Draw two layers
Sketch a firm layer above a softer one and put your river’s flow from left to right. At the drop, draw a small pool. Your first sketch needs the top shelf to rest on the softer rock under it.
Draw the missing support
In a second panel, let the water remove part of that lower layer near the pool. Draw the shelf projecting over the recess. In a third panel, cross out the unsupported piece that broke off. The new edge is behind the old one, to the left in this drawing.
Keep the two arrows separate
Draw one arrow for the direction the water flows and another for the direction the fall moves. The water still runs downstream to the right. The ledge has moved left, towards the river’s source, as rock failed near its edge.
For the underlying idea, read how a waterfall moves. Explore more routes in the water cycle hub.
Read the source: US Geological Survey · Waterfalls of Yellowstone.