Abstract: A rock slope in Nujiang River (China) was taken as an example to better understand the distribution of the gently-dipping discontinuities subjected to river incision. Based on field surveys, geological analysis and numerical simulation, a study on the density and evolution of the gently-dipping discontinuities was carried out to reveal the distribution and genesis mechanism. The results indicate that the gently-dipping discontinuities in the epigenetic reworking stage have evolved to distribute at irregular intervals: the discontinuities concentrate in certain zones (i.e., high-density zones), separated by some low-density zones. Totally, there are three to four high-density evolution zones of gently-dipping discontinuities. The density of discontinuities in these high-density zones ranges from 10 to 18 per 10 square meters, while the value in the low-density zones is 5 to 8 per 10 square meters. Moreover, the geometry and kinematic analyses of the system of discontinuities suggest that the genesis of the discontinuities can be synthesized into a progressive evolution model. The model suggests that the joint starts with the formation of an array of en-echelon cracks, with continued crack elongation and shearing; then they approach one another and interact, forming a complex joint system finally. According to the history of river valley evolution, a view is proposed that the river intermittent incision was the main factor resulting in the intermittently concentrated distribution of the gently-dipping discontinuities.