In this stage mass wasting of hillslopes is dominated by diffusive downslope processes, and therefore is weathering-limited. As river energy slopes increase due to continued rock uplift, stream power increases, but the mass flux of sediment fed to the channels remains roughly the same because local relief remains constant. Streams erode their alluvial beds to reach capacity and begin to downcut through bedrock, using the sediment load as abrasive tools. Local relief increases, thereby increasing sediment volumes delivered to the channel. The increased sediment load provides more abrasive tools and, as long as streams remain under capacity, bedrock wear rate continues to increase, further increasing relief.
Greater relief leads to increased frequency and magnitude of bedrock-involved landslides.
This run-away system is checked when hillslope processes begin to provide too much sediment to streams. Channels begin to alluviate, bedrock erosion slows, and local relief lowers. Ultimately, a landscape is reached in which rivers delicately adjust local relief to provide just enough abrasive tools to erode their bedrock floors at the bedrock uplift rate. This is a landscape where bedrock rivers are at grade. For plucking, none of this argument applies.