This study investigates the spatio-temporal changes in the downstream river bed during a dam construction on the Buyukkumla river channel in Bursa's Gemlik district in Turkey. To achieve this goal, an evaluation of the seasonal and annual effectiveness of dam construction activities in the study area, identifying geomorphological effects during dam construction, determining the spatio-temporal changes in the downstream river bed based on the comprehensive field observations, and analyzing the impacts on erosion-accumulation dynamics caused during the establishment of the dam is discussed. Measurements along the river stream were measured for different times, starting from the dam downstream. The two methods, satellite technology Global Navigation Satellite System (GNSS), also known as Global Positioning System (GPS) and terrestrial methods were used. GNSS was applied to fewer vegetation areas, whereas terrestrial methods were employed for areas with dense vegetation. Using the real point data obtained from the measurements, applying the kriging interpolation method, a Digital Elevation Model (DEM) was created by the Geographical Information Systems (GIS) programs, with similar resolution (each pixel with a 10(-5) x 10(-5) degree resolution). The results dividing the downstream side from the dam to the sea into two parts were obtained. In the upper zone of the study area, just downstream of the dam, accumulation in summer and scouring in winter were observed during the dam construction activities. Contrarily, in the lower part of the study area where the river flows into the sea, scouring in summer and accumulation in winter were dominant. One and half years later, after the dam excavation was over, with a little accumulation, the river bed reached an equilibrium condition. In the previous periods, while scours and accumulations higher than 6,000 m(3) were observed, the net change was, respectively, 933 m(3) erosion and 963 m(3) accumulation in the last two semi-annual periods.