Scientific Investigations Report 2006–5060

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5060

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“Preferred” Options

After estimating water-supply reliability, cost, regulatory feasibility, and considering input from water users and landowners, three options were designated as “preferred” options by the water management and water supply agencies involved and are undergoing more detailed analysis in an environmental impact statement (EIS). The USGS is involved only to the extent of providing scientific information so that planning decisions are made with a sound technical foundation. The three options are (1) no modification to the dam, (2) the 12.2  m (40 ft) dam raise with pump-back water, and (3) the 6.7 m (25 ft) dam raise with pump-back water. In this modeling study, the latter two options were evaluated with selective withdrawal capability. Results for these three options are included in the previous sections of this report, but will be summarized and compared here.

The reservoir with its current configuration starts the year isothermal, cold, and well-oxygenated (Sullivan and Rounds, 2005). A thermocline develops in early summer, and by late August to October dissolved oxygen becomes depleted in the hypolimnion. Ammonia production and accumulation occurs in the hypolimnion concurrent with the depletion of dissolved oxygen. Algal blooms occur in the lake, and appear to be limited in size by phosphorus availability, water temperature, and other factors.

Compared to this scenario with no modification to the dam, model results for a dam raise with selective withdrawal capability and pump-back water suggest that changes in water quality in Hagg Lake would occur. The modified lake was predicted to have cooler annual average lake water temperatures and a shallower thermocline. Less water with low dissolved oxygen (<1 mg/L) would be present, and the duration of conditions with low dissolved oxygen in the lake would be shorter. As a result, the concentration of ammonia in the modified lake was predicted to be lower. Orthophosphate and total phosphorus concentrations in the modified lake would be greater than the base case conditions. This is largely due to the source of the pump-back water, the Tualatin River, which has higher orthophosphate concentrations than the natural tributaries to Hagg Lake. The amount of the phosphorus increase depends in part on the volume of water pumped back; for instance, the larger volume of Tualatin River water used to fill the 12.2 m (40 ft) dam raise led to higher orthophosphate concentrations in the lake compared to the 7.6 m (25 ft), raise which required less pump-back water (table 5). The concentration of phosphorus in pump-back water varies and also affects lake concentrations; pump-back water had higher phosphorus concentrations in 2002 compared to 2001. Further study and understanding of phosphorus dynamics in the part of the Tualatin River where pump-back would occur could be helpful. The model showed a small increase in lake algal and chlorophyll a concentrations with the pump-back options, as some algal growth is phosphorus limited. However, the altered temperature regime in the modified lake, and other complexities of algal dynamics also could contribute to changes in their concentration or species abundance.

For the outflow, the modified dam was predicted to be able to meet downstream water-temperature criteria in Scoggins Creek if blending from multiple outlets is used; currently the outflow occasionally exceeds those criteria. Outflow dissolved oxygen concentrations were predicted to decrease slightly, with lower predicted minimum concentrations. Ammonia, orthophosphate, and chlorophyll a concentrations in the outflow were predicted to increase slightly. Water diverted directly into a pipeline would have low dissolved oxygen concentrations in the September to November time period, which could affect its use for flow augmentation downstream.

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