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