The secret is that there are WAY bigger carp in the main river than you will see in the sloughs and channels along the Columbia. The big river carp like bigger forage, and they tend to feed on drop offs, points, or deeper edges where they can dig larger clams etc. Granted, in the spring some of the big females move into the sloughs to spawn, but you will absolutely find more big carp in the main river when compared to the slough. I'd go so far as to say the "ordinary" carp in the sloughs will be in the 7 lb range, and the "ordinary" carp in the river is probably 9-10 lbs. The big carp definitely hang out in the river.
This was a 36 inch long, 26 lb carp I caught late summer out of the river. Not the big cobble on the shoreline...the bigger carp really prefer gravel/cobble bottom to a soft mud bottom. I think it is because there is bigger protein around...more calories for less work sort of thing. That type of bottom, plus proximity to deep water is where I find the biggest fish that I catch each year.
Disclaimer...i'm speaking of fly fishing only. I have to find and SEE the fish to catch them...it is possible you could bring a bigger fish to a rod with bait, so my theories on location and bottom type that the big carp prefer might just be applicable to fly fishing with artificials.
