|Abstract or Summary
- A framework is presented for a more causal explanation and ordering of
stream characteristics than traditional means have used. Patterns of stream habitat
distribution are related to particular characteristics of the geomorphology of
watersheds. Variability in stream characteristics can be explained by the spatial
distribution of properties of the watershed and by identifying stages in the
development of stream habitat. This has implications for the classification of streams
in a geomorphic context, emphasizing the concepts of capacity and genesis. Within the
context of a hierarchical watershed classification, stream segment is considered the
most useful classification level for understanding habitat distribution and dynamics.
Basins in basalt and sandstone geologic formations in the Oregon Coast Range were
investigated for stream habitat distribution.
In streams flowing through sandstone, repetitive patterns of large-scale
valley segments are identified in headwaters to large rivers. Segment boundaries are
identified by changes in valley morphology. Alternating patterns of wide valleys and
constricted valleys are common in some landtype associations. The formation of these
segments is influenced by bedrock stratigraphy and large-scale geomorphic processes
such as slump-slides. Valley dimensions are quantified by nondimensional indices of
valley width / channel width and the stream gradient index.
Pool distribution was intensively analysed in 2.8 kilometers of a fourth order
stream that flows through basalt bedrock. The mainstem was stratified by tributary
junctions, each of which was further divided into 3 equal zones. Distributional
patterns were analyzed by pool spacing, linear nearest neighbor, and by composition
of zones. Pool spacing does not conform to the expected normal distribution with a
median spacing at 5-7 channel widths. Pool spacing is strongly skewed towards
shorter spacings and has a median spacing at 2-3 channel widths. Pools of similar
size and type occur locally, which indicates that different areas of the stream have
different conditions for habitat formation and development. Habitat distribution and
composition is related to valley morphology, hillslope processes, location in relation
to tributary junctions, and time since disturbance. Debris torrents are the dominant
processes influencing the stream, these supplying sediment and instream structure of
wood and boulders, as well as influencing channel slope.
A balance between disturbance and recovery exists in unmanaged as well as
perturbed watersheds. Effective management of watersheds must maintain the
geomorphic integrity of the valley and stream. Classification of the stream and
habitat in a geomorphic context enhances explanation and the understanding of the
interaction between watershed form and process and characteristics of the stream.
Stream habitat formation, distribution, and dynamics are better understood in the
context of geomorphic concepts such as sediment storage and dynamics.