Trenton is the starting point for the Trent-Severn Waterway, which continues northwest to Peterborough and eventually Port Severn on Georgian Bay.

Around 1850, lumber from this area was being sent out on ships; the village was renamed to Port Severn in 1868.

Highway 103 was first designated during the Second World War, providing access from Highway 12 at Waubaushene to Port Severn.

At Port Severn, the highway meets the rugged Canadian Shield, and winds its way north through the granite, often flanked by towering slabs of rock.

Following the completion of Highway 400 to Port Severn, the next target became Parry Sound.

The municipal offices are at Port Severn.

Most loopers will take the Trent Canal from Trenton, Ontario to Port Severn on Georgian Bay.

Lock 45 at Port Severn was nearing completion, so it was finished as a small, "temporary" lock.

Beausoleil Island is a 8-kilometer long island in the Lake Huron's Georgian Bay, near Port Severn, Ontario.

It was at this time that Highway 69 was at its longest, from Highways 12 and 400 near Port Severn to Sudbury.