Mahagallaka Reservoir is considered to be one of the 16 largest reservoirs built by King Mahasen. Today it is known as Nachchaduwa Wewa. Nachchaduwa is located just outside the city of Anuradhapura. Nachchaduwa Wewa has three sluices to control the water level. The main sluice is located near the right end of the bund and the water is mainly released to Tissa Wewa. The other two sluices release water to the city.

In 1906, the Irrigation Department took over the responsibility of restoring the reservoir which had been destroyed in the past. In 1917, the department made further efforts to improve the capacity of the reservoir. The Great Flood of 1957 caused severe damage to the reservoir, but in 1958 rehabilitation work was completed. Today it draws water from the Mahaweli system. The reservoir covers a total capacity of 1,200 hectares (12 sq km). The length of the dam here is about 1646m. The height is 11m. It can store about 55.7 million cu.m of water. The water of the Nachchaduwa tank is used to provide farming and drinking water to the people of the area.