Change the DC
When a character makes an ability check, there is always a DC they must reach to succeed. The basic rules shows a table for different DCs, in which a DC of 5 is very easy, 10 is easy, 15 is medium, 20 is hard, 25 is very hard, and 30 is nearly impossible. Jumping is a lot harder with a waterfall spewing water on you, so you should set the DC accordingly.
Disadvantage works too
Although it is less customary, you can give them disadvantage on the jump. This is almost like changing the DC to be harder, as disadvantage in the books is commonly correlated with plus 5 to the DC. This is just making the jump harder in a different way.
