Abstract:
The characteristics and interactions of the riparian
stand, large woody debris (LWD), and channel morphology
were examined on five undisturbed, low gradient streams in
southeast Alaska. One first-, two second-, one third-,
and one fourth-order streams were studied. Stream
morphology variables were measured systematically at fixed
intervals of three to ten feet depending on stream size:
50-foot intervals were used for riparian forest
measurements.
The percentage of alder comprising the riparian
forest increased with stream size. The first- and second-order
streams had alder comprising approximately 8% of the
total basal area whereas, the fourth-order stream had 25
Likewise, the percentage of LWD pieces consisting of alder
increased from 129 in the two smallest first and second-order
streams to 319 in the fourth-order stream. These
findings, along with inspection of air photos indicate a
alternating 1'one sided" alder corridor exists along the
largest stream.
The proportion of large woody debris pieces with
rootwads in the channel increased from 2 and 6% in the two
smallest first- and second-order streams to 32° in the
fourth-order stream indicating the largest stream has
recruited LWD from bank cutting and/or lateral channel
migration. LWD oriented perpendicular (900) to general
stream flow was relatively frequent for all streams. No
significant (alpha=0.05) linear relationship was found
(r2<O.05) between piece length and orientation to flow.
Channel morphology changed with stream size. For
example, the length of stream with side channels and/or
braided reaches increased from 1% in the first-order
stream to 416 of stream length in the fourth-order stream.
The overall percentage of pools averaged 579 and showed no
changes with changes in stream size. However, the
relative proportions of individual pool "morphological
types" and "causal elements" did change with stream size.
"Underf low pools" comprised less than 10% of the
morphological types in the first-, second-, and third-order
streams but increased to 17% in the fourth-order
stream. Autocorrelations of the spatial distributions of
stream morphological variables (i.e., depth and width)
indicated that the streams are influenced by a wide
variety of interacting factors and processes. Thus
channel dimensions are characterized by high variability
and an absence of "memory" or repeatable" components.
A positive linear relationship (alpha<O.O1, r2=O.23)
was found between LWD volume and the standard deviation of
bankfull width. No such relationships were found for
other stream morphological variables including thalweg
depth, low-flow width, cross-sectional area, and width
depth ratio, indicating other variables besides wood
volume present are influencing variability in stream
morphology, or that the effects of large woody debris upon
channel morphology is not easily expressed by linear
regression techniques.
Autocorrelations and frequency histograms of an
upstream forested section, with instream LWD, and a
downstream meadow section with no LWD, noncohesive soils
and tidal effects, show that the meadow section had
greater "memory" and less diversity in "morphological
types" and "pool causal elements."
These results provide a quantitative assessment of
several stream morphology variables for first- through
fourth-order streams in southeast Alaska. Further
research is needed to compare these results for
undisturbed streams with streams influenced by management
activities.
