Last week, I decided to try something. I bought the JR 15mm to 28mm connector. It is a slightly strange piece of terrain that has the join for both their 15mm rivers and 28mm rivers on it with a big pooled out area in the middle. I also bought another 9" section of river as well. I decided that keeping the rogue piece of river is a good thing. I will add it on as an edge piece for the board.
Eventually I will run out and buy the 15mm to 6mm connector and get some 6mm river sections as creeks to run across the table. If I like the look, I may get some more 28mm river sections just to have varying widths of rivers on the table. It should work out rather nicely.