Never swim in Nyando River naked

When I was still very innocent and naïve, someone came up with the line 'Never swim in River Nyando naked'. For a long time, I used to pass by the river on my way to school looking at those swimming there naked and I always felt like detouring and spending some time with them just to warn them about the dangers of swimming in the river naked. That was until I got the real meaning of the expression. But why did it have to be River Nyando? Why not Athi River or the Red Sea?