1250, yawn.... 1500 better. 2000, the ark tends to wash. +2000, gets better and better. +4000, best.
Tough to exactly pinpoint what gets harder and easier as some stuff is harder at certain flows, and gets easier at higher and then gets tougher higher and then gets easier.....
The major rapids, Zoom, the Seven Stairs, tend to be tougher (but great fun) at higher flows. Big drop, Toilet Bowl can easily be sneaked at higher flows, so where the rapid might be tougher, you can always sneak. Like any river, the bigger the flow, the pushier it will be. If you really want to pucker up, try Numbers above 2500!