Landscape controls on forested headwater channel morphology
Mike Liquori
Ph.D., Forest Resources
Introduction
A growing body of work has been conducted on the dynamic nature of stream
channels. Some consider the channel to be the manifestation of quasi-equilibrium
conditions that develop in response to the magnitude, frequency, duration
and perseverance of various input processes. Others see a more random
signal of continuous chaos imposed through the stochastic nature of these
same inputs. In general, however, there appears to be some agreement that
channel conditions are dependent upon a range of processes that include
areas of geomorphology, hydrology, and to some extent, ecology.
Geomorphic processes include factors such as the dominant geologic material,
sediment size, distribution, supply, entrainment and transport mechanics.
Also important are landscape influences, which include network dynamics,
process routing, gradient, confinement and landscape position. The relationships
between channel and hillslope processes are also quite important, and
often control many of the geomorphic processes mentioned above.
Hydrologic processes include climatic conditions, flood frequency, duration
and magnitude. The channel's response to these flood conditions is thought
to be the primary driver of many of the geomorphic responses within a
channel system.
The recognition of the importance of riparian influence has increased
greatly within the last few decades. The role of large woody debris, in
particular, has been demonstrated to be critically important in providing
channel complexity, regulating in-stream sediment transport, creating
habitat spaces, and supporting other ecological functions. Also important
is the role of living vegetation in providing temperature regulation,
nutrient inputs, bank stability, and flood-associated sediment retention.
Problem Statement
Most of the work developed with regard to channels has focused on either
large rivers, or intermediate sized streams. Limited work has been conducted
on smaller, headwater channels (e.g. streams with contributing areas smaller
than about 100 hectares), despite the fact that these streams comprise
the majority of the stream network. Preliminary indications suggest that
process patterns are quite different in headwater channels, and extrapolation
of data from larger systems is not appropriate.
Headwater channel response to riparian inputs likely differs from larger
channels. Headwater channels tend to be both transport limited and supply
limited, and may be less capable of self-regulating channel morphology.
Additionally, riparian communities differ as compared to larger streams.
The function of LWD, sediment and other riparian inputs may translate
to different ecological roles in headwater systems.
Objectives of Research
My research is trying to understand how headwater systems in managed forests
process wood, water and sediment. I hope to establish some relationships
between landscape conditions and channel morphology, particularly as it
may affect ecological function of headwater channels. My focus will consist
of three important pathways:
- The morphologic response to physical inputs, particularly channel roughness elements such as LWD and coarse sediment
- A comparison of step-forming theories and evaluation of differences between stepped channels and other channel types
- Development of a conceptual model that seeks to explain why the large variation in observed headwater conditions exists.