River blockage by landslides is a common geological disaster in mountainous regions that threaten the safety of human lives. Therefore, it is necessary to better understand the formation and failure of landslide dams. In this work, flume experiments are conducted to study the influence of landslide mobility and river flow dynamics, both characterized by the Froude number, on the formation process of landslide dams and the degree of river blockage. Here, river blockage is classified as either complete or partial depending on the ratio between the minimum dam height over the river flow depth. A criterion based on the relative Froude numbers of landslides and rivers is proposed to judge the degree of river blockage. We further found that during breaching, the erosion rate is still mainly controlled by the Froude number of the breaching flood and less so by the degree of blockage. The results of this work are expected to enhance the understanding of the formation and failure of landslide dams.