In the Alps, debris flow deposits generally contain < 5% clay-size
particles, and the role of the surface-charged < 2 mu m particles is
often neglected, although these particles may have a significant impact
on the rheological properties of the interstitial fluid. The objective
of this study was to compare debris flow deposits and parent materials
from two neighbouring catchments of the Swiss Alps, with special
emphasis on the colloidal constituents. The catchments are small in area
(4 km(2)), 2.5 km long, similar in morphology, but different in geology.
The average slopes are 35-40%. The catchments were monitored for debris
flow events and mapped for surface aspect and erosion activity. Debris
flow deposits and parent materials were sampled, the clay and silt
fractions extracted and the bulk density, < 2 mm fraction bulk density,
particle size distribution, chemical composition, cation exchange
capacity (CEC) and mineralogy analysed. The results show that the
deposits are similar to the parent screes in terms of chemical
composition, but differ in terms of: (i) particle size distribution; and
(ii) mineralogy, reactivity and density of the < 2 mm fraction. In this
fraction, compared with the parent materials the deposits show dense
materials enriched in coarse monocrystalline particles, of which the
smallest and more reactive particles were leached. The results suggest
that deposit samples should not be considered as representative of
source or flow materials, particularly with respect to their physical
properties.