CONTEXT:

• The River Periyar in Kerala is witnessing decolorization.
• The water turned pitch black in one stream, it was milky near the Pathalam regulator-cum-bridge.
• The Pollution Control Board (PCB) has attributed the change in colour to the poor quality of water as a result of eutrophication.

BACKGROUND

• River Periyar flows in the state of Kerala. It originates in the Western Ghats and flows north for a short distance to Periyar Lake.
• Periyar Lake is an artificial reservoir created by Mullaperiyar Dam across the river.
• From the Periyar Lake, tunnel carries water through mountains to the Vaigai River in Tamil Nadu, where it is used for irrigation.
• urther downstream Idukki dam is constructed across the river. The river descending out of the mountains into the coastal plain and empties into the Arabian Sea.