Multivariate Geomorphic Analysis of Forest Streams: Implications for Assessment of Land Use Impacts on Channel Condition

Abstract

Multivariate statistical analyses of geomorphic variables from 23 forest stream reaches in southeast Alaska result in successful discrimination between pristine streams and those disturbed by land management, specifically timber harvesting and associated road building. Results of discriminant function analysis indicate that a three-variable model discriminates 10 disturbed from 13 undisturbed reaches with 90 per cent and 92 per cent correct classification respectively. These variables are the total number of pools per reach, the ratio of mean residual pool depth to mean bankfull depth, and the ratio of critical shear stress of the median surface grain size to bankfull shear stress. The last variable can be dropped without a decrease in rate of correct classification; however, the resulting two-variable model may be less robust. Analysis of the distribution of channel units, including pool types, can also be used to discriminate disturbed from undisturbed reaches and is particularly useful for assessment of aquatic habitat condition. However, channel unit classification and inventory can be subject to considerable error and observer bias. Abundance of pool-related large woody debris is highly correlated with pool frequency and is an important factor determining channel morphology. Results of this study yield a much needed, objective, geomorphic discrimination of pristine and disturbed channel conditions, providing a reference standard for channel assessment and restoration efforts.

Assessing the relations between aquatic habitat indicators and forest harvesting at watershed scale in the interior of British Columbia

“…The negative significant correlation between pool frequency and timber harvesting activity quantified by percent ECA and percent historical clear-cut area (Table 5) indicates that cumulative logging disturbance can lead to a decrease in pool numbers in forest streams, which simplifies stream habitat by converting more pools to riffles, a result also reported by Ralph et al (1994) and Wood-Smith and Buffington (1996). Removal of trees from upslope areas can increase sediment supply, resulting in bed degradation and smoothing through pool filling (Hogan, 1987;Montgomery and Buffington, 1998).…”

“…from an intact to a disturbed condition (Wood-Smith and Buffington, 1996). The impact of forest logging activity has raised concerns that repeated cycles of timber harvesting occurring over many decades and distributed over broad areas may have altered fundamental characteristics of basin hydrology, sedimentation, channel morphology and aquatic habitats.…”

“…Reeves et al (1993) used high harvest levels (>25% of the basin area harvested) versus low harvest levels ( 25% of the basin area harvested). Others used more general rules such as pristine/undisturbed versus heavily impacted by timber harvest or road construction (Lisle, 1986;Wood-Smith and Buffington, 1996), unlogged versus logged pairs (Hogan, 1986;Carlson et al, 1990), or used three groups-unlogged, logged with riparian buffer and logged with no buffer (Bilby and Ward, 1991;Gomi et al, 2006). All these studies have used different subjective assessments to divide their data into two or more groups, which makes it impossible to develop standards which can be applied generally.…”

“…Most existing studies just focused on the reach level logging activity of riparian forests (how wide it should be is also subjective) usually within a pre-determined period of time (e.g., Carlson et al, 1990;Bilby and Ward, 1991;Wood-Smith and Buffington, 1996;Fuchs et al, 2003;Chen et al, 2005;Gomi et al, 2001Gomi et al, , 2006. Although disturbances of riparian forests can have direct and prompt impact on some of the aquatic habitat indicators, it is also more subject to some uncertainties, since where and when the samples are collected can exert more impact on the outcome of a study.…”

Empirical evaluation of the conceptual model underpinning a regional aquatic long-term monitoring program using causal modelling

“…The extent of riparian forest influences the amount of large woody debris (LWD) available for recruitment (Burns, 1972;Lemly and Hilderbrand, 2000;Tschapalinski and Hartman, 1983;WoodSmith and Buffington, 1996) % Forest ! stream temperature !…”