The upper Skagit watershed lies in the transition zone between the wet, moderate climate of the North Cascades’ western slopes and the dry, more extreme climate of their eastern slopes. The overlap of eastern and western species, considerable variation in elevation from valley bottoms to glaciated peaks, areas of old-growth, and variation in local microclimate result in rich biodiversity. The north-south orientation of the Skagit also makes this valley an important travel and dispersal route for many species, and a connection corridor between populations in British Columbia and Washington State.
Six of British Columbia’s fourteen biogeoclimatic zones are represented within the BC Skagit Watershed:

  • Interior Douglas-fir (IDF)
  • Coastal Western Hemlock (CWH)
  • Coastal Mountain-heather Alpine (CMA)
  • Interior Mountain-heather Alpine (IMA)
  • Engelmann Spruce – Subalpine Fir (ESSF)
  • Mountain Hemlock (MH)

At least 100 bird species nest in the upper Skagit watershed, with more than another 50 using the valley and surrounding mountains as a migration corridor, feeding area, or wintering ground. No less than 65 species of mammals, 7 amphibians, and 4 reptiles also make their home in the habitats of the upper Skagit. Plant diversity in this mountainous transition zone is particularly high, with the area noted to have among the highest tree diversity of anywhere in British Columbia. Biological surveys continue to confirm the presence of additional species. More than 100 plant and animal species are listed as species of concern in British Columbia and/or species of national concern in Canada.

The effects of the transition zone are evident not only in the biology of the upper Skagit, but also in its geomorphology. During the ice ages, the alpine glaciers on the west side of valley received more precipitation than those experiencing the rainshadow effect to the east. The western alpine glaciers grew faster, carving out U-shaped mountain valleys, until they ran into the large tongue of ice from the continental glacier that was pushing down what we now call the upper Skagit Valley. As the glaciers receded at the end of the ice ages, they left “hanging valleys” where the gently sloping alpine valleys suddenly drop away to the floor of the Skagit, creating waterfalls where mountain streams abruptly tumble over the edge of the hanging valleys.

The precipitation deficient glaciers of the mountains along the eastern side of the Skagit didn’t grow enough to reach all the way to the main Skagit glacier. Rather than hanging valleys, there are steep-sided water-cut canyons on the east side of the valley.

Photos: AJ Fedoruk