The fluvial landscape of the Hengchun Peninsula in southernmost Taiwan is generally characterized by a narrow river and a wide valley plain. Such underfit stream features are unusual in tectonically active areas such as Taiwan. In this research, we hypothesize that the relative sea level change since the end of the last glacial period might be the main factor driving the formation of this type of landscape on the relatively recently emerged Taiwan Island. The Gangkou River is among the largest and most pristine rivers on the Hengchun Peninsula, and well-preserved river terraces and bedrock exposures occur along the river and river mouth. Therefore, the Gangkou River was thoroughly surveyed to investigate the factors shaping the landscape and to determine the landscape evolutionary history. Our research indicates that the average uplift rate of the Gangkou River is 1.5 - 1.7 mm yr^-1. Because of the low uplift rate, the rising sea level after the last glaciation caused widespread aggradation in the Gangkou River. At approximately 8.0 - 7.7 ka, maximum flooding occurred, and the sea level subsequently remained stable, while the aggradation stopped after 7.1 ka. Incision followed, forming the high terraces, and starting in the Little Ice Age, repeated aggradation and incision occurred, forming the low terraces. These processes produced the current landscape of the Gangkou River. Overall, the uplift rates calculated from this research are relatively low in this tectonically active area.