News and Opinion from Sisters, Oregon
News and Opinion from Sisters, Oregon
Thirty seconds later the torrent reached the Metolius River at Mile 29 leaving in its wake a stark seven- acre field of boulders. About a dozen large trees survived the onslaught. Up to eight vertical feet of bark were battered away by rocks and mud surged up the trunks leaving stains as high as 25 feet above the ground.
Exactly when the debris torrent rushed down the side of Green Ridge is not known. Nobody saw it happen. But it probably occurred early on February 7 when rainfall rates were at their highest. It was discovered a few days later by John Berquist, a Sisters Ranger District fisheries volunteer while he was walking the river observing flood- related effects.
The debris torrent began halfway up the side of Green Ridge, 1,000 vertical feet above the Metolius River. It happened after heavy, relatively warm rains began falling on two to three feet of snow, quickly melting the snow and overwhelming the ability of soils to absorb the sudden release of water.
The water rapidly collected and flowed down the channel of the little canyon. Perhaps a small dam of organic debris failed or soils along the channel began to suddenly erode and slump into the rising stream. Whatever initially happened is not known, but once a small surge or wave of water started down the channel, the surge grew and gained speed dramatically by scouring out and consuming ever- increasing amounts of water, wet snow, soil, rocks, and vegetation.
By the time the torrent neared the mouth of the canyon, the great slug of water and debris was riding high on the outer bank of every curve, like a bobsled pinned high on sharp turns. On the final curve the torrent climbed 70 feet up the outer bank but only 20 feet up the inside bank.
When the debris torrent gushed out of the canyon and onto the alluvial fan, it toppled and swept away more than 100 trees, stripped away most soil, and rearranged most rocks and boulders originally on the fan. On average, about one or two feet of debris, mostly rocks were added to the surface of the fan.
Along both sides of the boulder field are walls of toppled trees, like large, crudely built rail fences. Surprisingly little mud or debris can be found beyond these walls. Apparently the trees in the walls and the large boulders found within them were thrown to the side of the torrent, like a large ocean wave casting debris high on a shore.
At the lower end of the fan near the Metolius River, the debris torrent ran out of steam. A strip of forest of variable width separates the river and the end of the boulder field. Parts of tree trunks, branches, and boulders crashed into the strip, but in only three narrow places did the torrent plow channels through it. Apparently most of the torrent, now largely stripped of its energy, poured into the swollen Metolius River through the middle and northern channel. A relatively small area of debris left by the torrent has reduced the width of the Metolius by about one- quarter.
Visitors to the site of the debris torrent will find many features to marvel at and ponder. This torrent is the most recent of a great many that have swept into the Metolius River every few centuries.
The entire alluvial fan at Mile 29 was constructed by countless such torrents since the last Ice Age and probably long before that. The ancient fan continues to hold the Metolius at bay. Immediately upstream the river flows smoothly and silently behind the dam created by the fan in the river channel.
Then in a short distance it cascades down to its older well- established channel below.
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