Scientific Investigations Report 2006–5060
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5060
Henry Hagg Lake (Hagg Lake) is a man-made reservoir in the Coast Range mountains of the Tualatin River Basin, west of the Portland metropolitan area in northwestern Oregon (fig. 1). The lake was formed by the construction of Scoggins Dam, an earthfill structure that impounds Scoggins Creek, a tributary to the Tualatin River. Besides Scoggins Creek, Sain and Tanner Creeks also flow into Hagg Lake. The reservoir filled and began operation in 1975. At the current full-pool elevation of 92.5 m (303.5 ft), the reservoir has a storage capacity of 62,216 acre-ft and a surface area of 1.7 mi2 (Ferrari, 2001).
Hagg Lake is the major water storage facility in the Tualatin River Basin. It is used for recreation, flood control, flow augmentation in the lower Tualatin River, and as a source of municipal, industrial, and agricultural water. Twelve cities, with a current combined population of 460,000, lie in the basin. Their populations and associated water needs are projected to approximately double by the year 2050 (Montgomery Watson Harza, 2004). Current municipal water needs are met with multiple water supply sources, including ground water and surface water, and importation of water from the Trask River to the west and the City of Portland’s Bull Run River water supply to the east. Due to rising demand, however, future water needs will not be met by the current water supply system.
A host of options has been considered to meet future water demands, several of which include structural, operational and inflow changes to Hagg Lake. Options include raising the dam height by 6.1 m (20 ft), 7.6 m (25 ft), or 12.2 m (40 ft); increasing inflows to more reliably fill an enlarged reservoir; and installing other lake outlets, perhaps including a selective withdrawal tower. The additional inflows may be obtained by routing water from the upper Tualatin River near Haines Falls into the reservoir via a tunnel that discharges to Sain Creek, or by pumping water uphill and into the reservoir (pump-back water), from the Tualatin River at the Springhill Pumping Plant via a pipeline during winter high flows.
Water releases from an enlarged reservoir would change to meet increased downstream demands and updated instream flow requirements for Scoggins Creek below the lake. Some future outflows may be routed directly from the reservoir through a pipeline to the water treatment plant for municipal and industrial users. The installation of additional lake outlets, or a selective withdrawal tower, would allow water to be blended and released from different lake depths (with different temperatures and water quality) to meet downstream water temperature or water quality targets.
Water quality in Hagg Lake was documented and simulated with a model by the U.S. Geological Survey (USGS) (Sullivan and Rounds, 2005). The model was calibrated for the years 2000–03, a period that included a drought year (2001) in which the lake did not fill. The model successfully simulated the lake’s heat budget and captured the important components of processes affecting water quality, focusing primarily on nitrogen, phosphorus, algae, and dissolved oxygen. Water temperature was simulated with a root mean square error (RMSE) of less than 1°C, and dissolved oxygen was simulated with a typical RMSE less than 1 mg/L, measures which indicate that the model accurately simulated the dynamics and seasonal variations in the lake’s water quality.
Hagg Lake exhibits several annual limnological cycles. Each year, as the lake surface warms through the springtime, a thermocline develops in the lake by early summer. This thermal gradient isolates cold and dense water within the reservoir’s lower layer. As the summer progresses, dissolved oxygen in this lower layer of water (the hypolimnion) is consumed, typically leading to anoxia (no oxygen) by late September. At the onset of anoxia, ammonia begins to accumulate in the hypolimnion. As surface water temperatures cool in autumn, the temperature stratification eventually is eliminated and the lake mixes and reoxygenates, generally in mid-November; this vertical mixing is termed lake turnover. Moderate algal blooms (about 20 µg/L chlorophyll a), including a blue-green algae bloom in August, typically occur every year in Hagg Lake and influence the lake’s water quality. The seasonal temperature pattern of outflows from Hagg Lake is characteristically different from that of the tributaries to the lake, and the outflows occasionally exceed the downstream water temperature criteria. Water with low dissolved oxygen concentrations also is predicted to be released in autumn, but efficient aeration of that water by turbulence at the foot of the dam prevents violations of the minimum dissolved oxygen standard (Sullivan and Rounds, 2005).
The effect of the proposed structural, inflow, and operational changes on reservoir and outflow water quality can be evaluated before such changes are made. Issues of concern include temperature, dissolved oxygen, ammonia, and algae in the lake and in the outflows, including water released to Scoggins Creek and water delivered to downstream users via a proposed pipeline. In this investigation, the USGS Hagg Lake model was used to simulate the effects of these proposed structural changes on the quality of the lake and the water released from it.
This investigation resulted from a scientific and financial partnership between the USGS and Clean Water Services (CWS), one of a group of agencies investigating the various options for meeting future water‑supply needs in the Tualatin River Basin.
The purpose of this study was to develop a model of Hagg Lake that could (1) simulate lake hydrodynamics, temperature, and water quality; (2) help develop an understanding of the processes affecting hydrodynamics, temperature and water quality; and (3) predict changes in hydrodynamics, temperature and water quality that could result from a set of proposed modifications to the dam and lake. A previous report (Sullivan and Rounds, 2005) addressed the first two goals, and this report addresses the third by summarizing model results for hydrodynamics, temperature, and water quality under different structural, operational, and inflow conditions. The calibrated USGS Hagg Lake model for the year 2002, a normal hydrologic year, was used as the basis for most of the scenarios. Some scenarios were repeated for 2001, a drought year.
The model scenarios included combinations of the following elements:
Scenario results were analyzed with regard to the water quality of the lake, the quality of water withdrawn from the lake, and the type and placement of outlet structures. For the lake itself, analyses focused on thermal structure, ammonia and orthophosphate concentrations, algal population dynamics, and the extent, timing, and duration of hypolimnetic anoxia. The quality of released water in the model was analyzed with regard to water temperature, dissolved oxygen, ammonia, orthophosphate, chlorophyll a, and the downstream water temperature criteria in Scoggins Creek. The effect of various outlet structures and water blending schemes on lake and downstream water quality also was analyzed. These results can be used to help determine how modifications to the dam and reservoir might be designed and managed to optimize water quality.
For more information about USGS activities in Oregon, visit the USGS Oregon Water Science Center home page.
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