rules state - "players should push the disc by hand in the opposite direction of the direction the previous disc moved". 

It shouldn't be to hard to know which way it previously went as it only happened a moment ago.  Also the rules do state that the river is "usually alternating, but not always".  If a group really can't remember which way the river just went then just randomly decide the next direction.  

I find if a player gently holds the board down either side of where the board fold is and the river splits that prevents the river discs from getting jammed.

I use the fact the discs fall at the bottom of the river and leave a leave a disc there as a reminder which fell last. there are 3 spare discs so one pile always has 2 and the other always has one meaning the river should fall there next.