Mahagallaka reservoir (today known as Nachchaduwa reservoir) is considered to be one of the sixteen large reservoirs built by King Mahasen (276-303). Nachchaduwa lies just outside the city of Anuradhapura . Centuries later King Dhatusena (459-477) built a canal (Jayaganga) to bring water to this reservoir from Kalawewa. This reservoir is considered to be a bold scheme since huge flood water with very high velocity could come down the valley. The great chronicle Mahavamsa probably makes another reference to this reservoir during the time of King Moggallana II (535-555).
It is said that King Moggallana II, who lived 300 years later built Passapatanavapi reservoir by damming the Kadamba river (Malwathu Oya). This description matches well with Nachchaduwa. It is possible that Moggallana II could have expanded the work of Mahasen. Nachchaduwa Wewa has three sluices to control its water levels. The main sluice is located near the southern end of the bund and releases water mainly to Tissa Wewa . The other two sluices release water to Nuwara Wewa.
Accrding to H. Parker, Ancient Ceylon, “It was a bold scheme as floods estimated to amount to 11,000 cu ft per second were to be expected and there was no suitable rock over which they could be allowed to flow, but it was carried out successfully”.