Graduate Thesis Or Dissertation
 

Relationships among in-stream physical habitat, land use, and geology in small coastal streams of northern Oregon

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  • Salmonid populations in the Pacific Northwest are at historic lows and many populations continue to decline. Previous studies have linked salmonid declines to land use through degradation of in-stream physical habitat, but few of these studies have taken geology into consideration. This study relates habitat parameters known to be important to salmonids to land use patterns in different geologic settings of the northern Oregon Coast Range. Different geologic parent materials are expected to drive natural differences in stream physical habitat. Previous studies have found that streams on sedimentary bedrock have lower stream slopes, more pools, and more fine sediments than streams on volcanic bedrock. In this study a stratified probability sample of basins ranging in size from 3.6-6.3 km2 on three different geologic types (volcanic in the basin, sedimentary in the basin, and unconsolidated surrounding the reach) with four land use categories (<1% land use, 5-15% logged, 15-50% logged, and 5-15% agriculture) was used to study relationships among in-stream physical habitat, geology, and land use. During the 2000 low-flow season, data were collected on 48 first and second-order reaches using the United States Environmental Protection Agency Environmental Monitoring and Assessment Program (USEPA EMAP) protocols. Five streams were sampled early and late in the low-flow window to assess seasonal variability and sampling precision. This study found significant differences (P <0.10) in stream gradient among basins situated on fine sedimentary, coarse sedimentary, and volcanic bedrock. Differences in substrate composition among geological substrates were observed in basins without recent logging, but not in basins with recent logging. No significant differences in channel morphology (% pools, number of residual pools >50cm in depth, mean residual depth, width to depth ratios, standard deviation of the thalweg depth, or sinuosity) or the amount of large wood were found among geologic substrates either with or without recent logging in the basin. When all sedimentary and volcanic sites were examined, the number of residual pools >50 cm deep, mean residual depth, and the standard deviation of thalweg depth were positively associated with riparian logging. Percent pools, width to depth ratios, and sinuosity were not related to riparianor basinscale logging. The number of pieces of large wood in the active channel was positively associated with recent basin-scale and riparian logging. In addition, fine sediments were positively associated with riparian logging. Fine sediments were not significantly related to recent basin-scale timber harvest, probably because of the history of intense riparian logging. Relationships between stream physical habitat and logging differed among geologic substrates. In basins underlain by volcanic bedrock, fine sediments and wood in the channel were positively related to recent logging (1988-1998). However, in basins underlain by sedimentary bedrock, few relationships between in-stream physical habitat and basin-scale logging were observed. Instead, fine sediments and large wood were positively correlated with riparian logging. In basins containing unconsolidated deposits, fine sediments were positively related to large wood and negatively related to land use (riparian disturbance and basin-scale agriculture). However, in these basins land use was correlated with the percentage of alluvial deposits in the basin, therefore land use influences could not be studied independent of geology.
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