| Summary: | In the Pacific Northwest landslide inventories are routinely compiled by means of aerial
photo interpretation. When examining photo pairs the forest canopy, notably in old-growth
forest, hides a population of "not visible" landslides. The present study attempts to estimate
how important is the contribution of landslides not detectable from aerial photographs, to the
global mass of sediment production from mass failures on forested terrain of the Capilano
basin.
To achieve this, aerial photo interpretation has been coupled with intensive fieldwork
for identification and measurement of all landslides. In order to minimise bias in the
comparison and integration of field-collected and air photo-collected data it was decided to
define a 30-year time window. Incidentally, it has been possible to prove how landslide scars
that appear on a single photo set would date further back than 30 years.
Results show that "not visible" landslides can represent up to 85 percent of the total
number of failures and can account for up to 30 percent the total volume of debris mobilised.
Rates of sediment production differ greatly (one order of magnitude) between two sub-basins
of the study area, suggesting that such figures should be generalised with care within a
physiographic region. The difference in denudation rate is explained qualitatively by GISbased
analysis of slope frequency distributions, drainage density and spatial distribution of
surficial materials.
Fieldwork has demonstrated that gully-related failures have a greater importance than
one could expect from air photo interpretation. ANOVA and nonparametric tests indicate that
careful logging in East Cap Creek has produced no detectable effects on mass wasting.
Similarly, Sisters Creek, where timber harvesting stopped about 20 years before the start of
our 30-year time window, has apparently recovered from the signs of past extensive logging.
The existence of "not visible" events affected in a minor way conclusions about the
impact of logging on slope stability in terms of land use (management) effects. It had a major
impact on the nature of landslide magnitude-frequency relations and, finally is demonstrated
to have implications for British Columbia Terrain Stability Classification from the terrain
sensitivity point of view. === Arts, Faculty of === Geography, Department of === Graduate
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