Bhagya Sandun - Google
A massive water reservoir built in the 3rd-century was regarded as one of the sixteen great reservoirs of King Mahasen. The reservoir was built to fulfil the water requirements of the ancient cities and to prevent floods. Chronicles suggest that from 300 years after Mahasen, King Moggallana II had expanded the reservaiour and built a dam across the Malwathu Oya.
In 1906 the reservaiour was found in a ruined state and the Government of Ceylon took the responsibility of restoring the reservaiour. However in 1957 during severe floods the reservaiour was again damaged, and it got restored back in 1958.
To control its water levels there are sluice gates and channels being connected to Tissa Wewa and Nuwara Wewa. The reservoir has a water capacity of 1.5 million cubic meters, and represents the engineering marvels of ancient Sri Lanka.