Synthesis of Water-Quality and Benthic Community Analyses


Overall, the water quality and algal conditions, combined with a qualitative assessment of the benthic invertebrate populations, showed that the Molalla River, although exceptionally warm during summer, is a moderately nutrient enriched and productive stream capable of providing abundant food resources for salmonids. Despite the high amounts of primary productivity, based on the diatom species present, the river is not organically enriched. At all but the most downstream site, algal biomass levels were only moderately high, which could have resulted from high densities of grazing benthic macroinvertebrates, relatively low phosphorus levels, or some other factor.


Several factors contribute to the productive nature of the Molalla River. Nutrient levels in the river were generally moderate at all but the most downstream sites, and concentrations of biologically-available dissolved nitrate and phosphate remained above detectable levels despite the prolific growths of periphyton, indicating that the nutrient supply or regeneration is keeping up with demand by stream algae. Based on the great areal extent of periphytic growth in the river, it is expected, although not measured, that demand for dissolved nutrients would be substantial. Light is also readily available during summer when the absence of rainfall and runoff result in very clear water with low turbidity (less than 2 Formazin Nephelometric Units, or FNUs; table 8). The low flows in summer create expanses of shallow riffles that provide suitable habitat for diatoms and filamentous green algae to flourish. These algae feed a diverse assemblage of benthic macroinvertebrates and provide a solid base to the food web that is critical for fish. Although riparian vegetation is substantial in some sections, the riverbed receives direct sunlight in many areas, supporting photosynthesis and contributing to downstream warming. 


Even though inclement weather and slightly higher than normal streamflows delayed the 2010 growing season, algal growths and luxuriant bio-films of diatoms were observed throughout much of the river wherever the channel was open to sunshine and water depths were shallow enough to permit light for photosynthesis. A more favorable growing season might have produced higher biomass, and larger daily swings in dissolved oxygen and pH than those recorded in 2010. Nonetheless, evidence of active photosynthesis by algae, including supersaturated concentrations of dissolved oxygen and diel fluctuations in dissolved oxygen and pH were observed throughout the study reach, even at the most upstream site upstream of Glen Avon Bridge. Much larger diel swings in dissolved oxygen levels and pH were observed downstream at Knights Bridge, where the nuisance threshold of 100 mg chlorophyll-a/m² was exceeded in spotty but locally heavy growths of filamentous green algae (Cladophora glomerata). In previous years, decaying mats of Cladophora have clogged the drinking-water intake for the City of Canby water treatment plant (Brian Hutchins, City of Canby, oral commun., 2011). The annual cycles and severity of growth of Cladophora in streams and rivers is commonly highly variable from year to year (Powers and others, 2008), depending, in part, on the degree of grazing by benthic macroinvertebrates or severity of winter floods (Wooton and others, 1996).


First posted February 29, 2012