Ross Lake is a reservoir, formerly the Skagit River. It is 25 miles long running from the North Cascades Highway to the Canadian Border. The only road access is through Canada. From the North Cascades Highway, access is via Diablo Lake, portaging past the Ross Dam, or portaging down from the highway.
Both Diablo and Ross Lake are subject to high winds, the water is cold (Diablo is dangerously frigid) and beaching possibilities can be far apart. NPS requires a wilderness permit, which will have specific campsites on specific days for your trip. It is an idiotic system, which falls apart if you should run into bad weather and be forced to stay in camp. You can alter the plan by waving down the park service patrol boat (no, you do not have cell phone or radio access).
The south end has quite a bit of small motorboat action due to Ross Lake Resort, but the lake gets more peaceful as you move north. The scenery is spectacular, the campsites are, mostly, very nice and well maintained. In mid-summer start your trip to avoid the south end on weekends.
The Ross Lake Resort will portage your canoe/kayak via a flatbed truck from Diablo Lake for $25. There is a resort phone on the side of the powerhouse below the dam. The portage is 1 mile, 800 ft. up and 200 ft. down. No truck available if you portage direct from the highway.
Wilderness Permit and campsite plan required (free) from NPS Backcountry in Marblemount.
Fishing license, if you're into that.
In the US, Highway 20 from either I-5 or from Winthrop. It's the only road and it is well signed. You will pick up your wilderness permit in Marblemount on the way in, watch for the sign to the backcountry office.
USGS topos and NPS website info