Question

what is moving (erosion) the rock in this canyon?

Explanation:

In a canyon, the main agent that moves (erodes) rock is usually water (from rivers or streams) or wind. Water erosion is common in canyons formed by rivers, as the flowing water carries away rock particles. Wind erosion can also occur, especially in arid canyon environments, where wind transports sediment. Gravity can also play a role in mass wasting (like landslides) that moves rock. But for most canyons, water (river flow) is a primary erosional agent moving rock.

Answer:

Water (from the river/stream in the canyon) is commonly the agent moving (eroding) the rock. Wind or gravity - related processes (like mass wasting) can also contribute, but water is a primary erosional agent in many canyons.