This paper presents an experimental study of coupling fluvial erosion and cantilever failure with the slump block effect for cohesive banks. Two types of cohesive materials with different percentages of silt-clay content were investigated under similar hydraulic conditions using acceleration sensors to clarify failure phenomena. The experimental results showed that fluvial erosion of the lower part of the cohesive banks progressively undermines the upper part during the initial stage of a cantilever failure. Tension cracks then develop at the upper surface of the cohesive banks, and beam-type failure occurs thereafter. Furthermore, slump block failures and decompositions were observed as the new phenomena in this study. The geometries of slump block including its failure and decomposition mainly depended on the cohesive force of the cohesive banks. Therefore, a reduction of the silt-clay content leads to smaller slump block dimensions as well as faster decomposition.