Overview of River Conditions During 2010–11


The study period spanned two exceptionally wet years with higher-than-average streamflow compared to the 103‑year period of record (fig. 6). Water temperatures in the lower Clackamas River at Oregon City were about 2–3°C lower during the first half of both summers and, in 2011, about 4–6°C lower in mid-July compared with the previous 8 years (data not shown). These conditions may have limited the degree to which algae affected source-water quality during this study. Although the continuous FDOM monitor indicated sharp rises in DOC concentrations resulting from rainfall events, DOC concentrations in the mainstem were generally low, typically about 1.0–1.5 mg/L and occasionally up to about 2.5 mg/L during early-season storms. Summary statistics for selected water-quality, optical, and DBP data are presented in table 6.


The first monthly sampling was conducted in mid-April 2010 (table 4), during the last snowmelt period of the season. The first basin-wide survey was conducted in May when flows were still moderately high, about 3,500 ft³/s at Oregon City (fig. 7). The last significant storm event of the water year, in early June, produced nearly 3 in. of rain. Samples were collected near the peak in flow when streamflow was about 13,000 ft³/s at Oregon City (fig. 7). Later that summer, a blue‑green algae bloom in North Fork Reservoir (Anabaena flos-aquae and Microcystis aeruginosa) was sampled in August and again in September (table 4). This bloom prompted the Oregon Health Authority to issue a human‑health recreational advisory for the reservoir on September 2, 2010. The second basin-wide survey on October 10, 2010, followed the first initial runoff-producing rain event in autumn. About an inch of rain fell over a 2-day period that produced slightly higher turbidity (about 5 FNU) in the lower mainstem (fig. 7). A series of larger autumn storms in late October delivered 3.5 in. of rain that produced the major “flush” of the season, mobilizing material accumulated in the soil profile during the dry season; this event was sampled on November 1, 2010 (table 4).


The 2011 water year and growing season were similarly wet and cool; higher-than-average streamflow resulted in relatively low water-column concentrations of chlorophyll-a (fig. 8). Anabaena bloomed again in September 2011, leading to another advisory for North Fork Reservoir and Timothy Lake (fig. 1) on September 8, 2011. The bloom in North Fork Reservoir was not as severe as in years past (Carpenter, 2003), although the bloom did produce enough biomass to form a surface scum on the reservoir and may have contributed to reports of tastes and odors in treated drinking water. In addition to the depth-profile sampling at the log boom in North Fork Reservoir, one surface sample containing a high abundance of Anabaena was collected at Promontory Park during the bloom (see photograph 2b). This sample represents DOM enriched in algal-derived carbon and served as an “end member” in terms of carbon characterization.


The third and fourth basin-wide surveys were conducted near base-flow conditions in early and mid-September, respectively, a low-carbon period for the river. The final sampling was during the annual drawdown of Timothy Lake (table 4). In addition to the typical suite of main-stem sites, the fourth survey included the main-stem site at the Two Rivers Campground (fig. 1), a site not influenced by the Timothy Lake release. Although in years past the Timothy Lake drawdown contributed higher concentrations of DOC, TOC, chlorophyll-a, and Anabaena cells to the upper mainstem (Carpenter, 2003), in 2011, the observed effect was limited to a three-fold increase in TOC at Carter Bridge that continued downstream to the DWTP intakes. Although discrete sampling ended with this last synoptic, the in-situ FDOM sensors continued to operate through January 2012.


Algal growth in the Clackamas River and its impact on diel changes in pH and DO are affected by factors such as streamflow, water temperature, nutrients, and availability of solar radiation for photosynthesis. The higher flows during this study, especially during springtime (fig. 6), produced lower-than-average water temperatures and probably also delayed colonization by benthic algae in faster velocity zones. These conditions, exacerbated by clouds and scant sunshine, delayed and (or) truncated the algal growing season both years. While much higher-than-average rainfall and prolonged cloudy weather in 2010 might have resulted in less solar radiation available for growing periphyton, field surveys in June 2010 revealed areas of high algal biomass, especially in the lower mainstem where periphyton chlorophyll-a levels exceeded the commonly applied nuisance threshold of 100–150 mg/m² (Welch and others, 1988) at all four main-stem sites—Estacada, Barton, Carver, and Highway 99E (table 7). By early September, periphyton biomass was even higher at the tributary sites but lower at all lower-basin main-stem sites, possibly from slow, metered losses due to the scouring high flows, grazing by benthic invertebrates, or some other factor.


Despite the apparent reduction in benthic algal biomass in the lower mainstem, conditions either precluded or obscured the occurrence of any substantial sloughing of viable chlorophyll-a into the water column, or the material was not particularly prone to fluoresce. In fact, concentrations of chlorophyll-a in the water column remained low and, on average, were much lower compared with previous years (fig. 8). It is possible that higher flow, as well as higher water velocity, may have produced a more steady (and less punctuated) losses of periphyton algal particles into the water column.


First posted February 11, 2013