Ingham, Michael; 1992; Cascade Reservoir, Valley County, Idaho 1988-1991; Water Quality Status Report No. 103; Idaho Department of Health and Welfare, Division of Environmental Quality, Southwest Idaho Regional Office, Boise, Idaho; 17p + attachments.
Findings from the continuation of the monitoring initiated by the Citizen Lake Monitoring Program for Cascade Reservoir extending throughout the years 1988 to 1991. Data from 1988 was detailed in Water Quality Status Report No. 85 (Klahr, Patricia C.; 1989; Cascade Reservoir, Valley County, Idaho 1988: Water Quality Status Report No. 85; Idaho Department of Health and Welfare, Division of Environmental Quality,, Water Quality Bureau, Boise, Idaho; 12p + appendix.) Water quality variables monitored included nutrients (ammonia, nitrate, nitrite, total nitrogen, total phosphorus, ortho-phosphorus), chlorophyll' a, Secchi disk depth, and dissolved oxygen, from four different sites (North Fork Arm, Sugar Loaf Island, South Bay, and the buoy line at the dam). Findings validate data obtained earlier and confirm trends identified previously, namely that the North Fork arm of the reservoir continued to exhibit the best water quality overall with the lowest nutrient and chlorophyll a values. While it remained consistently higher in water quality overall, decreases in water quality at the other sampling sites within the reservoir were echoed to a relative degree by decreases in the water quality of the North Fork Arm. Significant nutrient concentrations and lower overall dissolved oxygen levels were observed at the sites in the southern reservoir. This may be attributed to increased residence time in the southern end of the reservoir, leading to longer periods of stratification. The data available for the buoy line sampling site provides an extensive (and valuable) time frame (16 years) for water quality evaluation. Early and late summer data was tabulated for the years from 1975 to 1991. Early summer dissolved oxygen showed a shallow decrease from 1980 to 1990, while late summer concentrations showed the opposite trend, however, since variability of the analytical method is not known, quantitative interpretation is not possible. The observed trend may be an artifact of method variability or environmental factors. Secchi depth measurements showed a similar downward trend over time (not accounting for the variability factors noted previously). Chlorophyll a concentrations showed a trend that was negatively correlated with the Secchi depths for the tabulated years (also subject to variability factors noted previously).
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