Streamflow, Water Quality, and Algal Conditions


Data collection for this 2010 study builds on previous water-quality investigations by the USGS in 2000 in the main-stem Molalla River (U.S. Geological Survey, unpub. data). Water samples were collected, field parameters were measured, and benthic community conditions were assessed twice at five locations in the lower Molalla River during summer low-flow conditions in 2010 (fig. 3; table 2). The water samples were analyzed to determine the amount of nutrients available to support algal growth, and field parameters such as dissolved oxygen and pH were measured to gage the relative effects of algal photosynthesis and benthic respiration on water quality. Excessive algal production can produce high (alkaline) pH and result in low concentrations of dissolved oxygen that can be harmful for aquatic life including fish. Sampling of benthic algae (periphyton) was conducted to provide information on the specific types of algae in the riffles and chlorophyll-a was used to estimate algal biomass levels. Qualitative surveys of benthic invertebrates were conducted to gage the health and quality of these organisms. Many invertebrates consume algae, and these secondary producers are important food resources for fish including salmon and steelhead.


A cool and wet spring in 2010 delayed and shortened the algal growing season, which may have limited the accumulation of biomass in the river and dampened algal-photosynthesis induced fluctuations in pH and dissolved oxygen levels. Streamflow at the time of the water quality and algal sampling in 2010 ranged from 2.5 to 3.6 m³/s (88 to 127 ft³/s), which was 12–18 percent higher in July and August and 22–28 percent lower in September compared with the long-term average flows at the USGS gaging station near Canby. Streamflow has a direct effect upon water quality and algal conditions by affecting the water temperature, dilution rate, and resulting concentrations of nutrients and other solutes. Also, streamflow and water depth affect light available for algae to photosynthesize, and the amount of light reaching the streambed dictates, in part, the areal extent of algal growth. Vast areas of shallow riffles appear during periods of low flow in summer that provide ideal habitat for periphyton. In some wider sections of the stream, periphytic growth occurs across the entire channel and in other areas it is concentrated primarily along the channel margins. 


Field Parameters


Field parameter data, including water temperature, dissolved oxygen (DO), pH, specific conductance, chlorophyll-a (water column), and turbidity were collected at each of the five water-quality sites using a calibrated Yellow Springs Instruments 6600 EDS multi-parameter sonde. Water temperature, pH, and DO measurements were made in the main flow in the early morning and again in late afternoon to characterize diel minimum and maximum values during sunny and warm periods in August and September 2010 (table 8).


The Molalla River exhibited the typical downstream trend of increasing water temperature and specific conductance (fig. 24) consistent with the “river continuum” of natural changes associated with downstream increases in channel width, reduction in of the amount of riparian shading, and other downstream changes that occur in many rivers (Vannote and others, 1980). The increase in specific conductance at the two downstream sites was most likely due to a greater amount of agricultural and urban influences (Cole and others, 2004) along with inputs in the lower river from tributary drainages with similar land uses and possibly inflows of groundwater.


Water temperatures continued to be high in the Molalla River during summer 2010. The early August afternoon water temperature at Glen Avon Bridge (Rkm 42.8) was 19.5°C and higher (22–24°C) at the downstream sites in 2010 (fig. 24; table 8). Temperatures were not as high in 2010 as they were in 2004 when ODEQ conducted a basin-wide assessment of water temperatures in the Molalla River that led to the development of a TMDL analysis (Williams and Bloom, 2008). A water-temperature survey of the Molalla River conducted in 2001 (U.S. Geological Survey, unpub. data) shows that the peak temperatures steadily increased from the Table Rock Fork to the Highway 213 Bridge with only a modest increase in water temperature between the Highway 213 and Knights bridges during this low flow water year (fig. 25). Inputs of cooler water in the lower reach, although not investigated during this study, are possible and could have helped offset further warming downstream at Knights Bridge.


Morning and afternoon values of DO and pH for 2000 and 2010 are shown in figure 26. Although the water temperatures exhibited an increase downstream of the Glen Avon Bridge, minimum diel DO and pH declined downstream in 2010. The maximum (afternoon) DO showed modest downstream increases owing to algal photosynthesis, whereas the patterns for maximum diel pH were not consistent, with observed increases between Highway 213 Bridge (Rkm 39) and Knights Bridge (Rkm 6.9) sites in 2010, but not in 2000 (fig. 26). 


In 2010, the minimum dissolved oxygen concentrations ranged from 7.6 to 9.8 mg/L, or 84.3 to 97.8 percent saturation, with the lower values occurring at the downstream sites during the morning hours (fig. 26). Longitudinal patterns in diel variations of dissolved oxygen and pH show a greater influence from algal photosynthesis in the lower river, particularly at the Goods and Knights Bridges sites. The maximum DO concentration (11.1 mg/L at Knights Bridge in September 2010) correspond to 122 percent of saturation (supersaturated conditions), which resulted from algal photosynthesis. 


Diel swings in pH, with maximum values up 8.3 and 8.4 units during the afternoon and lower values in the morning, are synchronized with dissolved oxygen swings caused by daily cycles of photosynthesis and respiration. The observed daily ranges in pH were not, however, as great as those in the Clackamas River to the north, where pH fluctuations caused by algal photosynthesis can exceed 1–2 units in a day (U.S. Geological Survey, 2011). Daily ranges in pH were closer to 0.5–1 units in a day during USGS samplings in 2010, and daytime maximum pH values were always within the State of Oregon standard of 6.5–8.5 units.


Periodic measurements of pH were taken by ODEQ in the lower Molalla River since 1965 at Highway 99E and more recently at Knights Bridge (fig. 27). Although pH has been as high as 8.5 units, the highest value in 2010 was lower—only 8.0 units—on August 12 (Oregon Department of Environmental Quality, 2010). The lower maximum pH value could have resulted from the cooler than average temperatures and higher flows that delayed the algal growing season in 2010. Note that although pH values in figure 27 appear to be higher in recent years, that the time of day measurements are taken has a large effect, as would be expected in a productive river showing diel fluctuations in pH and dissolved oxygen. The highest values of pH occur after 2 P.M.


The water-quality standards in effect for the main-stem Molalla River for water temperature and dissolved oxygen are dictated by the fish-use designation and whether salmon or steelhead are rearing and migrating, or spawning. The middle and upper Molalla River (upstream of the Highway 211 Bridge) and the North Fork Molalla River are currently designated as core cold water habitat for fish, and the lower main stem provides rearing and migration habitat for salmon and trout (Oregon Department of Environmental Quality, 2003). Spawning maps from Oregon Department of Environmental Quality (2005) indicate that salmon or steelhead spawn in the lower river (downstream of the Hwy 211 Bridge) from October 15 to May 15; in the short reach just upstream of the Hwy 211 Bridge to near the confluence with Dickey Creek from September 1 to June 15; and in the uppermost reach, which extends from near Dickey Creek to the headwaters, from August 15 to June 15. On the basis of this information, and relative to data collected for this study, the fish spawning criteria apply only to the Glen Avon Bridge site on September 15, 2010. At this time, the water temperature, pH, and dissolved oxygen values appear to meet water-quality standards although temperature and oxygen data were collected on just one day whereas the State standards are based on 7- and 30-day average maximum and minimum values, respectively. The remaining data collected for this study were evaluated against the water-quality standards in effect during non-spawning periods. In this case, only dissolved oxygen at Goods and Knights Bridges (7.6 and 7.9 mg/L) did not meet the 8 mg/L criteria in the early morning, but again, these were one-day measurements targeting daily minimum values, not 30-day average minimum concentrations.


Nutrients


Water samples for analyses of nutrients were collected and processed using USGS protocols (U.S. Geological Survey, 2006; Wilde and others, 2004) whereby width-integrated samples were collected from beneath the water surface at multiple locations in the cross section. Samples for analyses of dissolved ammonia, dissolved nitrite-plus-nitrate (hereafter referred to as nitrate), and soluble reactive phosphorus (SRP) were filtered through 0.45-µm Acrodisk™ filters using an acid-cleaned 60 mL syringe, and total nutrient samples were acidified with 1-mL of 4.5-N sulfuric acid per each 125-mL sample. Nutrient samples were shipped on ice to the USGS National Water-Quality Laboratory in Denver, Colorado, and analyzed using USGS protocols.


Compared with many other area rivers, nutrient concentrations in the Molalla River were generally low at most sites but did exhibit a pronounced increase in both nitrogen and phosphorus at Goods and Knights Bridges, presumably a consequence of greater inputs of nutrients from anthropogenic sources in the lower basin that entered the river from tributaries, agricultural irrigation returns, or groundwater. Most of the nitrogen in the Molalla River was in the form of either dissolved organic nitrogen (upper basin sites) or dissolved nitrate (lower basin sites). Concentrations of nitrate in the Molalla River at Glen Avon Bridge (0.022– 0.032 mg/L, fig. 28; table 9), although not exceptionally high, exceeded the seasonal average concentration proposed by the U.S. Environmental Protection Agency for reference streams in the Cascade Range of 0.005 mg/L (table 10). The total phosphorus (TP) concentrations at this site were slightly less than the reference concentration of about 0.010 mg/L. Near the end of the study reach at Knights Bridge, the nitrate concentrations were 0.22–0.33 mg/L (table 9), which exceeded by about 25 percent the seasonal average concentration for reference streams in the Willamette Valley of 0.15 mg/L (table 10); TP concentrations (0.013–0.015 mg/L) were well below the reference value of 0.04 mg/L. The SRP concentrations were higher in 2010 than in 2000 (fig. 28), possibly because of lower streamflow: 2.4–3.2 m³/s (85–113 ft³/s) at Goods Bridge in 2010 compared to 8 m³/s (283 ft³/s) in 2000 (fig. 24). The relatively low phosphorus concentrations in the Molalla River reflected the lack of naturally occurring geologic phosphorus in the upper basin that is found in the upper reaches of other Cascade Range rivers influenced by High Cascades geology, such as the nearby Clackamas River, where phosphorus levels during summer can be nearly twice as high (Carpenter, 2003).


Biologically available forms of dissolved inorganic nitrogen (DIN), which include both ammonium and nitrate, generally were less than 0.04 mg/L and SRP concentrations were less than about 0.01 mg/L at the two most upstream sites (table 9). This resulted in low DIN: SRP ratios, ranging from 1.9 to 4.4, which suggests that algae might be limited by nitrogen availability considering that ratios of about 7.2 indicate balanced nutrient availability, according to the “Redfield Ratio” (Redfield, 1934; Redfield, 1958; Hillebrand and Sommer, 1999). Nitrate concentrations at the downstream sites, however, increased 2–10 fold compared with upstream sites, resulting in DIN:SRP ratios up to 32 and 18 at Knights Bridge in August and September, respectively (fig. 28; table 9). These ratios suggest a possible switch from limitation by nitrogen to phosphorus limitation in the lower river, where heavy growths of filamentous green algae(Cladophora glomerata) were observed (see photograph 4). It is also possible that invertebrate grazing or some other factor limits the growth of algal populations, a topic discussed later in this report.


Although water-column nitrogen is probably sufficient to saturate growth rates in the lower Molalla River, this could be an important factor governing the algal biomass and (or) species composition in the middle reaches of the river. It should be pointed out, however, that although concentrations of dissolved nutrients are often used in water-quality assessments to identify the potential for nuisance algal growth, measured concentrations may not provide an accurate assessment of stream conditions because such measures may represent only the nutrients that remain after uptake by algae. Water-column concentrations of biologically available dissolved nutrients during summer can therefore underrepresent true enrichment from nutrients when algal abundance is high because low concentrations of dissolved nutrients can result from such uptake (Mulholland and Rosemond, 1992; Dodds, 1993; Peterson and others, 2001; Carpenter, 2003). To address this issue, other indicators of eutrophication, such as benthic algal biomass, diel fluctuations in pH and DO, and algal species and diatom composition can provide a more thorough, time-integrated assessment of stream conditions.


Analysis of ODEQ’s ambient monitoring data for the lower Molalla River show an apparent decline in the minimum concentrations of DIN in the past decade or so (fig. 29). Although lower DIN concentrations could be a sign of improved water quality, this trend also might be the result of greater uptake of nitrogen (primarily nitrate) by periphytic algae in the lower river, which use DIN and SRP for growth. Summertime DIN concentrations started to decline in 1997, the year after the floods of 1996 that caused widespread damage to streamside vegetation, reworked large gravel bars, and widened stream channels (figs. 16 and 17), and may have created additional habitat for benthic algae to colonize and grow over time. The observed reduction in shade in both flood plain and island bars, shown by the increase in area of bare bars from 1994 to 2000 (fig. 19) would have provided more light for algal photosynthesis in the lower river, particularly upstream of Goods Bridge. Although greater amounts of photosynthesis could have produced the observed decrease in DIN and higher pH values in recent years (fig. 27), as described above, more of the recent samples were collected after 2:00 P.M., which, given the diel cycle of algal photosynthesis, probably also contributed to these patterns.


Benthic Community Conditions—Algae and Macroinvertebrates


Benthic community conditions were assessed in the Molalla River in late July and mid-August in 2010. Benthic algae samples were collected from rock and cobble substrates in riffle areas and analyzed for biomass (chlorophyll-a, or Chl. a) and species composition. To randomize rock selection, flat metal washers with short lengths of orange flagging were hand-tossed from shore to 10 locations within the sampling zone, and the rock or cobble immediately upstream of the washer was carefully removed and brought to the bank, where algal material was collected using the top-rock/cylinder scrape method (Moulton and others, 2002). The top of each rock was covered with a plastic PVC cylinder to form a scribe having outside diameters ranging from 4 to 10.4 cm. The largest scribe possible was used on each rock to maximize the surface area sampled, and algal material outside the scribe was removed with a knife and/or plastic bristle brush and discarded. The circular patch of algae remaining on each rock was then scraped and rinsed with native water into a plastic basin, compositing approximately 1–2 L of algal material per sample. The algae and rinse-water slurry was transferred into 1-L Nalgene^® bottles and placed on ice until further processing. Periphyton biomass samples were processed off-site by homogenizing in an electric blender and transferring into a churn splitter. Subsamples of 5–10 mL for analysis of Chl. a were removed from the churn using a large-orifice pipettor and transferred onto 47-mm glass-fiber (GF/F) filters under vacuum pressure using a plastic filtration apparatus. Chl. a was analyzed at the Oregon Water Science Center laboratory in Portland, Oregon using a Turner Designs fluorometer with acid correction for the presence of phaeopigments. 


Samples for algae species identification and enumeration were obtained from the same churn splitter as the biomass samples, preserving 95 mL of the algal slurry material with 5 mL of full strength buffered formalin at 5 percent final preservative concentration. Samples were analyzed using methods described in Aquatic Analysts (2007). Cell density enumerations and biovolume measurements of a minimum of 100 live algal units were made under a Zeiss standard microscope equipped with phase contrast up to 1,000X magnification. Only viable cells, verified by intact chloroplasts, were counted. Algae were enumerated along a linear transect, measured accurately to 0.1 mm with a stage micrometer. Cell densities were calculated from the area observed (transect length × diameter of field of view), and taking into account the effective filter area and the volume of sample filtered used in sample preparation. Average biovolume estimates of each species were obtained from measurements on each taxon encountered in each sample.


Additional algal samples were removed from the same churn or collected separately at a site for supplemental qualitative microscopic evaluations of unpreserved material. Samples were examined with a Leica DM1000 light microscope equipped with phase contrast using a range of magnifications up to 1000X. These observations complemented the algal identifications provided by Aquatic Analysts by identifying to genus or species many of the larger algal forms, or “macro algae” that can be missed in the cell counts.


Benthic algal biomass values in the Molalla River were moderately high in July, ranging from 30 to 55 mg chlorophyll-a/m² and did not vary much longitudinally (fig. 30). Much of the river channel was covered with a glistening layer of diatoms and other algae (see photographs 5 and 6). In August, biomass levels were higher at the Goods and Knights Bridges sites than at the three upstream sites. A large population of Cladophora glomerata, a high-biomass forming type of filamentous green algae, developed in the lower river in July, producing a biomass of 113 mg chlorophyll-a/m² downstream of Knights Bridge in August, which exceeds the 100 mg/m² threshold commonly used to indicate nuisance conditions (Welch and others, 1988). Although Cladophora was present in the riffle sampled at the Knights Bridge site, the abundance was much higher in the deeper, slower flow run habitats in the reach downstream (see photograph 4). Cladophora also produces nuisance conditions in other Cascade Range streams, including the nearby Clackamas River (Carpenter, 2003), the North Umpqua River in southwestern Oregon (Anderson and Carpenter, 1998), and many other streams. As described further in the Evaluation of Quality Assurance Data section, because of the limited cell counts and issues related to either subsampling during sample preparation or magnification, some types of macroalgae, including filamentous green algae, were not detected during the cell counts but were observed in the river and noted during sampling (table 11). Cladophora, for example, was not identified in the cell counts from Knights Bridge, but was abundant at this and other downstream locations (see photograph 4), and was observed in the supplemental qualitative microscopic evaluations of samples collected at other sites, including those upstream of Glen Avon Bridge.


Benthic algal assemblages in the Molalla River included small, fast growing diatoms and very large stalked diatoms, filamentous green and blue-greens, and a few planktonic forms of green and blue-green algae (appendixes B and C). Just 33 algal taxa, mostly pennate diatoms, were identified in microscopic cell counts. Compared with other streams and rivers in the Pacific Northwest (Cuffney and others, 1997; Anderson and Carpenter, 1998; Carpenter, 2003; Waite and others, 2008), species richness in the Molalla River was rather low, averaging just 11–17 taxa per sample. This is partly due to the limited number of cells counted—100 “algal units” per sample—but also may reflect the similar condition of habitat sampled (riffle cobbles), particularly for sites between the Glen Avon and Goods Bridges.


An algal autecological indicator species analysis was conducted on the diatom assemblages from the Molalla River to gage the degree of nutrient and organic enrichment using species classifications, preferences, and tolerances published in Porter (2008). Table 12 lists all 33 algal taxa identified in cell counts and their respective autecological classifications for nutrients, dissolved oxygen, pH, salinity, and organic pollution. Table 13 lists the percentages of each indicator group (guild) that were tallied for each sample, considering only those taxa classified for a particular trait. For example, the percentage of eutrophic taxa was computed as the proportion of diatom taxa classified as eutrophic divided by the total percentage of diatoms classified for all trophic categories. Indicator percentages were computed on the basis of the cell density and algal biovolume for the July and August samplings separately, and averages for each time period are shown at the bottom of table 13. The average percent abundances for the combined percent density and biovolume of each guild for July and August are shown in figure 31.


The two most abundant guilds were pollution sensitive diatoms intolerant of excessive organic enrichment (Bahls, 1993) and diatoms requiring high concentrations of dissolved oxygen (table 12). These guilds comprised 50 percent or more of the relative abundance in both July and August samples (fig. 31). Five diatoms (Achnanthes minutissima [Achnanthidium minutissimum], Achnanthes linearis, Gomphonema subclavatum, G. angustatum, and Cymbella minuta) occurred in all samples (appendix B and appendix C), making up, on average, 77 percent of the cell density and 34 percent of the biovolume per sample. All five taxa are sensitive to organic pollution, which is consistent with the relatively low organic content of the Molalla River during summer. Sedimentation of riffles does not appear to be an issue as motile diatoms—those that often thrive in habitats affected by sedimentation—constituted less than 2 percent of taxa in all samples (not shown), consistent with the relatively “clean” riffles sampled that had low substrate embeddedness, generally less than 5 percent, indicating only minor infilling of fine sediments among the riffle cobbles. More fine sediments in the lowermost site at Knights Bridge resulted in somewhat higher substrate embeddedness of about 10–25 percent.


Other types of diatoms in the Molalla River are, however, indicative of waters having high levels of nutrients (=eutrophic, meaning “well fed”) and alkaline pH (greater than 7 standard units). Many species of algae, including several types of filamentous green algae such as Cladophora, Stigeoclonium, and Zygnema, and many diatoms require high nutrient levels. Eutrophic diatoms constituted between 77 and 57 percent of the average algal biovolume and between 50 and 44 percent of the average cell density, considering all samples in July and August, respectively (table 13). High-pH-indicating diatoms constituted about 25 to 65 percent of the relative abundance, and exhibited a modest downstream decline in value along with the percent eutrophic diatoms between the Highway 213 and Goods Bridges (fig. 31). Increases in oligotrophic, pollution sensitive, and high dissolved oxygen indicators occurred in this reach in July (fig. 31), largely due to increases in two Cymbella species, C. affinis and C. cistula (appendixes B and C). The “improvement” in algal indicators at Goods Bridge is somewhat unexpected, given that water temperature, specific conductance, and nitrate concentrations were higher, and dissolved oxygen concentrations were lower at Goods Bridge than at the Highway 213 site, although the differences were small (less than 10 percent), except for nitrate, which was 2–3 times higher at Goods Bridge. Such discrepancies between measured and inferred water quality from algal indicators can sometimes occur because algae integrate conditions over time (Carrick and others, 1988; Lowe and Pan, 1996), whereas water samples characterize conditions only at the time of sampling.


Compared with other guilds, the abundance of oligotrophic diatoms, or taxa indicative of low nutrient conditions, was less than 15 percent of the average biovolume and approximately 35 percent of the average density in July and August (table 13). Although higher abundances of oligotrophic taxa were observed at individual sites—the highest abundance was at Goods Bridge from two Gomphonema species (G. angustatum and G. ventricosum)—they never comprised more than half of the relative density or biovolume in a sample (table 12). Oligotrophic taxa are typical of low-nutrient “reference” streams, so their relatively low abundance in the Molalla River suggests some degree of nutrient enrichment, despite the apparently low nutrient levels measured.


In contrast to many other Cascade Range rivers such as the Clackamas or North Umpqua Rivers (Anderson and Carpenter, 1998; Carpenter, 2003), the abundance of nitrogen-fixing taxa were notably low in the Molalla River. As the sole exception, Aphanizomenon flos-aquae, a colonial blue-green algae, was identified in samples from the Highways 211 and 213 sites in July. Aphanizomenon is typically planktonic and occurs in lakes, reservoirs, and other ponded water bodies, where it sometimes forms blooms, so its occurrence in the Molalla River periphyton is unusual. It is possible that the Aphanizomenon originated from a bloom in a stagnant side channel or alcove; drainage from off-channel ponds is another possible source. 


The absence of benthic nitrogen fixing algae combined with the relatively low concentrations of SRP and detectable nitrogen in the form of nitrate suggest that algae in the lower river are probably not limited by nitrogen. The occurrence of filamentous green algae, a type that requires high amounts of nitrogen, also supports this hypothesis.


Other types of macroalgae, including the large stalked diatoms Gomphoneis herculeana and Cymbella cistula, and the filamentous blue-green algae Oscillatoria sp. were observed in the supplemental qualitative microscopic evaluations. The occurrence of G. herculeana, C. cistula, and other stalked diatoms in the Molalla River is noteworthy because they are becoming increasingly notorious for fouling streams with high biomass and producing nuisance conditions (Spaulding and Elwell, 2007). In New Zealand, and in Montana and other northwestern states, a similar type of stalked diatom (Didymosphenia geminata, so-called “rock snot”) is blamed for decimating fish populations in blue-ribbon trout streams by smothering benthic invertebrate populations with a felt-like blanket of stalk material. Although not yet identified in the Molalla River or the Clackamas River to the north, Didymosphenia has been identified in the North Santiam River to the south of the Molalla River (USGS, unpublished data). The stalks of these diatoms are made of polysaccharide mucilage (Wustman and others, 1997) that can become thick (as much as 20 cm), covering whole areas with white-to-brown or golden colonies. Although not overly abundant in the Molalla River, several types of stalked diatoms were identified in cell counts at all sites in both July and August samples (appendixes B and C). Stalked diatoms have the potential to cause significant impacts to stream ecosystems by altering habitat, reducing food resource quality for benthic macroinvertebrates, and smothering fish spawning redds. Because these types of algae contain large masses of mucilage, they can survive partial desiccation, so management strategies to control the spread have included, for example, educational efforts aimed at anglers and other river users to encourage careful washing of wader boots or avoiding the use of felt-soled waders that retain moisture.


Although not quantified during this study, benthic macroinvertebrates were qualitatively assessed during a July 2009 float trip and again during the two algal samplings in 2010 (table 11). A previous survey of benthic macroinvertebrates by Cole (2002) found downstream declines in richness and abundance of sensitive species in the Molalla River, with lower diversity and more tolerant species in the lower basin. In 2009, a diverse benthic assemblage consisting of mayflies, stoneflies, caddisflies, and other taxa were found in riffles in the reach between the Highway 211 and Highway 213 bridges, and in 2010, benthic macroinvertebrates were abundant at all sites, with a trend toward fewer mayflies, stoneflies, and caddisflies, and more snails and Chironomid midges at the downstream sites. Very large abundances of the large stone-cased caddis fly larvae Dicosmoecus gilvipes were observed on the river bottom in the reach between the Glen Avon and Highway 213 bridges during mid-summer float trips in 2010 (see photographs 7–9). Dicosmoecus adults have one brood per year (univoltine), and early instars that overwinter have lighter cases that make them more vulnerable to scouring high flows compared with the larger adults. If they do survive winter, larvae rebuild cases made of small stones in the summer that make them virtually invulnerable to predators, but the large size and heavy weight of their rock casings also makes it difficult for Dicosmoecus to find shelter in stream bed interstices during streambed-moving high flow events (Wooton and others, 1996 and references cited within). 


The moderate abundance and patchy growth patterns of periphyton, combined with highly abundant invertebrate grazers, demonstrated the high degree of secondary production in Molalla River riffles. Dicosmoecus and other Limnephilid caddisflies and several types of mayflies are active grazers of benthic algae, and their high abundance in the Molalla River suggested that the accumulation of periphyton biomass may be regulated by these herbivores during summer, as was previously found in experimental stream channels (Walton and others, 1995), natural streams in the Pacific Northwest (Lamberti and others, 1992), and northern California (Wooton and others, 1996). 


First posted February 29, 2012