The best time to visit Fort Thompson is late spring through early fall (May to September). The Missouri River country is dramatic in any season, but summer gives you access to Lake Sharpe for fishing and boating, and the days are long enough to explore the surrounding reservation back roads properly.
Spring (April–May) is particularly good for birdwatching — waterfowl migration peaks then, with snow geese moving through in huge flocks. Fishing is excellent spring and fall, when walleye are more active in cooler water. Summer temperatures regularly exceed 90°F (32°C), and the plains can be exposed and harsh in high summer heat. Winters are severe: temperatures well below freezing, wind, and occasional heavy snow make travel difficult and most outdoor appeal disappears. If you are visiting primarily for the landscape and archaeological sites rather than fishing, late May or early September gives the best combination of mild weather and open access.