You can't raft the South Platte
Assuming the reference is to the SP in Denver: You can most certainly raft from C-470 (South Metro Parks is the managing agency at the put-in) to below Confluence Park. The section from C-470 to Union Chutes is actually much more pleasant than one would think (just don't go in with high expectations) with herons, ducks, and enough vegetation to help you forget you're in an urban setting for much of the reach. There's one mandatory portage at Xcel's Zuni Power Plant where there's a low-head dam blocking the river. Hopefully that dam(n) will be removed with planned river restoration on the Zuni reach - the only section above the Metro outfall that hasn't been restored yet. The Corps of Engineers has done the pre-construction environmental clearance work and has the restoration plans on the shelf - we just need our elected leaders to push for the work to be done (about a $25M - $30M pricetag). It'd be good to get boater feedback on turning drop structures into whitewater features, etc.
Whether you'd actually WANT to raft the South Platte is another matter as the water quality declines significantly below the Englewood wastewater plant outfall and then goes downhill from there. The water quality improvements are envisioned for sometime in the future and will take a lot of public pressure on our leaders to make that happen.
Back to trailering rafts - Here's a photo of my boat en route to Flagstaff/GC after driving over Vail Pass in a March blizzard:
When I got to Flagstaff I just hit a carwash and washed it off.