On YouTube there are several versions, one in Afrikaans, with pictures and lyrics. Josef Marais is well represented.
The river is the Mooi, and mealies (or mielies) is maize.
"Mealie-meal is a relatively coarse flour (much coarser than cornflour or cornstarch) made from maize which is known as mielies[1] or mealies in southern Africa, from the Portuguese milho. The Portuguese had originally brought corn from the Americas to Africa." - Wikipedia