This study investigates the spatio-temporal changes in the downstream river bed during a dam construction on the Büyükkumla 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. The measurements for March of 2014, June and October of 2016, March and November of 2017, and April of 2018 were recorded from the beginning to the end of the dam construction. 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.