The Champlain Sea clays of Eastern Canada are incised by numerous
rivers. Their slopes have been modified by landslides: on the Chacoura
River near Trois-Rivières (Quebec), several large landslide scars,
more or less recent, are visible. The role of erosion (channel incision,
lateral channel migration and erosion of slopes due to agricultural
drainage) as a trigger of these landslides is important. The aim
of this study is to understand how erosion and landslides are related
to valley development. From a detailed analysis of aerial photographs
and DEMs, a map of the phenomena has been drawn by identifying various
elements such as landslides, limits of the slope, position of the
channel, and the area covered by forest. It is shown that channel
change and erosion are strongly linked to landslides by the fact
that they change the bank morphology in an unstable way. A slide
in itself is a natural way for the slope to achieve stability. But
when it occurs in a stream, it creates a disturbance to the stream
flow enhancing local erosion which may change the river path and
generate more erosion downstream or upstream resulting in more slides.
Cross-valley sections and a longitudinal profile show that landslides
are a major factor of valley formation. It appears that the upper
part of the Chacoura River valley is still unaffected by landslides
and has V-shaped sections. The lower part has been subject to intense
erosion and many landslide scars can be seen. This shows that the
valley morphology is transient, and that future activity is more
likely to occur in the upper part of the river. Therefore the identification
of areas prone to erosion will help determine the possible location
of future large landslides just like the ones that occurred in the
lower part.