The South Arm is the southern branch of Sydney Harbour, a saltwater inlet on Cape Breton Island. It is a significant marine feature, separating the community of Westmount from the city of Sydney. The arm is formed where the Sydney River estuary meets the harbour, with the river's mouth located between Amelia Point on the Westmount shore and Battery Point on the Sydney shore. Petersfield Provincial Park, located off Route 239, provides a public shoreline access point overlooking the South Arm, featuring picnic tables and trails. On the western shore, the Dobson Yacht Club is situated at Dobson's Point (formerly Shingle Point) in Westmount, directly opposite the Sydney downtown area. This club has a basin protected by rock breakwaters, a wharf, a travelift, and a clubhouse; gasoline is available there. The harbour, including the South Arm, is having easy access. The area has historical significance, with Petersfield Provincial Park containing remnants of four cultural periods from the late 18th to mid 20th centuries. The low valley of the Sydney River, which feeds into the arm, was a traditional Mi'kmaq canoe portage route for travelling between Sydney Harbour and the East Bay of Bras d'Or Lake.