An upgrade of Kangaroo Creek Reservoir in the Adelaide Hills has offered a rare glimpse into the past — a bridge that has mostly been under water for the past 50 years.

The 19.1-megalitre reservoir has been drained for upgrades and its water levels are at 1 per cent.

That means a bridge hidden by water when the dam was flooded between 1968 and 1969 is now exposed.

The old Gorge Road bridge before it was submerged. ( www.christmaslane.info )

Known as Batchelor's Bridge, the old Gorge Road bridge is one of three inside the reservoir.

It was built in the 1920s before the dam was constructed and spanned 28.8 metres.

The structure known as Batchelor's Bridge as seen today. ( Supplied: Cudlee Creek Restaurant Tavern )

Crouch's Bridge further east is occasionally exposed during low water levels, while the Old Prairie Bridge, even further east, was rarely submerged but washed away during floods in the 1990s.

Construction of the Kangaroo Creek dam began in 1969.

It covers 103 hectares and, when full, has the capacity to supply South Australia with water for nearly 33 days.

An upgrade of the dam began in late 2015 to widen the spillway and raise and strengthen the dam wall.

The work is being undertaken to manage major floods and improve the structure's ability to withstand earthquakes.

It is expected to be completed in 2019.

Batchelor's Bridge was also exposed during low water levels in 1994.