Like glaciers worldwide those in the Cascade Range in Oregon have shrunk substantially as a result of a warming planet and since the end of the Little Ice Age (O'Connor et al. 2001). In areas where glaciers have receded in the Cascades hundreds of lakes have formed. Most are in stable basins, but about 30 are dammed by unconsolidated moraines that are susceptible to failure. The highest concenration of these lakes that have dams susceptible to failure in the United States are in the Mount Jefferson and Three Sisters Wilderness Area in central Oregon. In this area there have been 11 debris flows from 7 partial and 4 complete emptyings of moraine-dammed lakes. Most occurred between 1930 and 1950, with some as recent as the 1970's. Although deglaciation and resulting debris flows are not unique to the Oregon Cascades, they have been more severe in this region because these volcanoes are steep, extensively glacierized and composed of highly altered loose rock and fragmented material The Cascade Range extends from northern California to northern Washington and range in width from 50 to 120 km. It is composed primarily of upper Eocene to Quaternary volcanic, volcanoclastic, sedimentary and igneous intrusive rocks (O'Connor et al. 2001) The crest is at at elevation of 1500 to 2000 m, with several high volcanoes exceeding 3000 m.
South Sister And Broken Top near Cascade Lakes Hwy, Oregon
In this earthcache I will be describing a debris flow that occurred in October 1966 in the Soda Creek drainage mainly upstream from Highway 49. It was triggered by the over-topping of a moraine-dammed lake at the East Bend Glacier located on Broken Top. During maximum neoglacial advances East Bend Glacier covered .24 square km and was attached to Bend Glacier before deglaciation separated them. At its largest size the lake created from the melting glacier contained 340,000 cubic meters of water and had a maximum depth of 18 m.
On October 7,1966 140,000 cubic meters of water spilled over the outlet and partly breached the moraine dam. The resulting debris flow came down an unnamed tributary, then joined Crater Creek 6 km downstream from the breach. It then entered Soda Creek 1 km downstream. The debris flow followed Soda Creek another 3 km until it crossed Highway 49 (Cascade Lake Highway) and spread out over the large meadow near Sparks Lake. The meadow ended up being covered with approximately 250,000 square meters of sand and silt. The total distance the debris flow traveled between the breach and Sparks Lake was over 10 km.
Within a week after the event reporters from the Bend bulletin made a trip to the site and reported that the dam breach was caused by the calving of a large block of ice into the lake. The debris flow began by eroding the moraine material that formed the dam, and then depending on slope, channel characteristics, and geologic properties of the bed material the flow began incorporating bedrock, trees, outwash and colluvium. Boulders as large as 2 m in diameter also moved with this mass of mud, rocks and vegetation. Estimates of discharge, based on channel dimensions and slope calculations, ranged from 380 cubic meters/sec to 20 cubic meters/sec along the 10 km flowpath. Some reaches were badly eroded where others acted as depositional zones, and by the time it reached Highway 49 and the meadow it ceased acting like a true debris flow, but rather a greatly enlarged Soda Creek heavily laden with a large sediment load.
Moraine Dams are especially susceptible to rapid failure that leads to debris flows for the following reasons:
- Deep lakes that form behind moraine dams exert significant hydrostatic force.
- Width to height ratios are generally small.
- Outlet slopes are generally very steep.
The failure of these dams is usually caused by overtopping, piping or gravitational collapse. The most common type is the one I previously described; ice falls, rock falls, and avalanches into the lake causing large displacement waves that exit the outlet and collapse the dam.
References
O'Connor, J.E., J.H. Hardison and J.E. Costa. 2001. Debris Flows from Failures of Neoglacial-Age Moraine Dams in the Three Sisters and Mount Jefferson Wilderness Areas, Oregon. US Geological Survey Paper 1606.
To log this earthcache:
Answer the following questions and email them to the developer of this earthcache:
There is a set of interpretive signs at the coordinates given for this earthcache. Use the information given on the signs to answer the following questions.
- The first onto the scene following the debris flow ruled out what three environmental events/catastrophes that “did not” occur that could explain this mysterious flood?
- 30 years after the event a geological/hydrological study was conducted on Soda Creek to determine a restoration plan. Describe the different activities conducted on Soda Creek in the restoration process.
This logging requirement is optional. Take a picture of yourself and the members in your group, and showing your GPS unit, with the mountains to the west in the background. This would assure me that you were at the site.
Basic Visitor Use and Public Safety Information for Public Lands in Oregon
All visitors need to plan ahead and prepare for outings in Oregon's public lands by:
- Knowing the regulations and special concerns for the area you are planning to visit. Such as obeying the laws that prohibit the collection or destruction of artifacts.
- Carrying a map and GPS unit and/or compass. Maps are available for purchase at all BLM and US Forest Service Offices
- Staying on existing roads and trails.
- Planning for extreme weather, hazards, and emergencies.
- Carrying a full size spare tire, extra food and water, and warm clothes.
- Being aware that cell phones DO NOT usually work in the rural areas away from major highways.
- Leaving your travel plans with a responsible party, including the date and time of your return.
- Linking to 'Tread Lightly and leave No Trace' websites (www.treadlightly.org or www.int.org).
- Remember the geocaching motto of 'Cache in Trash Out'.