Spring can be a magical time to be on the South Platte River, with the Deckers stretch providing some truly incredible and consistent fishing over these next few months. Like any river within an easy drive of Denver, don't expect to have this river to yourself. That said, seeing as how Deckers doesn't receive a run of lake fish, you can almost always expect to have some elbow room through mid-April. As spring warms up, the river should start to fish well from the town of Deckers clear to the confluence with the N. Fork of the South Platte so don't be afraid to get out and explore some new water if you've never done so here.
Deckers is home to a very healthy population Rainbow Trout that can exceed 20" and you can expect to see them moving up onto the shallow flats with regularity as the spawn approaches. If you happen find fish up on their redds- LEAVE THEM ALONE!. My approach to fishing around spawning fish is to always find the closest available deep water (drop off/shelf/bucket/etc) below the spawning fish. These areas will be congregating fish that have either A) spawned and are seeking shelter, or B) are holding fish that are still in pre-spawn mode. Additionally, these locations will typically also be packed with Brown trout that are picking up the eggs being washed downriver from those upstream spawners.
Speaking of Rainbows and Browns - these are the predominant species of trout in the South Platte and can effectively be targeted in the shallow riffles, buckets, drop offs, rock gardens, etc. I always say that if an area looks like it's holding fish it probably is, so cover water and cast into anything that looks 'fishy'.
From a bug perspective the Midge / BWO / Stonefly / Caddis game will be coming alive throughout the next few months and you should match your flies accordingly. As spring wears on however, these four bugs will become the name of the game and fishing patterns from these four families will always be a safe bet.
Lastly, when it comes to flies the biggest rule is to not overthink it. Fish something that 'makes sense' (i.e. matches what you see getting active) and cover water. If you're nymphing, fish your flies close- like 10" apart- and use plenty of weight. Streamer fishing can also get quite good in the Fall, particularly on cloudy days. Stick with the small / midsized stuff that is a little slimmer in profile to find the most consistant success.
Fly recommendations: San Juan Worm (brown/red/wine), eggs, leeches (olive/black/purple/rust), scuds, Barr's BWO emerger, Juju Baetis, Sniper Baetis, Batwing BWO, Shotglass baetis, Barr's graphic caddis, buckskins, Barr's PMD emerger, Jujubee midges, miracle midges, wd40's, rs2's, foamback emergers (chocolate thunder), pure midges, flashbang midges, mercury bead black beauty's, bling midges, pheasant tails, copper johns. Sculpzillas, slumpbusters, sparkle minnows (white/olive/black), thin mints and double mints.