Yan Yean Reservoir, lake in Australia

Yan Yean Reservoir is a water storage facility in Victoria, built in 1857, and the oldest water supply system in the region. The dam is an earthen and rock structure measuring approximately 10 meters (33 feet) high and stretching nearly 1,000 meters long, capable of holding over 30,000 megalitres of water. Water enters the system through rainfall and rivers flowing from nearby forests, while a protected catchment area surrounding the reservoir ensures water safety for drinking.

The reservoir was completed in 1857 and was the largest of its kind in the world at that time. Engineer James Blackburn, an English convict sent to Australia for theft, designed the structure, which took four years to build and represented a massive financial investment for the era.

The name Yan Yean refers to an Aboriginal leader who signed Batman's Treaty in 1835 under the name "Yan Yan", recorded as meaning "young male". The reservoir sits in the locality of Yan Yean, near Whittlesea, and the nearby settlement of Mernda grew up during the dam's construction.

The reservoir and surrounding park are open daily from early morning until evening, with extended hours during daylight saving time. Walking paths are easy to follow, picnic areas with barbecue facilities are available, and toilets including accessible facilities, parking, and a playground near the Valve House are provided for visitors.

The reservoir's water originally came from the Plenty River, but stock crossings and rural pollution spoiled its quality. From 1883 to 1885 the Toorourrong Reservoir system was built to divert water from Wallaby Creek and Silver Creek in the Great Dividing Range, and an aqueduct has carried that mountain water to Yan Yean ever since.