The power of water on Catbells

We found this remarkable channel on the path up to Catbells from Grange. I assume it was cut on the floods weekend in December. Look at the stones piled up against the wall at the bottom. Let's say 1 metre high by 10 metres wide 1 metre deep. That's a volume of 10 cubic metres of rock. The density of rock is about 3 tonnes per cubic metre. That's 30 tonnes of rock shifted. Maybe not as much as I'd thought. What impressed me the most is that the water had such power given the short run-off. It looks like it gets about 150 metres of stream bed. I'm thinking about possible equations. I'd like to apply Newton's Second Law as F = v (dm/dt) where the final term is mass flow rate but the velocity of the flow is not constant because it is accelerated by gravity.