GRAND TETON NATURE NOTES

MUDFLOWS IN THE TETONS 1941.

By Carl E. Jepson

During the month of August, 1941 there occurred in several of the canyons of the Grand Teton National Park a great number of landslides, or probably a better name would be mudflows.

Very heavy rains during the early part of the month, and a total of 3.83 inches for the month, which is more than two inches above the average for the month for the past several years, as well as fairly heavy rains during July, no doubt were the immediate causes of the mudflows.

The slides occurred in nearly all of the canyons of the park at presumably the same period, believed to be the nights of August 8th and 9th. Probably the most significant flows are those in Leigh and Moran Canyons, yet other very large and important mudflows occurred in Cascade, Death, Open and Granite Canyons, with minor flows in other canyons.

Just how many of these mudflows there were in all is a matter of conjecture. It is my guess that there were at least seventy-five, and probably a hundred would be nearer the correct number. Since only a portion of the terrain in which these mudflows occurred has been visited by human beings since their occurrence, there is really no way of determining their number except by estimation on the basis of the number found in the canyons that have been investigated.

As far as known, no such phenomena have been reported in the past, and there does not seem to be any evidence to support a theory that such mudflows are of a common occurrence.

The cause of these mudflows, as has been previously stated, was without question due to the unprecedented heavy or torrential downpour of rain during July and August. Several days of heavy rains preceding the flows had no doubt saturated to great depths the material in many of the alluvial deposits at the base of the numerous side canyons or gullies on both sides of the main canyons.

Then as the great deluge of water coming down the couloirs and gullies of the steep slopes of the mountains washed over these alluvial deposits, great gullies were formed in them and much of the material was carried along and spread out in great flows of mud, sand, gravel and boulders that spread out fanwise as it appreached less steep grades of the lower canyons.

Most spectacular of all the mudflows is the largest one occurring in Leigh Canyon. It was described by Dr. F. M. Fryxell as follows:

"In lower Leigh Canyon, converging mudflows floored the canyon and pushed downstream a quarter of a mile, wiping out scrub forest. Above this point the canyon was dammed in two places, one mudflow 700 feet wide impounding a lake 1000 feet long."

The writer visited this particular mudflow some time after Dr. Fryxell had reported its presence and found that the lake so formed by the mudflow still remained. To all indications the dam seemed very substantial and in slight danger of being washed out. The tremendous size of the flow was astounding and hardly conceivable.

The vast amount of material that had been moved and the size of some of the boulders that had been swept along by the flood was most surprising.

As far as known no one witnessed the actual movement of any of the mudflows. A few individuals reported hearing what they termed avalanches, but no doubt it was the roar of the water and mudflows that were heard.

A few people encountered very high water in Cascade Canyon and some reported having to take to the steep mountain slopes to avoid the floods.

Further study of these mudflows is required before a complete or more comprehensive report can be given. Enough to say at this time, that it was a most unusual occurrence and resulted in a great deal of damage to the trails of the park, many plants, shrubs and trees were destroyed and no doubt many of the small animals suffered, if not loss of life at least loss of a home, especially in the case of conies, marmots and chipmunks.

Many of the mountain climbers reported a very hazardous condition existing in the couloirs of many of the favorite routes up several of the peaks as a result of the flood that loosened many of the formerly securely bedded rocks and left them precariously perched in the gullies or couloirs.

We regret that we do not have the names of the park visitors who reported their adventures with the floods up Cascade Canyon and hope if they, by any chance, may read these Nature Notes, they will send us any information they may have in regard to these mudflows or their experience with the floods at the time of these mudflows.