exactly... trying to claim prominence between waddington and robson, you would have to figure out the elevation of some hydrographic divide between the fraser river and homathko river, somewhere near redstone reserve.... a junction point where, nonetheless, you would either have to go DOWNHILL into the Fraser Basin, or take a detour of thousands of kilometers along a divide, to get to Robson from.  anyways like you pointed out, the next highest point after robson is probably somewhere in the USA, Colorado or something.... and perhaps the two drainages are not even contiguous. i don't even want to know what you do then. please don't tell me. or calculate the prominence difference between everest and Olympus Mons.