RELATIONSHIPS OF REACH-SCALE AND WATERSHED-SCALE VARIABLES TO FINE SEDIMENT ABUNDANCE IN FORESTED STREAMS OF THE OREGON COAST RANGE

Increased loading of fine sediments (sand size and finer) has been associated with the degradation of stream ecosystems throughout the United States. Such degradation affects the entire food chain from macroinvertebrates to salmonids. Evaluation of fine sediment pollution and related factors in forested streams often uses reach-scale data, but collection of this data is often expensive and time-consuming. Many studies now begin with an evaluation of a growing collection of remotely sensed and previously encoded field data that can be analyzed together using geographic information systems (GIS). This GIS data has the potential to better focus field work, and to further evaluate the environmental influences of fine sediment deposition across different spatial scales.

We have used a relatively large dataset (60 reaches, 120 km. of stream) in the Oregon Coast Range to investigate the explanatory power and correlations of indicators of fine sediment abundance using both field-collected reach-scale and GIS-derived watershed/valley-scale variables. Study reaches were chosen to minimize riparian anthropogenic disturbances and differences in stream gradient. All of the reaches we selected were underlain entirely by either volcanic or sedimentary lithology, and had pool-riffle channel morphology.

Preliminary results obtained from multiple linear regression models indicate that a suite of watershed morphology variables have a significant correlation with a visual estimation of fine sediment abundance, (R² ~ 0.60), and the addition of reach scale variables indicative of roughness and potential sediment storage, improve the correlation, (R² ~ 0.80). In general lower elevation watersheds with, steeper hillslopes, flatter overall topography, and lower main channel lengths, tend to have more abundant fine sediment. Similarly reaches with more pools, more beaver dams, and wider active channels tend to have more abundant fine sediment.

These results should prove not only useful for prioritizing field work, but for understanding the natural, background variability of fine sediment loading between watersheds in a geographic region.