Executive Summary

This report presents results from a 2009–10 assessment of the lower half of the Molalla River. The report describes the geomorphic setting and processes governing the physical layout of the river channel and evaluates changes in river geometry over the past several decades using analyses of aerial imagery and other quantitative techniques. 


The peak-flow hydrology in the Molalla River has been characterized by a series of large floods during the 1960s and 1970s, a period of relatively small peak flows from 1975 to 1995, and a relative increase in severity of events in the past 15 years. Although incomplete, the gaging record for the early 20th century showed only modest high flows. The flood chronology since 1960 has affected the geomorphology of the river corridor, principally by increasing the active-channel width. The area affected by channel migration in the late 20th century, however, was reduced by the construction of revetments along the river corridor which acted to contain channel movement.


The study area along the Molalla River was divided into six unique geomorphic reaches. The upper-most reach, designated GR6, is a narrow, bedrock-controlled reach with ample shade and large riffles. The next downstream reach, GR5, is also largely bedrock controlled but has a wider flood plain and active channel-migration zone. The longest geomorphic reach, GR4, has a wide channel-migration zone with many strategically placed revetments that work in concert with bounding bedrock to the northeast to suppress overall channel movement. In contrast, GR3 is a wide, active reach that responds more dramatically to flood and non-flood periods than the other geomorphic reaches. The anthropogenically confined GR2, adjacent the City of Canby, has relatively little historical channel movement and relatively few gravel bars. Finally, the farthest downstream reach, GR1, is an actively meandering reach that most closely resembles its pre-development state.


Detailed analysis of aerial imagery from 1994, 2000, 2005, and 2009 showed that channel-migration activity and active-channel widths were greater in GR3 than in any other geomorphic reach and were related directly to the timing and magnitude of high flows. Similarly, the revegetation of exposed bars is significant in GR3 and elsewhere when large floods do not occur. A qualitative analysis of older aerial imagery dating back to 1936 showed that the recent channel-migration activity in GR3 is no greater than it was historically. Channel-migration activity in GR2, GR4, and GR5 was reduced relative to historical rates as a consequence of the construction of revetments and encroachment along the river corridor.


Analyses of the longitudinal water-surface profile first suggested a possible accumulation of alluvium in GR3, but subsequent analysis of the shape of the longitudinal profile juxtaposed against bedrock outcrops in the river channel showed that the river is largely flowing over a shelf of bedrock and not filling with sediment. 


Water-quality, benthic algae, and benthic invertebrate conditions were examined during summer low-flow periods to determine the overall health of the river and to provide possible insights into the physical or chemical influences on diatom assemblages. 


A wetter than normal spring in 2010 resulted in higher-than-normal flows in July and August that may have delayed the algal growing season and limited the accrual of algal biomass in the river. Longitudinal changes in water quality, including downstream increases in water temperature and specific conductance, were observed in the Molalla River during August and September. Such patterns are typical of many rivers receiving inputs from anthropogenic sources in the flood plain, including agricultural and rural residential lands (Milk and Gribble Creek basins) as well as some urban runoff in the lower river.


Nutrient concentrations in the Molalla River were generally low at most sampling sites but did increase at the Goods Bridge and Knights Bridge sites, presumably from a greater influence from anthropogenic sources that enter the river from tributaries, agricultural irrigation returns, or groundwater in the lower basin. Nitrate concentrations at Glen Avon and Knights Bridges exceeded their respective reference values for streams in the Cascade Range and Willamette Valley. Although the nitrate-nitrogen concentrations were somewhat elevated, phosphorus, in contrast, is relatively much less abundant in the Molalla River. N:P ratios for soluble, biologically available nitrogen and phosphorus were lower in the upper middle reaches (less than 5), but the absolute concentrations of orthophosphorus (0.010 milligrams per liter or less in July) suggest that attached periphytic algae in the river may be limited by phosphorus concentrations or some other factor, but probably not by nitrogen. The Molalla River has lower phosphorus concentrations than other rivers draining the Cascade Range because the phosphate-rich rocks of the Oregon High Cascades, prevalent in other drainages, are not present in the Molalla River basin, which is wholly contained within the Western Cascade Range geologic province.


The 2010 algal growing season was delayed due to an unusually cold and wet spring, which produced streamflows 12–18 percent higher than normal in July and August and could have limited the accrual of periphyton biomass in the river. Nevertheless, a healthy biofilm of diatoms and other types of algae developed in the shallow riffle habitats during July, covering the entire stream channel in some areas. Generally, riffle habitats appeared healthy, with little sediment and low substrate embeddedness (that is, the degree of infilling of fine sediments around gravels and cobbles) was less than 5 percent at all sites except the Knights Bridge site, where embeddedness was about 10 to 25 percent higher.


Algal biomass levels in July were moderate, ranging from 30 to 55 mg of chlorophyll-a per square meter, and the high densities of benthic macroinvertebrate grazers in the riffles suggests that the accumulation of algae (biomass levels) may have been limited by these herbivores. In August, however, a benthic bloom of filamentous green algae (Cladophora glomerata) increased algal biomass in the lower river, with nuisance levels at the Knights Bridge site. Higher nutrient concentrations (both nitrate and orthophosphate) combined with fewer invertebrate grazers (mostly snails) likely contributed to the higher biomass at this site. Long filaments of Cladophora also were observed in the area near the Canby drinking-water treatment plant, where in previous years, algae have clogged water intakes during periods of senescence when algae detach from the river bed and enter the intake. In 2010, algal biomass conditions were not as severe and the intakes were not affected.


Distinct fluctuations in concentrations of dissolved oxygen and in pH levels from algal photosynthesis were observed at all sites sampled, with the largest diel changes and highest daily maximum values occurring at the two most downstream sites, particularly at Knights Bridge. Although some relatively high pH values were measured (as much as 8.4 units), none of the pH measurements exceeded State of Oregon water-quality standards, even in the afternoon hours on warm sunny days. Dissolved oxygen concentrations at Goods Bridge and Knights Bridge did not meet the 8 milligrams per liter criteria in the early morning hours, but compliance with the standards is only evaluated with 30-day average minimum values, which were not available. Relative to the salmon spawning criteria, for which the data collected during this study applies only to the Glen Avon Bridge site in September, water temperature, pH, and concentrations of dissolved oxygen all met the state standard in effect.


Thirty-three species of algae were identified in the Molalla River, including fast growing small diatoms and very large stalked diatoms, filamentous green and blue-greens, and a few planktonic forms of green and blue-green algae that may have washed into the river from an upstream pond. The occurrence of high-biomass forming types of algae in the river, including filamentous greens such as Cladophora and large stalked diatoms such as Cymbella and Gomphoneis, could be a concern for fish populations because of the potential for smothering fish redds or by impacting benthic invertebrate populations that feed fish.


Together, most of these algae (and overall algal biomass) are typical of generally high quality waters with little organic pollution, high concentrations of dissolved oxygen, and alkaline pH. The relatively high percentage of eutrophic taxa does, however, suggest some degree of nutrient enrichment in the river, despite the relatively low concentrations observed at most sites. 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. 


First posted February 29, 2012