Yellow swirls on the lake have intrigued visitors and scientists.

A sure sign that spring has finally arrived in the high country around Crater Lake is the first appearance of broad yellow swirls of material floating on its surface. These canary yellow deposits stand out vividly against the deep cobalt blue of the water. For summer visitors who gaze down on the placid water below, such a sight can be unsettling. What is this yellow stuff, sulfur from the old volcano, pollution from all the cars and boats? Park staff are well aware how the public demands that Crater Lake, the deepest, bluest, and clearest lake of its type in the world, must be kept that way. Because of this concern, Park Naturalist Earl U. Homuth wrote an article about these yellow deposits and presented it in the first edition of Crater Lake's Nature Notes in July 1928: Obviously, these early park visitors too were concerned about the lake. Rangers have been repeating Homuth's explanations to tourists ever since.

Close examination of this yellow substance shows it is pollen. But where is it from? Each year, warming temperatures awaken trees in the region, which have been locked in a deep dormancy since the preceding fall. By late May and June, warmer weather finally reaches the Rim around Crater Lake, causing the pine and fir forest to come alive. On the lower branches of these conifers, catkins, which appear like small cones, begin to develop. Each catkin will produce millions of pollen grains no larger than a tenth of a millimeter. Once mature, a gust of wind will free the pollen from the branches and carry it off to other trees so that seed cones can be pollinated. Clouds of yellow pollen may be carried for miles by the wind. Some pollen grains support wing-like appendages that help keep them airborne. However, most of the pollen never finds a seed to pollinate. As wind currents die, the pollen falls out onto the ground, onto parked cars, and onto the surface of our lake. (Those of us who suffer the effects of hayfever each spring will be glad to know that pollen produced by conifers does not affect our allergies). Once the pollen reaches the lake, water currents carry the yellow grains towards the shoreline where they become concentrated into the many curious swirls we see from the Rim.

If pollen settles on the lake each spring, where does it go? Some of the pollen washes ashore creating a giant "bath tub" ring and some sinks to the bottom. As the pollen settles to the lake bottom it becomes a part of a growing layer of lake sediment that began accumulating here when the lake first formed. Scientists would like to know exactly when that process began. The caldera, which holds the lake, was formed following the eruption and collapse of Mt. Mazama 7700 years ago. By dating the pollen grains in the oldest and deepest lake sediment, scientists can tell when lake water first appeared. The analysis has yielded a surprising result. Our lake is very old. Water began covering the basin of the caldera almost immediately after it was created. It took only 300 years for the entire lake to fill.

Pollen grains from different trees can be distinguished in core samples. Pollen released by true firs doesn't travel well. The grains are relatively heavy and tend to fall out close to their source. Their appearance in core samples from the lake indicate exactly when these and other trees began reforesting the lower part of Mt. Mazama following events 7700 years ago. The carbon dating of fir pollen suggests that forests began appearing around and beneath the Rim after only 400 years. But just as the fir and pine forests took root, they abruptly disappeared. For nearly 1000 years, fir and pine pollen is nearly absent in lake core samples. In their place are pollens from trees found at much lower elevations today like juniper, oak, and Douglas-fir (not a true fir). What happened? The most likely explanation is that the local climate both warmed and dried, permitting these species to replace those favoring a cooler and moister environment. These conditions reversed 6000 years ago and have stayed much the same since.

Is it possible that, during these centuries of warmer weather, reduced snowfall dropped the lake level below what we now see? Today, 67 inches (170 cm) of precipitation annually is needed to keep the lake 1,932 feet (589 meters) deep. If the lake level dropped 30 feet (9 meters), Wizard Island would be connected to the rest of the caldera!

The yellow swirls on the water of Crater Lake have become an annual sign of the arrival of spring in the Southern Cascades of Oregon. In addition to attracting the attention of casual visitors, scientists have found a reason to analyze the pollen once it reaches the lake bottom. Ranger Naturalist Earl U. Homuth would surely be pleased to know that lake pollen continues to tell interesting things about this magnificent place.