How do you get Gyarados in Pokemon snap?

When you first enter the valley, start shooting pester balls at the edge of the river in the direction of the crazed mankey. If you keep shooting at the water, a magikarp will pop out and flop towards the mankey. The mankey will then punch it and send it flying through the air. Keep going through the valley until you reach a waterfall. Look to the left and the magikarp will fall onto the ledge near you. Throw another pester ball at it and it will flop into the water towards the waterfall, where it will suddenly evolve into gyarados. That's when you take your snapshots.