Morphometry, Water quality and Sedimentation of Phewa lake, Pokhara, Nepal SHRESTHA, Gyan Kumar Chhipi 1 and PRADHAN, Ananta Man Singh 2 1 Lecturer, Central Dept. of Environmental Science (CDES), Tribhuvan University (gshrestha@cdes.edu.np) 2 Engineering Geologist, Dept. of Electricity Development, GoN; Guest Faculty, CDES (anantageo@gmail.com) Abstract The Phewa lake is one of the prominent tourist attractions of Pokhara close to Annapurna Mountain Range. The morphometric parameters were determined by the field survey in September and October, 2007 whereas for the analysis of water quality, the field survey was carried out in October, 2009. The depth of Phewa Lake were measured by randomly dipping Heavy Grab sampler at 107 sites of the lake and the site was located using GPS (Global Positioning System). These sites and depths were plotted in digital topographic map and calculated morphometric parameters using GIS software ArcView 3.3. Altogether, 25 sites were selected for water quality determination and Von Dorn water sampler was used to collect water samples from different depths of each site. The dissolved oxygen of surface water is above 5 mg/L indicating suitable for aquatic life however it is lower than that at deeper water. This indicates that the water quality of the lake is moderate but is gradually started its deterioration at some sites due to the inflow of open drainage containing sewage and solid wastes. The Phewa lake having water of volume of 30,358,882 m3 and maximum depth of 22.75 metres has high sedimentation that is triggered by bank erosion of the inlet streams, intensive land use practice without due consideration to soil erosion and frequent debris flow in the steep slopes. Therefore, there is an urgent need to control sedimentation and also organic load inflow into the lake. For this, complete catchment should be taken into action and local funds can be generated through Payment for Ecosystem Service (PES) mechanism. Key words: Bathymetric map, morphometry, sedimentation, water quality. 1. Introduction Phewa watershed lies in the Mahabharat Range and is situated in the western part of Pokhara Valley, Kaski District of Nepal. It lies within the latitude of 28 ° 11′37″ to 28 ° 17′26″ N and longitude of 83 ° 48′02″ to 85 ° 59′18″ E which drains into Phewa Lake [1]. The topography varies from 850 m at the lake to 2500 m at Panchaase peak. The major river of the catchment, the Harpan Khola drains into Seti River and thus forms part of the Gandaki (Narayani) river basin. The climate is humid sub- tropical to humid temperate by a monsoonal rainfall pattern. Mean temperature at Pokhara (850m) ranges from 12 ° C in Dec/Jan. to 25 ° C in July/August [2]. Average annual rainfall is 3850mm (Airport, elev. 854m) to 5200mm (Lumle,1662 masl) [1]. Pokhara valley is an anticlinorium whose axial plane runs NW-SE almost parallel to Seti river. Pokhara velley is fluvio-glacial and partly lacustrine deposit whose parent materials throughout the watershed is grey- phyllitic schist. Sub-tropical Riveraine forest is found near the Lake and lower reaches of Harpan khola where Castanopsis spp., Alnus spp ,and Schima spp. are dominant. The land cover map of the Phewa watershed is shown in Fig 1. Fig 1. Land cover map of Phewa watershed Phewa Lake is one of the prominent tourist attractions of Pokhara close to Annapurna Mountain Range It supplies water for Hydro Electricity Power Station and irrigation to approx. 320 hectares of agricultural land. Conservation of Phewa Lake from siltation and pollution has become a topic of national interest. The lake watershed lies in the fragile physiographic region which experiences intense monsoon rainfall events. Intensive land use and construction of roads without due consideration to conservation measures are the major causes of erosion process in the watershed. This has transported enormous amount of sediments to the lake, thus reducing its capacity [3].