The fastest visible changes are related to the colors and patterns of the hot springs. Most of the bright colors of the springs are produced by bacteria living in hot water, and each species of bacteria has its own color and temperature range. A change in temperature in a pool leads to a change of bacteria and then a change of its color. The more shallow the pool, the higher the possibility of short-term change.

The activity of the geysers can change significantly in very variable periods. Though some geysers, like Old Faithful, have had regular eruptions for more than a century, others are always unpredictable. Others have bursts of activity for a few years, during which eruptions can happen daily, and then suddenly become quiet for decades. The cones of the geysers, however, are extremely stable. Their structure grows so slowly that it is impossible to see significant differences in 17 years.

The growth of the vegetation and particularly of young trees is observable over a 17-year period. In Yellowstone, where the winters are long and hard, this growth is slower than in other places. As mentioned before, this phenomenon is well visible on the pictures taken 10 and 25 years after the fires that burned most of the forest in 1988.

The trees that are killed by hot water overflows from the springs and are partially petrified afterwards are remarkably resistant and can stay unchanged for decades; this amazing phenomenon can be seen on some of the pictures presented here....

-from the Introduction