History of Secondary Highway 521:
Secondary Highway 521 was a local highway in the County of Renfrew, which served as an access road to the small communities lining Round Lake, southwest of Pembroke. Highway 521 was originally much longer when it was first established in the mid-1950s, but was gradually shortened as sections of the highway were renumbered during the 1960s.
Highway 521 was first depicted on a Preliminary Route Plan prepared by the Department of Highways of Ontario (DHO) in April 1956, and was designated as a Secondary Highway by an Order-in-Council dated May 9, 1956. The route of Highway 521 originally began in Brudenell and then headed north to Killaloe via the Brudenell Road. The highway then extended northwesterly from Killaloe via the Round Lake Road to join Highway 62 at Bonnechere. Highway 521 was approximately 21 miles (34 km) in length when the route was first established in 1956. The road from Brudenell to the Highway 60 Junction at Killaloe was already paved before it was designated as Highway 521 in 1956. The balance of Highway 521 from the Highway 60 Junction at Killaloe to the Highway 62 Junction at Bonnechere had a primed bituminous gravel surface when the highway was designated in 1956.
In 1960, two sections of Highway 521 were renumbered. The section of Highway 521 from Brudenell to the Highway 60 Junction at Killaloe was renumbered as an extension of Highway 512. The route of Highway 62 was signed concurrently with Highway 60 between Barry's Bay and Killaloe starting in 1960. In addition, a 4-mile section of Highway 521 from Killaloe to the southeastern corner of Round Lake was also renumbered as a part of Highway 62. It was originally envisioned that a new diversion would be built for Highway 62 along the eastern side of Round Lake and that the old road around the western side of Round Lake would remain as Highway 521. However, other highway construction priorities prevailed and the Highway 62 Diversion along the eastern side of Round Lake was ultimately never built. In July 1967, the DHO decided to renumber the remaining portion of Highway 521 around Round Lake as Highway 62. The new Highway 62 route number at Round Lake was first reflected on the 1968 Official Ontario Road Map, which suggests that the route number was changed sometime in the Fall of 1967. The route around the western side of Round Lake continued to be operated as a provincial highway until the late 1990s, when Highway 62 was transferred to the County of Renfrew. The former route of Highway 521 is now part of today's Renfrew County Road 58.