Koga River is one of the major tributary of Blue Nile (at the head), it is said to be Gilgel Abay which flows to Lake Tana. The river has a Catchment area of 27850 ha approximately 50% is under cultivation. As it was indicated in the project document most of submergent area was a plain land (wetlands) which was used as a grazing land by the community in the dry season, as wetlands are rich in biomass production, moreover, the animal manure that were added for a numbers of years will have a significant impact on improving water water quality in its nutrient load. So it seems potential to carry sufficient fish population through trophic structure. Water quality assessment is the overall process of evaluation of the physical, chemical and biological nature of water in relation to natural quality, human effects and intended uses, particularly uses which may affect human health and the health of the aquatic system itself. It includes the use of monitoring to define the condition of the water, to provide the basis for detecting trends and to provide the information enabling the establishment of cause-effect relationships. Important aspects of an assessment are the interpretation and reporting of the results of monitoring and the making of recommendations for future actions. As in the case of fisheries resources most of our rivers inhabit some common types of fish species like tilapia, cat fish and barbs, and Koga reservoir may have those species because of its interconnection with the lake system. Food-web structure regulates the productivity of lakes other than nutrients and light. Fish yield, for example, strongly depends on primary production of Phytoplankton. The primary production of organic matter, in the form of phytoplankton and macrophytes, is most intensive in lakes and reservoirs and usually more limited in rivers. Since the project envisages the creation of a reservoir with an impoundment area of about 1859 ha at full supply level. It is ideal that the newly created reservoir could be used for the development of fisheries. It is a justifiable and economical method to improve the availability of animal protein especially for local population. The failure to utilize reservoirs as fresh water resources for fisheries development constitutes a waste in potential production. This study will enable to determine both the water quality and its status of productivity and also to determine the fish species present in the reservoir and their abundance, distribution and temporal variation to facilitate the future management plan options for the benefit of the rural people and the Region at large.