The Kelani River is a 145-kilometre-long (90 mi) river in Sri Lanka. Ranking as the fourth longest river in the country, it stretches from the Sri Pada Mountain Range to Colombo. It supplies approximately 80% of the water used in Colombo. In addition, the river is used for transport, fisheries, sewage disposal, sand mining and for production of hydroelectricity. Through these factors, many people depend on the river for their daily routine in life.
Depending on the operation of three reservoirs, the river flow varies from 20 m3 (706 cu ft) to 25 m3 (883 cu ft) in the dry seasons, and 800 m3 (28,252 cu ft) to 1,500 m3 (52,972 cu ft) during the monsoons. The annual sand extraction from the river is approximately 600,000 m2 (6,458,346 sq ft) to 800,000 m2 (8,611,128 sq ft).
From a barge, people dive to the river bed, from where the sand is lifted to the barge in a bucket, and when the barge is full, it is taken to the river bank and unloaded by a separate team. The sand mining causes the river bed to sink by approximately 10 cm (4 in) per year.
At present, two main concerns in connection with the river are flooding during the monsoon and saline intrusion in the dry season. The problems are related: the saline intrusion is enhanced by the deepening of the river caused by the sand mining. Regulation in order to prevent the saline intrusion can reduce the water quality in other ways, and can increase the flood risk. Sand mining is economically important nationally and to the many involved people.