Faculty member at CSU since 1989. Research primarily on mountain rivers and bedrock chanyons; within these environments, projects on hydraulics, sediment transport, channel morphology, physical-biological interactions, and human effects. Currently involved in several projects on wood in Wyoming, Colorado, and Costa Rica. Books include Virtual Rivers (2001, Yale University Press), Disconnected Rivers (Yale, 2004), Mountain Rivers (AGU Press, 2000).
Wood dynamics in streams of the Colorado Rocky Mountains
Ellen Wohl, Dept of Geosciences, Colorado State University
A monitoring study begun in 1996 is designed to evaluate mobility of wood in
5 channel reaches in Colorado Rocky Mountain streams. Only sites with no timber
harvest for at least 50 years were included in the study, although one site
had a forest fire in 1977. No mass movements occurred at the study sites during
the period of monitoring. Each channel reach is 40-70 m in length. Initial surveys
included channel dimensions and orientation of each piece of wood within the
reach. Annual re-surveys then focused on wood orientation. Range of values for
channel parameters are as follows: bed gradient (0.013-0.096 m/m), bankfull
discharge (0.64-3.90 m3/s), D50 (12-22 cm), and wood loading (no. pieces/m2
channel; 0.02-0.32). Minimum piece size for wood to be measured was 1 m in length
and 5 cm in diameter. Average diameter of 16 cm and length of 3-4 m varied little
between sites. Each wood piece was categorized as a bridge (resting on both
banks; mean length 6.9 m), ramp (resting on one bank; mean length 3.6 m), sunken
(partially buried in streambed; mean length 2.2 m), or floater (unattached to
bed or banks; mean length 2.7 m). Average yearly mobility (no. of mobile pieces/total
no. pieces) ranged from 5% to 132%; this can also be expressed as an average
residence time of 2-3 years per piece of wood, where residence time represents
total no. of pieces/no. of mobile pieces. This was contrary to our initial assumption
that wood in these snowmelt-dominated streams would be stable between major
disturbances such as fire or flood. No significant correlations were found between
wood mobility by channel reach averaged over the period of the study, and the
potential control variables of R/D84, shear stress, channel width, bed gradient,
average wood loading, and average bankfull discharge. Annual wood mobility within
each reach did not correlate with annual bankfull discharge or wood loading.
We did observe, however, that the two years with the greatest wood mobility
at most sites (2003 and 2006) were characterized by large snowpacks that melted
unusually rapidly. This situation created a high magnitude, short duration flood
peak, as well as high moisture contents in stream banks after the flood receded.
The latter characteristic may have been responsible for high wood recruitment
through bank collapse. Wood mobility by piece type indicates that floaters are
significantly more mobile. These results suggest that piece mobility is predominantly
a function of piece orientation, rather than piece length, piece diameter, or
flow and channel parameters.
Richmond and Fausch. 1995. Characteristics and function of large woody debris
in subalpine Rocky Mountain streams in Northern Colorado. Canadian Journal of
Fisheries and Aquatic Sciences, v.52, no. 8, pp. 1789-1802.