The catchment area of the Tuul River is located in the centre of Mongolia. The study has been done by two datasets SRTM and ASTER, using two different models of ArcHydro and ILWIS. At the end of the catchment definition, sixteen parameters of the Tuul River and its catchment area have been redefined. The water quality assessment study has provided a comprehensive water chemistry assessment of the Tuul River in the area surrounding UB city, Mongolia, using dataset collected between 1998 and 2008. It presents the spatio-temporal assessment and seasonal pattern of 15 hydro-chemicals at 15 monitoring sites. The level of pollution in the downstream section of the river strongly depends on CWTP. There is a process of natural purification within the river, but even 50 km downstream of the city pollution can still be detected. In order to change this situation, improvement of CWTP and artificial increment of DO become crucial to improve the water quality. DO concentrations can be artificially increased using bull stone wall, which has big enough holes that fish and sediment can easily pass through. This method is more eco-friendly and works more effectively over the long period.