Graduate Thesis Or Dissertation
 

Large woody debris and channel morphology of undisturbed streams in southeast Alaska

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  • 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.
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