MULTIVARIATE GEOMORPHIC ANALYSIS OF FOREST STREAMS - IMPLICATIONS FORASSESSMENT OF LAND-USE IMPACTS ON CHANNEL CONDITION

Multivariate statistical analyses of geomorphic variables from 23 fore st stream reaches in southeast Alaska result in successful discriminat ion between pristine streams and those disturbed by land management, s pecifically timber harvesting and associated road building. Results of discriminant function analysis indicate that a three-variable model d iscriminates 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 po ol depth to mean bankfull depth, and the ratio of critical shear stres s of the median surface grain size to bankfull shear stress. The last variable can be dropped without a decrease in rate of correct classifi cation; however, the resulting two-variable model may be less robust. Analysis of the distribution of channel units, including pool types, c an also be used to discriminate disturbed from undisturbed reaches and is particularly useful for assessment of aquatic habitat condition. H owever, channel unit classification and inventory can be subject to co nsiderable error and observer bias. Abundance of pool-related large wo ody debris is highly correlated with pool frequency and is an importan t factor determining channel morphology. Results of this study yield a much needed, objective, geomorphic discrimination of pristine and dis turbed channel conditions, providing a reference standard for channel assessment and restoration efforts.