From Chur take road 13 south to Bellinzona. Take exit 36 Mesocco-Sud and take the paralel road. A little further you can see Cascata del Rizeu dropping down over a hundred meter at the left side of the road. Unfortunately it is impossible to stop along the road.
July 2021 I tried to find out if there was a better way to view Cascata del Rizeu but the only save view I got was from a parking at the “exit 36 Mesocco-Sud”. A shame because it look like a nice waterfall.
There is no official path to get nearer to the waterfall but maybe some day somebody has a suggestion…..