Step 2 goes fast. Put a bead of caulking in the center of the river bed. Smooth that out with your finger. You'll want to keep paper towels on hand as this is messy.
Take the handy dandy caulk river texture smoothing device (aka cheap paintbrush from Home Depot) and dip it in some water. Run the brush over the caulking to get rid of the large ridges and rough areas from your finger smoothing. You'll now have something that looks like the photos below.
Why not just leave the tile surface and paint it as water? Well....water has texture; it moves, it dips, it raises. It's not flat like a sheet of steel. Using caulking gives that movement texture to the river area.
Let this dry overnight, and give the caulk "water" areas a heavy spray of Dullcote or whatever your favorite spray fixative is. Sealing the caulk will keep bank texture sand from sticking to the water area.
More in the next post as we move the river tile construction forward.