Contributors (in alphabetical order)
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Dissolved OxygenThe lowest oxygen concentrations recorded during 1997 typically occurred during September. At station SNR-40, which is in the tailwater section below the Lower Monumental Dam, the lowest water column concentrations ranged from 6.7 to 6.8 mg/L, or less than 75% saturation from the surface to the bottom with an overall depth of 8.0 meters. The next downstream station, SNR-18, which is in the Ice Harbor Reservoir, had DO concentrations ranging from 6.9 to 7.1 mg/L or roughly 76 to 79% saturation at depths above 20 meters during the same time interval. A few locations had lower concentrations at depth. Station SNR-108, which is at the deepest point in the Lower Granite Reservoir, had low readings of 5.3 and 3.4 mg/L or 59 and 38% saturation at depths of 30 and 35 meters, respectively. At depths above 20 meters, DO levels ranged from 7.0 to 7.6 mg/L or 80 to 87% saturation. In 1994 and 1995, the lowest readings recorded at SNR-108 at a depth of 34 meters were 2.3 and 4.9 mg/L, respectively. A comparison of DO data collected in 1975 and 1977 at approximately the same time of year suggests that concentrations of DO at depth have decreased over time. Station SNR-83 in the Little Goose Reservoir just below the confluence of Deadman Creek also had relatively low values of 5.5 and 4.7 mg/L or 62 and 52% saturation at depths of 10.0 and 30.0 meters, respectively, during early September 1997. Above 10 meters, DO levels ranged between 5.8 to 7.2 mg/L or 67 to 84% saturation. However, prior to this event and by early October, DO levels at all the Snake River stations were near or above 90% saturation. Total Suspended Solids (TSS)Table 14describes the mean TSS concentrations (mg/L) and 95% confidence intervals for a select number of sample sites within the project area, covering a period of up to 24 years. These data indicate that in most cases the average concentrations for each site either increased slightly or were relatively similar. However, it is noteworthy that the 18 mg/L mean determined for SNR-148 in 1997 was due, in large part, to the 65 mg/L value that was observed during early June, when runoff was close to maximum; otherwise, concentrations were less than 10 mg/L most of the time. Table 14. Average and 95-percent confidence intervals for growing season total suspended solids concentrations (mg/L) at 1 m for selected sampling sites and years (Corps 1999).
It is generally thought that larger particles transported by the rivers settle out in the transition zone in the vicinity of Lewiston, Idaho, and downstream into Lower Granite Reservoir. Finer material that passes Lower Granite Reservoir remains suspended. As such, the data suggest that there may have been a decrease in the larger fraction of the suspended solids transported by the in-flowing rivers, yet the amount of fines that travel down through the series of dams has remained about the same. Occasionally, elevated concentrations near the surface occur in the reservoirs as a result of localized algal blooms, port operations, and tributaries. Typically, TSS concentrations are highest during the spring freshet and then decline as flows diminish through late summer and into the fall. Within the Lower Snake River, the upstream station (Station SNR-140) had peak TSS levels of 60 and 65 mg/L at the surface and bottom depths, respectively. Station SNR-129, at the uppermost portion of the Lower Granite reservoir, appeared to have the highest peak level of 72 mg/L at a depth of 20 meters, and an average concentration of nearly 50 mg/L throughout the water column. Discharge in the Lower Snake River was around 175 kcfs at the time, and the average TSS level throughout the remaining portions of the system was about 30 mg/L with no distinct differences between the impounded and non-impounded reaches. Again, the highest levels were generally observed at the greater sampling depths. By the June 29 sampling date, the average TSS level declined to just below 20 mg/L, except at Station SNR-83, which had an unusually high level of 70 mg/L at the surface and much lower levels below. This high TSS level was likely the result of patch conditions that often occur on the reservoirs. For the remainder of the sampling season, TSS levels were consistently below 20 mg/L, and most often below 10 mg/L. There are no state water quality standards for TSS. However, turbidity standards in Idaho and Washington limit increases to 5 nephelometric turbidity units (NTU) when the background is less than 50 NTU except when the flood exceeds the 7-day, 10-year flood frequency. Turbidity levels of the Snake River exceeded state water quality standards in June 1997 at most stations (Table 15). Table 15. 1997 turbidity measurements (FTU¹) in surface waters at selected Snake River stations (Corps 1999).
None of the TSS concentrations observed in 1997 would have lethal effects on adult or juvenile salmon (Newcombe and Jensen 1996). Concentrations of 25 mg/L for 4 hours have been shown to reduce feeding rate; higher concentrations up to 1,000 mg/L have shown no deleterious effects on adult salmon other than coughing and apparent stress. One study showed 50% mortality of juvenile coho salmon at 509 mg/L TSS (Newcombe and Jensen 1996). Of the various soluble inorganic forms of nitrogen, nitrate plus nitrite (NO3 + NO2) was the principal component, often comprising more than 90% of the soluble fraction. Nitrate nitrogen concentrations exhibited inter-annual variations at several of the sites, but long-term trends were not apparent. However, two important issues were identified regarding the inorganic nitrogen species. Nitrate concentrations were consistently greater than ammonia values at almost all stations. The two upstream Lower Snake River stations, SNR-140 and SNR-148, had median NO3 levels that were much higher, ranging between 0.33 and 0.35 mg/L, while the median NO3 levels throughout the Lower Snake River reach ranged from 0.13 to 0.19 mg/L. These data suggest that the high levels contributed from the middle Snake River reach are slightly diluted by the low levels in the Clearwater River, resulting in moderately high NO3 levels in the Lower Snake River. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||