Geomorphic Setting, Aquatic Habitat, and Water-quality Conditions of the Molalla River, Oregon, 2009-10

Download or read book Geomorphic Setting, Aquatic Habitat, and Water-quality Conditions of the Molalla River, Oregon, 2009-10 written by and published by . This book was released on 2012 with total page 77 pages. Available in PDF, EPUB and Kindle. Book excerpt: [Cont.] Uptake of dissolved nutrients by algae, and inputs of additional nutrients, complicates interpretations regarding nutrient concentrations in the river, especially because samples were collected during summer growing season. Although the bulk of the diatom species generally were similar among at least the four upstream sampling sites, the multivariate ordination suggests a downstream trend in assemblage structure from the Glen Avon Bridge site to the Highway 213 Bridge. The next downstream site, at Goods Bridge, near the downstream end of the alluvial GR3 reach, however, plotted closer to the most upstream site at Glen Avon Bridge, which indicates a change in assemblage structure. The algal indicator species analysis showed a change in species composition at the Goods Bridge site, including decreases in eutrophic diatoms, increases in the relative abundance of oligotrophic diatoms, and an increase in diatoms sensitive to organic pollution that suggests an improvement in water quality conditions. Although this may be related to the enhanced water exchange into and out of the streambed in the alluvial reach, and such hyporheic activity could work to clean the river of organic compounds and nutrients, small decreases in water quality (lower concentration of dissolved oxygen, and higher conductance and nutrient concentrations) were observed between the Highway 213 and Goods Bridge sites.? The multivariate analysis relating the diatom species composition data to the geomorphic and water-quality variables indicated that the presence of local gravel bars, bedrock, exposure to the sun (open canopy), and pH had a significant role in shaping the diatom assemblage structure. Although there was a high percentage of similarity among samples, many of these factors have the potential to affect diatoms and other algae through various interrelated mechanisms that relate to channel mobility and associated effects on light available for algal photosynthesis, for example, and other potential factors. Although only qualitatively addressed for this study, benthic macroinvertebrates, including mayflies, caddisflies, and stoneflies, were abundant in the Molalla River and indicate a high degree of secondary production in the riffles throughout the study reach. Snails, another voracious grazer of algae, also were relatively abundant at the Goods Bridge and Knights Bridge sites. Additionally, large numbers of the large caddisfly larvae Dicosmoecus were observed throughout most of the lower river in a range of depths and habitats. The large densities of these grazers, combined with the moderate level of algal biomass, suggest that invertebrate grazers could have limited the accrual of algae during summer 2010, an assertion that could be evaluated with further study. In northern California's Eel River, high abundances of Dicosmoecus were detected in summers following winters that lacked bankfull flow, as was the case for the Molalla River in water year 2010. The lack of disturbance might explain the high abundance of these herbivores in the Molalla River. The information from this study can be used to adapt management strategies for the Molalla River and its flood plain. These strategies may assist in developing and maintaining a healthy river environment that includes high-quality water for aquatic life and human consumption."--Executive summary.