A stream's ability to accumulate and transport soil and weathered rock is aided by the work of raindrops, which knock sediment particles loose(see Figure 4.19, p. 99). When the ground is saturated, rainwater cannot infiltrate so it flows downslope, transporting some of the material it has dislodged. On barren slopes the sheet flow will often erode small channels, or rills which in time may evolve into larger gullies(see Figure 4.20 on p. 100).