Zola Fel river folk have songs that propel them upstream, their god aiding his followers.
That's what your Water Rune is for isn't it?
I'd say they swap boats at the falls, carrying their goods up. Or if the boats are small enough carrying them up too. Likely different River folk clans control the upper and lower stretches. For example I have the Deep Water clan as the majority folk in the Corflu area.
I can't remember how the pcs were meant to deal with the Falls in the River of Cradles campaign. Their boat was powered by undines.