By the looks of an image posted by loaloa on Disney Central Plaza, the ride would be built behind the Frontierland Train Station. There have been rumours of Splash Mountain being built in this particular area but I personally think that a Rapid Ride like this would be a great addition as long as it has a more "Wild West Theme" than the DCA version. The original ride contains a section where riders flow past exploding geysers, which would fit perfectly having the geysers next to Boot Hill. For me Splash Mountain would edge Disneyland Park nearer to becoming a generic "Magic Kingdom Park", something which scares me. But altogether I think that "Grizzly River Run" would be a good move from Disney!Have a magical day!