ABSTRACT: Quantitative estimations of climate changes on hydrological processes help to understand and  manage waterresource problems such as floods and droughts in the future. This study aims to present the preliminary results of astudy to determine the mpact of climate change on river flow to develop sustainable watershed management plans and to evaluate the possible urban water use and environmental impacts to prepare policies to mitigate its negativeimpacts. In this study the Soil and Water Assessment Tool (SWAT) was used as a hydrological model to predicthydrological effects of climate change to better understand and solve future water resource problems. In this context,firstly the Soil and Water Assessment Tool (SWAT) was operated on the e Yuvacık Dam Basin in Marmara Region toreveal the present hydrological situation. The hydrological model was then simulated by using the climate changedata that have 20 km spatial resolution between 2021-2099 years based on RCP 4.5 and RCP 8.5 scenarios thatwere produced by the General Directorate of Meteorology. Thus, the effects of climate change on the amount ofwater and sediment were estimated between these years. According to the RCP 4.5  and  RCP 8.5 scenarios, 2.23% and  2.062% decrease in the average annual rainfall  1.24  0.03 of increase in temperature values werepredicted, respectively. However, an increase in precipitation was expected between April  August. The averagemonthly flows (between 2006-2014) of the e Kazandere, Kirazdere  Serindere rivers recharging the Yuvacık Damin the basin is 0.55, 1.28 1.94 m3/s, respectively. Corresponding modelled flow values based on RCP 4.5  RCP 8.5 scenarios for 2021-2099 period, for Kazandere, Kirazdere  Serindere are 0.14  0.17 m3/s, 0.41  0.33 m3/s, 0.86  0.68 m3/s, respectively. These values show significant decrease in flow rates o the rivers in the future. Moreover the peak flow values of the streams especially for Kazandere  Kirazdere decrease from 8 to2 m3/sec The amount of the monthly average of sediment measured at the Serindere stream station for the years2010-2013 was 49 tons, while it was calculated as 247.58 tons based on RCP 4.5  332.21 tons based on RCP 8.5for 2021-2099 period. According to both scenarios, an increase in the predicted rainfall between April  August,  an increase in temperature will cause mechanical erosion in the basin. Thus the amount of sediment carriedby flow is expected to increase. The Yuvacık Dam is used for providing irrigation  drinking water requirementsof Sakarya  Izmit provinces,  for the prevention of flood as well as for water management during droughtperiods. Therefore,the decrease in the flow rates of the streams recharging he dam will cause a decrease in thewater volume of the dam, hence the future water management planning should be made for the crop pattern in theirrigated areas of the dam  for the drinking water usage. Furthermore, the decrease in  the flow rates will causeto decrease the forest areas in the basin  which in turn will lead to an increase in the amount of erosion. This willcause an increase in the amount of sediment coming to the reservoir thus the service life time of the reservoir willbe shortened