Stream corridor encroachment and its consequences: the case of Pinga Oya tributary in the upper Mahaweli River in Sri Lanka

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The expanding human interference with stream corridors creates many undesirable impacts on the riverine ecosystem at different scales. Population growth and demand on land due to suitability of location for economic activities are pushing factors for encroachment in stream corridors. This issue is critical in the context of all headwater stream corridors in Sri Lanka. The objective of this study is to identify and highlight the nature and intensity of the encroachment and its consequences. Within the context of this concern, the Pinga Oya catchment located within the upper Mahaweli stream corridor, was selected as the study site. Relevant literature sources, legal documents, image classifications, chain survey, stream cross-sectional survey, field observation, questionnaire survey and stakeholder interviews were used to conduct the study. Encroachment in the stream corridor resulted in changing natural functions of the stream corridor ecosystem in many ways. Given that, artificial constructions conceal 100% area within 40 m distance from the stream’s left bank and 90% area within 40 m distance from the stream’s right bank within the stream’s highly populated reach. The average populated reach within the area of 40 m distance from the stream’s left and right banks is covered with 80% of artificial constructions. The less populated reach within the area of 40 m distance from the stream’s left bank is covered 45% by roads and 55% with natural area s. The right bank is totally natural, with 95% of vegetation and 5% of rock and no artificial constructions. There is less species density and diversity in the highly populated segment of the Pinga Oya stream corridor. The trend has begun since the 1980s after the transformation of Akurana as a trade centre. Stream water quality in terms of turbidity, biological oxygen demand (BOD) and nitrate concentrations has exceed the tolerance limits. The recorded nephelometric turbidity units (NTU) is 18.5 in highly populated reaches and 12.4 NTU in less populated reaches during the rainy season. BOD levels are 14, 10.3 and 9.2 mg L−1 respectively, showing decreasing pattern from highly populated to less populated reaches. In the less populated reach both seasons represent high (Nitrate) N levels, with 27.5 mg L−1 in the rainy season. This is particularly due to agriculture-related waste leachate containing nitrate. To overcome the major problems, the government should give more power to relevant authorities to take legal actions to demarcate the stream corridors, at least streams in urban premises. Awareness programmes for stream dwellers to maintain the stream corridors and strict rules and regulations are essential for restoration of the Pinga Oya stream corridor to make it sustainable.