After taking a few trips to the South Platte River in downtown Denver I have realized that the really good fishing there is not done for carp, although that is what the river is known for, the best fishing is for large rainbows. Not to say that the carp fishing on the Platte is not amazing, those epic endless river flats that are loaded with 8-20lb carp still get much of my attention, but catch a few of these bows and you will change your tune.
Typically these fish are found in the "trout" water, meaning the structures that are at the top of those great carp runs. This type of water will hold a few fish and man are these fish hot! So my favorite thing to do lately is to take my 6wt and rig that up for carp, fish up the flats and then when I get to the head of the run, I grab my 10'6" 3wt and Czech nymph the deeper faster water for a few of these healthy fish.
You might have to share your newly found secret of large trout in Denver with a few of the local meat hunters. They are easy to spot, typically found with 2-4 rods set up along a run, lawn chairs, and the stereotypical white bucket and Styrofoam worm can. Just smile and wave and move up to the next run.