By chance I had some time looking at some old aerial oblique photos taken in 1925. It is always very exciting to have a look at an 'historic landscape' and compare it with the present as can be done so easily with Google Earth. As early as 1921, Canadian Royal Air Force pilots returning from WWI played an important role in exploring Canada from the air. Their legacy is stored in Canada's National Air photo Library, now one of the most important environmental baseline data bases in Canada. This early and important contribution from the 'men in their flying canoes' forms a unique baseline from which environmental change can be measured as well as visually shown.
The Wekusko Maps area (63J) in Manitoba, Canada is just North of Lake Winnipeg. It is an interesting area ecologically and geomorphologically. A very large portion of the land areas is covered by wetlands predominantly fens and bogs. They were formed after the drainage of Glacial Lake Agassiz. The organic material typically formed over glacial lacustrine clays.
Permafrost in Peat plateaus and palsas is common in the wetlands in this area. It has been speculated that the maximum extent of the permafrost in wetlands was reached in this area during and just after the 'little ice age' (Thie 1973).
Permafrost in peat plateaus and palsas can be forming (aggrading) or melting (degrading). Both processes can occur simultaneously on the same landform or in the same landscape. If aggradation exceeds degradation permafrost is (slowly) expanding and consequently the Black Spruce forest cover would expand. This process could take several life cycle of Black Spruce. When degradation dominates, permafrost disappears. The degradation process appears much faster than the aggradation part. The melting scars are quite prominent and can be easily monitored with air borne and satellite images taken over a period of time. The examples below cover a rather unique 85 year time line.
PHOTO 1 below: 1925 Oblique aerial photo of a long peat plateau near Waskik Lake and the Minago River in Northern Manitoba. The numbers 1, 2, 3, 4 all indicate permafrost areas on peat lands. The large peat plateau (2, 3) is about 3400 meters in length. Although small collapse/ melted areas can be seen in the center of the plateau, the edges of the peat plateau are still intact- no significant melting visible. The area around 4 was burned perhaps about a decade before (around 1915): significant melting has already occurred here (i.e. before 1925) but the center core still has permafrost and has regenerated in black spruce with some white birch.
PHOTO 2: The 1984 Google Earth image, source SPOT image, shows approximately the same area. Significant melting has occurred in the 59 year period. Some of the melting areas are identified by blue arrows. Melting and collapse of the edges of the peat plateaus is most common. The light green arrows show mature stands of Black Spruce. These areas have not been burned in this period. The area just surrounding (3) was burned in the early 1970's, but permafrost is still present, although the 'tail' area -blue arrow- point to a large melted part. The gradient of the wetland is in the order of 1 meter per kilometer, drainage flow indicated by the grey dotted arrow. Typical active melting edges show a loss of 20-35 meters of permafrost over a 59 year period which averages out to between just over 0.3m to 0.6 meters per year.
Photo 3: 1955 vertical air photo. The peat plateau has retained the same shape and form. Melting around the edge is not visible- around 1,2,3 the situation appears unchanged. Some collapsing and melting has taken place between 1925 and 1955 but appears insignificant compared to the melting before 1925. Even the burned area around 4 does not seem to have changed much. The green arrow shows a solid cover of fire-origin Black Spruce.
PHOTO 4: 1979 air photo. Significant changes are visible. The blue arrows point to areas which have clearly melted since 1955. Also the area around 3 has been burned. Significant melting is visible in the 'tail' area. Many of the inside collapse areas have increased as well, but not as much as the 'melted edges'. It is remarkable that the area around the green arrow has changed so little since 1955.
PHOTO 5 above: Part of the peat plateau in 1968. No melting around the edge can be seen, as was also noted in photo 4 -green arrow. Melting can be seen in the collapsed areas in the centre (see also more detailed image below), but melting rates are quite slow.
PHOTO 6: A detail of the above photo 5- red outlined area. Active melting areas are identified by blue arrows. Some indications of surface water can be seen there.
PHOTO 7: 1984 Satellite Image. This Google Earth image shows a significant amount of change from the 1968 photos. Melting around the peat plateau edge is prominent. The small palsas B and C have disappeared.
PHOTO 8: Most recent Apple Maps Image- winter. This selection is comparable to photo 7 above. The snow cover enhances the difference between the denser Black Spruce on permafrost and non frozen areas. It is clear that melting has continued since 1984, but not in a dramatic way.
PHOTO 9: Most recent Apple Maps Image- winter. This image should be compared with photos 2,3 and 4 above.
Figure 1 : Longitudinal elevation profile of the wetland and peat plateau: The length of this profile is 7.18 km, with an elevation loss of 3.03 m. It is interesting to see that the peat plateau with the mature Black Spruce appears level and is distinctly higher than the surrounding areas. Of course, the Digital Elevation Model and data base used by Google Earth is not really accurate enough to use for analytical purposes. But it is at least more interesting the the minimal contour information on the official topographic map for this area (MAP 2 below)
Figure 2: Maximum extent of permafrost: probable situation around 1800-1850. This back casting scenario is based on the analysis of present permafrost and old melting scars. Time period of maximum extent is based on the 'Little ice Age' influence in this area. The past permafrost is mapped on the most recent, high resolution Apple Maps winter image: Photo 9 above
PHOTO 10: 1925 oblique air photo shows the tail end of the peat plateau and a series of smaller palsas in a large tamarack fen wetland. While the area north of the wetland shows all signs of a fire regeneration, the palsas and the tail were not touched by that fire. Note that the edges of the of the palsas and do not show any sign of melting---yet. In fact, this area appeared quite stable until 1955.
PHOTO 11: 1925 oblique of the area marked with a (4) on photos 1,2,3 and 4 above. This areas was burned a number of years before 1925. Much of the permafrost has melted: a regeneration of black spruce can be seen on the remaining permafrost. Also the light dots of the white birch foliage can be clearly seen in this low sun angle photo.
PHOTO 12: This is a typical example of a "flying canoe". Float planes were essential to cover Canada's extensive north where no airfields were available.
This could actually be the flight crew which took the remarkable aerial photos in 1925. This picture was apparently taken in 1924 before take off from the flight base near Winnipeg. (from Alex J. Milne, "Pioneer Survey Flights in Northern Manitoba and Saskatchewan", Canadian Aviation Historical Society Journal 21, no. 4 (1983)). S Bernard Shaw in his book "Photographing Canada From Flying Canoes" gives a detailed historical overview of this period in Canada's air photo aviation history.
It is rather interesting to think that this crew may actually have taken the 1925 oblique aerial photos which are such an important part of this analysis. Certainly their work contributed to the making of the old topographic map (MAP 1 at bottom of page) which shows the actual flight lines and centres of the oblique air photos taken by the crew. The peat plateau was actually mapped as a prominent landform in the surrounding 'muskeg' (see point of the pen). The modern version of this map (MAP 2 Below) Does not provide this interesting detail.
MAP 1: Old Topographic map showing the flight lines taken by the "flying canoes". Note that the elongated peat plateau described on this page is actually mapped - just at the tip of the pen.
MAP2: Section of Present Topographic map available from Natural Resources Canada
1. Milne Alex J., 1983. "Pioneer Survey Flights in Northern Manitoba and Saskatchewan", Canadian Aviation Historical Society Journal 21, no. 4 (1983)
2) Thie J., 1974. Distribution and Thawing of Permafrost in the Southern Part of the Discontinuous Permafrost Zone in Manitoba. Arctic 27 3 189
3) Shaw S. Bernard 2001."Photographing Canada from Flying Canoes" ISBN 1-894263-42-1, Published by GSPH -General Store Publishing House, Canada.