The most rarely paddled section on the White River, because at most flows it has a terminal canyon that is a strenuous portage. The main reasons to paddle this section are to link together the full White at low flows, or to tick the portage drop off your list. The portage canyon is totally runnable at the right flows (low), it is marked as a portage here so that it is taken seriously. Paddling the full White makes for a nice day of boating finishing at Sayward Junction: Canyon (IV-V), Upper (III-IV (P)), and Lower (III). While you're in the area, hit up the other quality runs such as the Adam (III+) and Middle Memekay (III-IV). Look for these runs and others to be described in an upcoming guidebook produced by the Vancouver Island Whitewater Paddling Society.
There is no online gauge on the White River, but the gauge on the Salmon River near Sayward can help to estimate flows. To consider paddling the Upper White, the White River Canyon should be on the low side, which means probably not much more than 50cms on the Salmon River at Sayward gauge.
The takeout is found just over an hour north from Campbell River, by following Highway 19 to Sayward Junction, turning left across from the gas station, doing a little zig zag to find the start of the White River Main which begins with a steep hill, and following this up the river right side of the White River for ~12km to a small pullout and quarry on the left. Across the road, there is a steep fisherman's trail down to the water. It's strongly recommended to hike in to the takeout ahead of time to scout the last rapid before committing to this run. Take the takeout trail partway down, then follow a forested bench upstream on river right and try to get a glimpse down into the canyon. The put in is ~6km further upstream at a bridge over the White River (avoid Victoria Main). Expect to encounter loaded logging trucks, which have the right of way.
From the put in bridge, the paddling is read and run class III with both boulders and some wide bedrock shelves. Gradually, the ledges get a bit taller and more complex, catching out the inattentive boater. Where the canyon tightens up, stay on your toes and look for a beefy riverwide ledge above a narrow canyon, often full of wood. This is the portage rapid, and to portage it the whole canyon must be portaged, probably easier on the left. Don't overcommit. If you are considering running the portage canyon, you should have scouted the hazards beforehand, which include but are not limited to: a beefy entry ledge, a very deep cave river left, many old growth logs, and a terminal eddy at the bottom right. Below the portage, the steep take out trail is immediately on river right, but it is recommended just to continue down the Lower White (III) to Sayward Junction.