It's obvious in the reds and oranges flecking the forests, the crispness of the air, and even in the slow gait of the old bison. Summer's long gone in Yellowstone National Park, and winter isn't far off.

Underfoot, the seasonal change is evident in the crunch of dried leaves, while along the runoff trough from Imperial Geyser the rimming grasses and plants have seen their supple green stalks and leaves go the color of straw and umber. You can smell the change in the slow decay of forest duff.

I haven't seen any chevrons of Canada goose or ducks passing overhead, though in Yellowstone some of these waterfowl stay year-round, and others no doubt took flight when I wasn't watching.

For the old bison we encounter along the Grand Loop Road, the coming winter might be his last. The bull is slowly following the road north, heading perhaps to Gibbon Meadows with its dry fall forage. He pauses here and there to bite a bunch of dried grasses along the shoulder, oblivious to the traffic stopping for photographs. He won't be hurried, lacks the interest or energy to butt the vehicles, and has an intestinal ailment of some sort that leaves a decidedly un-buffalo-chip mark on the roadway.

Warm weather is fleeting in the Rockies, and the transition to winter can be quick. That was seen at Glacier National Park, where an early October snowstorm left a fresh coat of white on the mountains that frame Saint Mary Lake. It likely won't stick, but it sure is pretty.

Change is rampant across the National Park System this time of year. Many of Yosemite Valley's waterfalls are tricklers now at best, and in the Appalachians the rouge of fall is rolling down the flanks in Shenandoah and Great Smoky and along the Blue Ridge Parkway. In Maine, the hardwoods of Acadia National Park are dazzling. Soon these mountains from Acadia to the Smokies will grow more angular, falling leaves taking the roundness off the forests and exposing cliffs and outcrops.

True, the redrock doesn't lose its hue Arches and Canyonlands with fall, though the crowds thin out decidedly, making it easier than just a month ago to drive up and land a campsite in the parks' front-country campgrounds.

This is the season in many national parks to best experience nature's transformation from vibrancy to dormancy, from the robustness of life to the decay of age. It's not a season to turn your back on the parks. But it is one for heavier layers when you head out on a hike, for watching the metamorphosis play out, for a comfortable chair in front of a cracklin' fire and a hot drink when you return.

In truth, there is no bad season in the parks.