The Lower White is a nice intermediate canyon right off the north island highway, runnable at many different water levels. 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. A rough minimum flow to consider the Lower White is 50cms on the Salmon River at Sayward gauge, but 100cms will be a more enjoyable medium level. Local whitewater canoeists have a visual staff gauge at the White River Campground, on which the red zone indicates their preferred range. The canyon accommodates high water well, and there's no reason expert kayakers shouldn't try it at flows of up to 500cms or higher, although it is above the class III range at these flows. This gauge is on the Salmon River downstream of the White River confluence; the actual flow in the Lower White will be on the order of 30-50% of these values.
The takeout is found just under an hour north from Campbell River, by following Highway 19 to Sayward Junction, turning left across from the gas station to a bridge over the White River. To get to the put in, do a little zig zag to find the start of the White River Main which begins with a steep hill, and follow 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. Expect to encounter loaded logging trucks, which have the right of way.
The Lower White starts just below a riverwide 5m ramp drop. Initially the riverbed is open with some boulders and wide bedrock shelves. Then, the river narrows up into a series of remarkably straight canyons. The whitewater at low flows is predominantly technical boulder gardens and slalom moves. At higher flows, this section turns into a freight train of huge wave trains and holes to dodge, but can still be read and run. After the canyons, chill out as the river bends right into the Salmon River valley, take out at the second bridge, and start looking for real estate in Sayward: "Nature's Playground."