Why do I believe it to be a debris-flow remnant?

Here are some descriptions of debris-flows associated with glaciers. One in particular sounds as if it is describing this very feature (second citation below)! Debris-flows associated with glaciers are very common occurrences.

First a general description from http://geohazards.cr.usgs.gov/factsheets/html_files/debrisflow/fs176-97.html

Debris-Flow Hazards in the United States

By Lynn M. Highland, Stephenson D. Ellen, Sarah B. Christian, and William M. Brown III

U.S. Geological Survey Fact Sheet 176-97

Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly. Debris flows, sometimes referred to as mudslides, mudflows, lahars, or debris avalanches, are common types of fast-moving landslides. These flows generally occur during periods of intense rainfall or rapid snowmelt. They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that are typically about 10 mph, but can exceed 35 mph. The consistency of debris flows ranges from watery mud to thick, rocky mud that can carry large items such as boulders, trees, and cars. Debris flows from many different sources can combine in channels where their destructive power may be greatly increased. They continue flowing down hills and through channels, growing in volume with the addition of water, sand, mud, boulders, trees, and other materials. When the flows reach canyon mouths or flatter ground, the debris spreads over a broad area, sometimes accumulating in thick deposits that can wreak havoc in developed areas.

Doesn't that sound like a plausible description of what probably happened here in Kyle Canyon? Snow melt plus a torrental downpour could have caused liquification of sediment and debris on the steep upper slope. Then it came roaring down over the waterfall, picking up more material along the way down to the main canyon where it sloshed across to the other side, damming it up, temporarily. With about half of the mixture being water, at a speed of perhaps several tens of miles per hour, the word "slosh" may be appropriate although the picture of water sloshing from side to side in a moving container is not correct. It was a "one-way slosh." When the soggy, turbulent, and massive mess of water and debris slowed, water drained rapidly, stopping the mass' movement. What was left was a solid dam, with hardly any sorting of its content.

The next description, one that tells what happened in an actual debris-flow not long ago, could also have been written about the event in Kyle Canyon a long, long time ago:

This is from http://vulcan.wr.usgs.gov/Volcanoes/Hood/RiverDrainages/Polallie/OFR84-578/framework.html
-- Gary L. Gallino and Thomas C. Pierson, 1984, The 1980 Polallie Creek Debris Flow and Subsequent Dam-Break Flood, East Fork Hood River Basin, Oregon : USGS Open-File Report 84-578, 37p.

At approximately 9 p.m. on December 25, 1980, intense rainfall and extremely wet antecedent conditions combined to trigger a landslide of approximately 5,000 cubic yards at the head of Polallie Creek Canyon on the northeast flank of Mount Hood. The landslide was transformed rapidly into a debris flow, which surged down the channel at velocities between about 40 and 50 ft/s, eroding and incorporating large volumes of channel fill and uprooted vegetation. When it reached the debris fan at the confluence with the East Fork Hood River, the debris flow deposited approximately 100,000 cubic yards of saturated, poorly sorted debris to a maximum thickness of 35 ft, forming a 750-ft-long temporary dam across the channel. Within approximately 12 minutes, a lake of 85 acre-feet formed behind the blockage, breached the dam, and sent a flood wave down the East Fork Hood River. The combined debris flow and flood resulted in one fatality and over $13 million in damage to a highway, bridges, parks, and a water-supply pipeline.

Note that a "poorly sorted" debris-flow created in the upper reaches flowed down very rapidly and created a dam across a river. A temporary lake formed that overflowed and then breached the dam. This is exactly the scenario I envision concerning the alleged moraine's history: it is a debris-flow remnant with evidence of temporary, one-time dam overflow.

This is the end of the story. What follows below the line is some pictures of a debris-flow off Mt. Hood. I get the impression, perusing these volcanism websites, that glaciers without debris-flows is a very unusaul thing. So the debris-flow remnant in Kyle Canyon is actualy pretty convincing proof of glaciers on the slopes above it!

At the very end of this page is an EPILOGUE discussing the timing of ice ages and why this glacial debris-flow remant may be about 10,000 years old rather than the 20,000 years stated in the newspaper article.

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I picked several pictures of one spectacular debris-flow off Mt. Hood in Oregon, just for fun and because you have suffered through enough words already. These are from http://vulcan.wr.usgs.gov/Imgs/Jpg/Hood/WhiteRiver1998/hood_white_river3.jpg and from http://vulcan.wr.usgs.gov/Imgs/Jpg/Hood/WhiteRiver1998/hood_white_river1.jpg, respectively:

Some more words remind us that it is warm weather that causes glacial debris-flows. These are very recent (summer of 2001) observations: http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/2001/current_updates_20010815.html

Cascade Range Current Update
Submitted at 5:30 PDT, August 15, 2001
Hot weather makes for high runoff from Rainier glaciers

Hot weather in the Pacific Northwest has resulted in increased melting of glaciers around Mount Rainier, producing more melt water than usual. Around 9 pm yesterday evening (14 August), Mount Rainier National Park rangers reported a flood event near Christine Falls in lower Van Trump Creek within the park. Observations today by Mount Rainier National Park rangers and USGS scientists indicate that meltwater from Kautz Glacier had saturated loose glacial debris in the upper Van Trump Creek drainage area. Portions of this saturated material slumped last night, resulting in small debris flows that moved down Van Trump Creek to the Nisqually River. Minor rise in the Nisqually River ensued for several hours. Continued slumping of the loose saturated material throughout today (15 August) has caused additional minor debris flows down the creek. It is expected that more small events like those of the past few days will occur in the the Van Trump Creek drainage as long as the hot weather lasts.
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EPILOGUE: In my always-appropriately-humble opinion, these types of observations could have been made ten to twenty thousand years ago in Kyle Canyon. Which reminds me, the last ice Age ended about 10,000 years ago, and if the deposit is really 20,000 years old it is from the height of the last ice Age which ranged from about 24,000 years ago to around 10,000 years ago. If it is 20,000 years old as the newspaper interviewees suggest, it shows there was a major warm spell during the last Ice Age right after its deepest period. That is credible enough, most ice ages are a series of advances and retreats. The reference to an ice age going back 1.6 million years is probably a misprint: it probably meant to say "ice ages" in the plural, since there were several ice ages during the last 1.6 million years. Just as there will be several during the next million years, starting sooner than we think, perhaps.