Lucky Peak Master Plan
Volume 2 - Technical Appendix
Section 3 - Project Inventory and Analysis
3.02. Ecological Factors.
f. Hydrology (Boise River Basin)
Approximately 78 percent of the runoff of the Boise River and its tributaries is from snowmelt. High flows in the spring result from temperature increase and snowmelt. The annual high water periods begin with slow to gradual flow increases in March, culminate with a peak discharge usually between 15 April and 15 June, and terminate with a gradual recession to base flows during July. Low flows prevail from late July through February. Approximately 78 percent of the total average annual runoff volume occurs between March and July. The peak discharge of the flood season contains the greatest volume from the several flood pulses resulting from weather fluctuations. Occasionally, during the winter months, the pattern is interrupted by high flows of short duration caused by locally heavy rainstorms. These, however, are generally small in runoff volume and duration when compared with the larger snowmelt flows. Nevertheless, they do constitute an important water management factor in the Boise river Basin (USACE, 1983).
Figure 3-2 (unavailable) shows the natural streamflow pattern of the lower Boise River. Above the Diversion Dam, built by USBR 2 miles downstream from Lucky Peak to divert irrigation water into New York Canal, winter and spring flows have been reduced and the summer and autumn flows increased by the regulated storage of Lucky Peak, Arrowrock, and Anderson Ranch Dams. further downstream, irrigation canals divert up to 6,000 cfs during the irrigation season. Return flows from irrigated lands, which enter the river in its lower reaches, increase the downstream flows significantly during the autumn and early winter period. Two drought years have occurred (1966 and 1977) since Lucky Peak Dam was constructed. The 1977 drought year brought the lowest runoff volumes on record; while the 1966 drought year had the seventh lowest runoff volumes on record (USACE, 1983).
Years of low runoff from the upper watershed can critically affect recreation at Lucky Peak. Low lake elevations reduce the usability of most of the launching ramps, and also affect operation of the marina. Visitation figures indicate that, on the whole, recreation attendance is less during years of low lake levels.
Lucky Peak is a lake of low biotic productivity. Inflowing water is nutrient poor and, except for a small contribution from agricultural activities on Mores Creek, watershed inputs are minimal. No serious water quality problems have been encountered in 4 years of monitoring. Since most of the land in the drainage area is unsuitable for agricultural development, and is managed for wildlife conservation, rapid changes in water quality are not expected under the present operational scheme.
Lucky Peak is a polymictic lake, a lake characterized by frequent or continuous temperature circulation. Although adequate temperature profiles exist only for the past 2 years, the data indicate that a long-term thermal stratification does not occur in the summer months. During calm periods in the summer, distinct thermal gradients are exhibited. However, the short time the lake is at full pool, coupled with wind-induced circulation, preclude the formation of stable stratification.
Without thermal stratification, oxygen depletion in the lake is primarily dependent on the oxidative processes generated by materials transported in from the drainage area. Although low dissolved oxygen concentrations have occasionally been measured, an oxygen depletion problem does not exist in the lake. Extremely low (less than 50-percent saturation) dissolved oxygen levels have been observed in what seems to be a basin depression immediately upstream of the dam. The water mass in this depression does not readily circulate and, consequently, exhibits lower dissolved oxygen concentrations than the rest of the lake. This relatively small volume of oxygen deficient water does not affect the overall water quality of the lake.
The major turbidity input to the lake is Mores Creek. Normally, high turbidities are measured at the mouth of the creek only during the spring runoff period. During the extreme 1977 drawdown, the concentration of debris contributed to heavy turbidities at the upper lake stations in September and December. Turbidity generally decreased from upper to lower stations in the spring. No obvious trends were apparent during the summer-fall period when phytoplankton populations (floating or drifting green plants - mostly algae) influence turbidity.
Except for the 1977 fall and winter measurements, levels of algal nutrients in the lake were below those thought necessary to support nuisance algal blooms. Nitrate nitrogen and total phosphorus values have remained relatively stable throughout the sampling period, averaging 0.13 mg/l and 0.029 mg/l, respectively. September and December nutrient levels in the extremely low 1977 pool were two to three times the average values.
The highest total coliform and fecal coliform bacterial numbers occurred at the mouth of Mores creek during the spring runoff. Throughout the rest of the year, coliform bacteria were noted infrequently at all lake stations.
The phytoplankton population in Lucky Peak Lake is dominated by diatoms, an algae characteristic of the open illuminated zone of a lake. Diatoms are good indicators of water quality, and are generally characteristic of nutrient-poor waters. Copepods and cladocerans are representative members of the zooplankton (animal plankton) population. Sampling frequency has not been sufficient to analyze specific population trends. Generally, phytoplankton numbers have increased in the spring, reaching maximum in mid-June. Fragilaria, a diatom, was the most abundant genus at all stations. Zooplankton numbers were highly variable, and mid-summer peaks with counts of up to 45/m3 were observed in 1975 and 1977. The peaks were transitory, generally dropping to normal levels of less than 7/m3 within a short time.
The Sandy Point swimming area is created by backflow from the project outlet works. As a "backwater" area, it does not possess a natural circulation system. As a result, floating accumulations of algae appear in the beach area and coliform content increases, though not to hazardous proportions. Based on the low bacteriological counts documented during a 1976 intensive survey (USACE, 1976) and the marginal to low counts shown in historical data, the water in the Sandy Point swimming area is considered a high quality primary contact water according to defined water quality standards [Idaho Department of Health and Welfare (IDHW), 1980]. A study is currently underway to monitor water quality at Sandy Point swimming area due to the fill area for the new powerhouse across the Boise River.
A major interest in the lake's water quality is its influence on the Boise River below the dam. Downstream water quality deteriorates as irrigation returns and municipal discharges contribute pollutants to the river. Water discharged from Lucky Peak has generally been of excellent quality with sufficient time available to utilize or break down nutrients, and to reduce wastes somewhat before they flow into the Snake River. During low flows, nutrients transported by the Boise River have contributed to water quality problems in Brownlee Reservoir. With the probability that adequate waste treatment facilities will not keep pace with the projected population growth in the Boise River Basin, protecting Lucky Peak's water quality from degradation should be a prime objective.
The primary problems on the lower Boise River are the addition of oxygen-demanding wastes from municipal and industrial discharges in the Boise Valley and low streamflows resulting from irrigation withdrawals. Outfall from the frequently overloaded Boise sewage treatment facility has created extensive growths of slimes, algae, and Sphaerotolis, a plant-lie bacteria associated with sewage effluent. Biological oxygen demand (BOD) is a standard method of pollution assay that determines rates of change and the balance between use and production. Meat packing and potato processing plants, in addition to other manufacturing and sewage plants, account for more than half of the BOD load in the Boise River below Caldwell. More than half of the phosphorus loading was due to irrigation return flows.
Good quality drinking water is provided by wells at Sandy Point, Barclay Bay, Robie Creek, and Spring Shores. Additional well development, or the cultivation of high-grade springs, is currently needed to increase supplies of water, particularly at Spring Shores. Lake water, though generally of high quality, should not be depended upon as a drinking supply.
The water quality of Lucky Peak is good, and no problems are expected in the future. Development and management practice should be executed in a manner that will ensure continued high water quality.