The reason Casper exists in the first place, and still its best amenity. The North Platte runs cold and clear past the city — fed by Wyoming snowpack and tightly regulated below several upstream dams — which makes it one of the country's better tailwater trout fisheries. Rainbows, browns, and the occasional cutthroat are the quarry; the "Miracle Mile" section about an hour upstream is the most famous stretch.
Drift-boat fishing trips with local guides run from March through November. In summer the same guides will run scenic floats for people who do not want to fish. The Platte River Trail along the banks gives free access to roughly ten miles of paved riverside walking and cycling.