During which stage of erosion do small channels, or "rills," begin to form?

The formation of small channels, known as "rills," occurs when sheet flow, or the thin layer of water that flows over the surface of the soil, begins to concentrate in low-lying areas. This process typically happens when the water that moves across a surface loses its uniform coverage and starts to funnel into defined paths due to the topography of the land. As this concentration occurs, the flowing water begins to erode the soil, creating small, often temporary channels.

In contrast, while excessive water infiltration, removal of vegetation, and heavy rainfall events contribute to erosion in various ways, they do not directly result in the formation of rills themselves. High levels of infiltration can lead to saturation, but it's the concentration of flow that specifically initiates the rill development. Vegetation removal can certainly exacerbate erosion by removing root structures that stabilize soil, but the actual formation of rills is primarily driven by the concentration of water flow. Heavy rainfall events can lead to increased sheet flow but do not directly trigger rill formation without the specific condition of flow concentration in low areas.