As for your question - I generally figure the water in Colorado gets going in May. There's boating before this (you can run Glenwood Canyon all year), but it's cold and most runs won't have water before this. Early runs include Cross Mountain (Yampa), Escalante Creek (the one in Colorado), and Piedra.
Late May through June and into July is the normal boating season. If you're planning a trip for boating, June is generally when you want to be here. But Colorado is a big state, drainages vary in elevation, some runs (like the Big South) are accessed by roads that are closed until the water is about gone, so you'll want to be flexible. You didn't say anything about the level or kind of rivers you're interested in, to this is all general. If it's a big year and you're an intermediate boater, you might find high water in mid-June is more than you bargained for. If you're a Cl V boater, just go for it. Either way, you'll want a copy of the guide - Whitewater of the Southern Rockies.
I don't think we'll be out of lockdown before June, so plan accordingly.