The Upper Mamquam is a classic III+ run near Squamish. It's best run at higher flows -- peak snowmelt or after rain.
To get to the river, drive up the Mamquam FSR from Highway 99. About 11m from the highway (after you cross Raffuse Creek and near a brand new -- 2020 -- clearcut), you'll see a gate on the left leading to the Upper Mamquam Hydro Project. This is the lowest takeout. 2km up from here, take a left over a bridge across the Mamquam. If you're looking to skip the class IV section, you can take out here. Continue on this road across Skookum Creek and stay right at all upcoming intersections. The final spur road (another 9km up from Skookum Creek) has some deep water bars, but it's a short walk to the put-in from there if your vehicle can't make it.
The river starts out small but gains flow after the Crawford Creek confluence. There are a few rapids right at the put-in, but it quickly flattens out for a handful of kilometres. The Mamquam goes through a short "canyon" section with a series of class III+ rapids before opening back up. Watch out for trees in the more open, mellow sections.
Eventually, you reach the Skookum Creek confluence and the river picks up a substantial amount of volume. From here down, the rapids build to class IV before all-too-quickly going flat above the takeout dam. You can run this section at any water level; at low, late-summer flows, it's fun III+. When the normal Upper Mamquam is runnable, this section has a big water feel to it.
The Mamquam gauge is newly online so we have yet to establish a solid correlation. 45-55cms translates to a nice low or medium-low level (though note that this still has a big volume feel for the last two kilometres). The gauge is quite far downstream (near town), so the correlation may not be consistent.