Construction of dams across rivers is a common practice worldwide. However, obstructing free-flowing rivers could result in many negative impacts on riparian ecosystems, and are highly site-specific. In the present study, we evaluate the potential impacts of river regulation on the riparian vegetation by enumerating the vegetation at different distances from the dam/ reservoir complex from upstream to downstream reaches of a tributary of the river Mahaweli in Sri Lanka. The tree-dominated riparian vegetation has been evaluated using belt transects located in the immediate and 8 km away from the reservoir/dam towards both upstream and the downstream of the tributary (US-0, US-8; DS-0, DS-8, respectively). A total of 150 species belonging to 58 families were identified, of which 43% were tree species. Approximately 77% of species were found exclusively in the upstream while 8% were recorded in the downstream. Both canopy and understory layers showed a significant decline in terms of richness, abundance, diversity and stem density from upstream to downstream. Leuceana leucocephala (Lam.) de Wit, an invasive tree species, showed higher relative abundance in downstream reaches, while riparian species such as Terminalia arjuna (Roxb.) Wight & Arn. and Pongamia pinnata (L.) showed no marked decline from upstream to downstream. The conditions triggered by reduced water discharges and severe river bank erosion may have contributed to these changes. The results indicate that the extreme water management practices in storage reservoirs have incurred negative impacts on the composition of the riparian vegetation. Introduction of well-coordinated flow management practices may help to mitigate some of these negative impacts.

River regulation as a result of construction of dams caused global-scale ecological changes to riverine ecosystems. Studies have shown that vegetation alterations may take place due to changes in the water flow, flooding frequencies, and fluctuating water table associated due to dam construction. The Accelerated Mahaweli Project (AMP) which started in 1970's involved construction of five major dams across the Mahaweli River. However, no study has been undertaken so far to assess the dam-induced impacts on downstream riverine forests. The present study was aimed to investigate any dam-induced impacts on the riverine vegetation in the immediate downstream areas of the AMP. In order to achieve this objective, a stretch of Mahaweli River with a regulated water flow (in Minipe area) and a tributary of Mahaweli river with an unregulated water flow Badulu Oya was selected for the study. A complete vegetation sampling was carried out using eight 10 m x 30 m belt transects laid perpendicular to the river flow. Each belt transect was divided into three 10m x 10m zones (hereafter named as lower, mid and upper slopes). The vegetation was quantified under three categories canopy vegetation, understory and ground vegetation. Paired t-test (95% CI) in Minitab 16.0 revealed that the canopy vegetation abundance in the lower slope is significantly higher (p ≤ 0.039) at Badulu Oya than that of Minipe. However, no significant difference was observed in other two zones in between the two rivers. The lower slope showed similar trend in terms of canopy vegetation diversity though it was not significant. Terminalia arjuna and Ficus racemosa dominated the canopy vegetation in Badulu Oya and Minipe, respectively. Both abundance and diversity of the understory vegetation was significantly higher at Badulu Oya than that of Minipe and this effect is more prominent in the lower-and mid-slopes than in the upper slope. Dimorphocalyx glabellus dominated the understory vegetation at Badulu Oya while at Minipe it was Cipadessa baccifera. Riverine vegetation in both rivers was dominated by native species, however no significant difference observed in the abundance of invasive plants. Interestingly, at Badulu Oya the abundance of the riverine vegetation decreased towards the upper slope but at Minipe no such trend was observed. The preliminary findings of the research indicate that the regulation of rivers through damming can have long-term impacts on the riverine vegetation by changing its composition and diversity. However, further studies will authenticate this inference.

Abstract:Systomus martenstyni is an endemic and endangered fish of Sri Lanka. This species was earlier known to be restricted to the Amban Ganga River basin. Recently two populations of S. martenstyni were located in two major tributaries of River Mahaweli: Loggal Oya and Badulu Oya in the Uva province of Sri Lanka. These new populations are relatively small and are facing the threat of habitat quality degradation and habitat loss. Effective conservation actions should be undertaken to conserve this endangered fish species.

No abstract

No abstract