IMPACT OF MAHAWELI DIVERSION SCHEME ON BIOLOGICAL DIVERSITY OF DOWNSTREAM FLOOD PLAINS AND ASSOCIATED VillUS

Abstract

Flood plains of Mahaweli River cover an area of 50,000 hectares. The flood plain extends fromthe river mouth in Trincomalee at its bottom end, to a little way south of the old settlement ofYakkure situated southwest of Manampitiya in the Polonnaruwa district. The flood plain iscomposed of various ecological zones consisting of river channels, villus

(Riverine marshes) seasonally flooded grasslands and swamp forest. Villus are shallow andsaucer-shaped depressions on ground their water level is maintained by the annual cycle ofrainfall and river flooding. Most of these are directly connected to the Mahaweli River bychannels. There are 38 identified Villus located in the Mahaweli River floodplain system.A study program was initiated in early 2000 to evaluate the changes that are taking place in thebiotic components of this importantecosystem due to the Mahaweli Development activities suchas reservoir construction and water diversiou.

Villus and floodplains have direct or indirect connection with the river or its tributaries. Whenthe river is high, water flows into the Villu or floodplains, and when the water level falls thereverse flow may occur towards the river. It was anticipated that upstream diversion schemes ofMahaweli River would reduce the river flow at Manampitiya by 50%. However fieldinvestigation carried out during our survey indicated that water levels of the Villus have notchanged significantly due to the diversion scheme.

Accelerated Mahaweli program provided irrigation water for large areas of undeveloped landaround villus, thus converting them in to paddy fields. All These paddy field receive water forboth 'Yala' and 'Maha' rice growi l.g seasons. Today, vast extents of paddy fields drains in toVillus of the Mahaweli System. As a result, villus in the Mahaweli flood plain now receive ayear-round supply of water. Before the river diversion scheme villus received water from theriver only during the flooding season (i.e. January.) This has brought changes in the waterregimes of the villus.

At the moment there is a booming development of Eichhomia crassipes (Japan jabara) due toexcessive loading of nutrients in villus.

The study found that the river diversion schemes have resulted considerable changes in the biotaof the Villus and other ecosystems associated with the river. Therefore, steps should be taken torelease floodwater to Villus during the natural floor period of the river in order to sustain thenatural faunistic and floristic diversity in the villus and associated flood plains.