Rex Allen #17 (1953)

Rex Allen investigates suspicious activity at a dam construction site where dynamite has been used to blast rocks, suspecting foul play by criminals trying to sabotage the valley's water supply. When Rex spots a mysterious figure fleeing on horseback, he pursues the lead and discovers that the criminals plan to keep the valley flooded with water from the dam until their leader, Plate, escapes from jail and can retrieve stolen loot hidden nearby. In a secondary story, "Koko Takes Charge," Rex's horse Koko proves his worth by bucking off a young boy who was riding recklessly, demonstrating that the animal is a better teacher than any human could be.