Hi Doug,

Your selection of permafrost slide markers is really very interesting. Each of these sites warrants further studies. Your photograps would certainly be of interest. Why not add them to this discussion thread.

Your red marked site is very interesting since it shows very recent as well as 50 year old slide scar areas. The relative recent disturbed bare soil is clearly visible on both sides of the creek. Also the adjacent areas on both sides show significant sliding area which appear to have been stabilized and are recovered with vegetation.

I have taken some time to study your red marked site in more detail using air photos from 1950, 1953, 1972 and 1977, as well as Landsat. the image below shows my interpretation . You are right that the pre 1950 slide caused the formation of the little lake upstream. The pre 1950 slide appears to have been a sudden event. The pink arrow shows the slide path, which also blocked the creek flow, and created a small lake upstream. The the water erosion from the displaced creek started to undermine the north facing slope which showed a steady regression for the next 27 years before stabilizing. Just before the 1990's additional regression occurred on the south facing slope (thin orange line) also occurring as a sudden failure. The new slide areas (yellow dotted line) developed between 1990 and 2000.

Retrogressive Slide on 1950 and 1953 Aerial Photos
The 1950 photo ( A12847-144) was taken on August 2nd, 1950 at a height of 20'000 ft. Original scale 1:40'000. The pink arrow shows the flow direction. The slide, on the south facing slope occurred as a sudden event blocking the flow of the creek and creating a small lake upstream. The drainage channel was pushed into the opposite slope which started to collapse. Both sides continued to melt, but most of the regression in the following decade is in the north facing slope. The 1953 airphoto (A13753-22) was taken on 27 July from 35'000 ft.

1972 and 1977 Air photos
During the next 20 years the North facing slide is regressing. The 1972 and 1977 air photos show that the creek is pushed back again (yellow marker). In this 5 year period the North slide appears stable, and some further sliding occurred on the south facing slide (from blue dotted line to the light blue-grey dotted line). There are no signs of fire scars in the vegetation on these photos, but a fire scars ( (dated between 1958-1978) were identified in close to the Salter Hill area, about 5 km distance from this slide area. 1972 Airphoto (A22972-119) taken on 11 July, from 35'000 ft; 1977 air photo (A24761-133) taken on August 1, from 35'000 ft.

NASA Landsat Pseudo Colour 1990 (left) and 2000 (right). The pink arrow on the 1990 satellite image shows that the south facing slope has become active again. Also the light blue arrow shows the start of the next phase on the north slope. The 1950 and 1970 slide areas still look reasonably stable and are covered with vegetation again. The red arrow on the 2000 image shows a part of the 1950 slide which has become active again: the small dark black area- a mudflow The yellow arrow on the 2000 image points to the dark purple active slide area essentially very similar to the Google Earth image (2007). No fire scars are visible on each of the images.

Fire History for the Yukon 1946-2004 . The map below depicts the wildland fires since 1946. Source: Source: Yukon Energy, Mines and Resources . The Caribou River area described here is marked with the red marker near (1). The closest fires on this map was (3) which occurred between 1990 and 1999. The Trevor Range Site (green marker) is identified with a (2). It will be interesting to study the blue markers provided by kningpointnorth in relation to fire history. A cursory review of some of the sites in recent and older burn areas seems to imply a link with the fires. Legend: Grey= 2000-2004; Red=1990-1999: Pink=1980-1989; Green= 1970-1979; Orange= 1960-1969; Yellow=1950-1959; Light Green= 1946-1949 (not on this part of the map)

Doug, you could add the fire map as overlay to your collection of markers. As you already can see, many of your markers are in burned and non-burned areas. I have looked at a number of the sites in recent burn areas, where the fires appear to be the cause of the slide. Fires can also have an impact on increased stream flow and therefore increased erosion.

Fires are already so abundant in this part of the world. If they double in frequency as result of climate warming, retrogresive thaw slides would certainly become more abundant. It would be interesting to study this with your markers and the existing fire history,

It is so far my impression from your markers and these preliminary analysis that ther could be twice as many slides today than there were 60 years ago.

Cheers,

Jean