Contributors (in alphabetical order)
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NitrogenTotal nitrogen (total-N) levels, which include both the inorganic and organic components, were relatively high in the Snake River stations. Total-N levels at the upstream Snake River station (SNR-140) were generally higher than those observed at the other sampling stations. In general, concentrations decreased throughout the Lower Snake River. In the spring and summer, the total-N levels increased from about 0.30 to 0.60 mg/L at the Lower Snake River stations. The total-N levels increased considerably in the fall with peak levels at the Snake River stations reaching 0.8 to 1.1 mg/L in October. This late-season increase may be due to a reduction in plant uptake associated with aquatic plant and algae dying back or going dormant as well as agricultural harvesting in the watershed. Early fall rains after prolonged dry periods also increase nutrient concentrations. A corresponding increase in TSS levels was not detected. The seasonal pattern of nitrogen concentrations is also apparent in nitrate data collected in 1971 at SNR-107, prior to construction of the Lower Granite Dam and SNR-108 in 1995. Nitrate levels were generally highest in spring and fall, likely due to the lower biological uptake during the non-growing season. Concentrations of nitrate were generally similar during the growing season for the 1971 and 1995 data. This late-season increase in the Snake River levels most likely caused the levels at Station CLR-295, in the McNary Reservoir, to nearly double from around 0.20 to 0.40 mg/L, while the upstream Columbia River station remained constant at just under 0.20 mg/L through the fall period. Ortho-PRecent and historical data suggest that ortho-P levels in the Lower Snake River tend to be highest in the spring and fall, with relatively low concentrations in the summer. The lower levels during the summer are most likely due to biological uptake by aquatic plant and algal growth. As plant growth diminishes in the fall, the phosphorus levels increase, which was most evident at the reservoir stations where algal growth is usually most abundant. Throughout the Lower Snake reach, ortho-P levels through mid-August peaked at 0.018 mg/L and increased to 0.022 to 0.063 mg/L from mid-September through October. Clearly, ortho-P is much more available throughout the Lower Snake River reach relative to other major rivers in the area. Total PhosphorusThe highest levels in the study area were measured in the upper portions of the Lower Snake reach. During the spring freshet, TP levels (water column average) throughout the Lower Snake River ranged from around 0.060 to 0.11 mg/L. The high TP levels during this time of year are most likely attributable to the suspended sediment contained in the peak flow period. For much of the 1997 growing season, TP levels generally ranged from 0.035 to 0.060 mg/L and then steadily increased in the fall. Similar concentrations were observed in 1994 and 1995, where concentrations ranged from 0.025 to 0.060 mg/L in the summer and then increased to around 0.09 mg/L in the fall. According to the Washington state water quality standards, total phosphorus levels above 0.020 and 0.035 mg/L are considered to be critical thresholds in terms of preventing excessive algal growth when ambient trophic conditions are considered to be in the lower and upper mesotrophic categories, respectively. Oligotrophic conditions represent high quality waters with good water clarity and low algal production and eutrophic conditions represent high nutrient levels, excessive algal growth and poor water clarity. Mesotrophic conditions are somewhere in the middle and typically represent moderate levels of algal production, water clarity and light transparency. Limnological conditions in the Lower Snake River impoundments have generally been considered to be in the upper mesotrophic to eutrophic category (Normandeau 1999a). Based on a review of the 1997 data, the average phosphorus levels throughout Lower Snake River appear to be in the 0.030 to 0.040 mg/L range during the mid-summer and slightly higher to near the 0.060 to 0.070 mg/L range during June and fall months. This would suggest that the average phosphorus levels in the Lower Snake River for much of the entire growing season would likely be above the WDOE phosphorus guideline of 0.035 mg/L that was established to maintain existing conditions and prevent eutrophic conditions. SedimentA total of 487 grab sediment samples were collected. Of the 487 grab samples, 356 were sieved to develop particle-size distributions. The remaining 131 samples (or 26.9%) were not sieved either because there was no sample recovery or because the sample consisted only of gravel and/or cobble. The average grain size distributions for the sediment samples collected from above the Ice Harbor, Lower Monumental, Little Goose, and Lower Granite Dams are summarized in Table 16. The mean grain size for the channel bed sediments range from very fine sand to silt/clay. The highest concentration of relatively coarser sediments (fine to medium sand) was found in Lake Sacajawea, above Ice Harbor Dam. The highest concentration of silt/clay size sediments was found in Lake West, above the Lower Monumental Dam. Fine-grain sediments are concentrated on the channel bottom in Lake West. The concentration of these fine grain sediments is most likely associated with the discharge of the Palouse River into Lake West. This contribution is evidenced by the elevated TSS concentrations in the water quality samples collected from the Palouse River (refer to TSS section of this report). Soil erosion within the Palouse River drainage basin has been documented as a chronic problem due to historical land use practices (Ebbert and Roe, 1998). Recent studies have also documented that the adoption of erosion control practices within the drainage basin has resulted in an observable decline in suspended sediment concentrations in the Palouse River (Ebbert and Roe 1998), and as a result, probably also into Lake West. Four sediment samples collected from the Lower Granite Lake were analyzed for dioxins (tetrachloro-dibenzo-p-dioxin or TCDD). The reason for only collecting samples from this portion of the study area was that it is located immediately downstream of the Clarkston-Lewiston area, where industrial discharges may have released this organic compound. Table 16. Summary of sieve test results for sediment samples collected from the Lower Snake River in 1997 (Corps 1999).
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