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Winter of course—Yellowstone is known for its long, cold winters!

Even so, the water in much of the Firehole River stays relatively warm year around. The coldest temperatures actually occur in mid June when the runoff from melting snow is greatest. The warmest period is in August when the snow melt has gone and the summer sun is heating the water. Therefore, the shift in rainbow trout spawning time serves two purposes—it avoids the highest temperatures of late summer and allows the young to grow throughout the winter before the river reaches its hottest temperature again. This means the fish are older before they have to cope with the stresses of high water temperature.

Why don’t the fish take refuge in tributaries of the Firehole during the summer?

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Cast of characters - Maya Crowbes, Dr. A Beadle, Gray Fischer, Viola DeRama
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Q: When is the water in the Firehole River coldest?

GLOSSARY: tributary(ies)—a stream that flows into a larger stream or body of water