Henry Hagg Lake Water-Quality Model

Science Center Objects

Henry Hagg Lake is a reservoir located in the foothills of the eastern slope of the Coast Range Mountains of northwestern Oregon. The lake is used for recreation in the summer and flood control in the winter.

Henry Hagg Lake is a reservoir located in the foothills of the eastern slope of the Coast Range Mountains of northwestern Oregon. The lake was formed by Scoggins Dam, an earthfill structure that impounds Scoggins Creek, a tributary of the Tualatin River. Hagg Lake was filled and began normal operation in 1975. At a maximum water elevation of 93.2 meters (305.8 feet) above sea level, the lake's total storage capacity is 64,812 acre-feet, with a maximum surface area of 1.8 square miles. The normal full pool water elevation is 92.5 meters (303.5 feet), with a capacity of 62,216 acre-feet, and a surface area of 1.7 square miles. The dam was built and is owned by the Bureau of Reclamation, which contracts with the Tualatin Valley Irrigation District for operation and maintenance. More information about the Scoggins Dam project is available from the Bureau of Reclamation.

The objectives of this study were to develop a model of Hagg Lake that:

  1. simulated the circulation, temperature, and water quality in the lake,
  2. improved understanding of lake circulation and water quality and the processes affecting them, and
  3. predicted changes in circulation, temperature, and quality that might result from a suite of proposed dam modifications.

  1. Lake levels were highest in late spring and early summer and decreased through the summer and fall as downstream users required water for irrigation, drinking water, flow augmentation, and municipal uses. Lake levels were lowest in November and began to rise with the onset of winter rains. A drought during the winter of 2000-2001 caused the reservoir to not fill in 2001. The annual cycle in lake level was an important factor that affected lake temperature and water quality. 
     
  2. Spatial and temporal patterns in water temperature in Hagg Lake were similar in all 4 years modeled in this study. A thermocline developed each year by early summer, isolating cold, dense water near the bottom, below the lake's outlet structure. Withdrawals from the lake, as well as other seasonal factors, tended to draw the thermocline down to the level of the lake's outlet structure by mid-summer. Hagg Lake typically turned over in November and remained uniformly mixed and isothermal until early March, when temperature stratification began. Meteorological factors (solar energy, air temperature, wind) and reservoir operations (lake stage, elevation of the withdrawal) were found to have significant influences on the lake's water temperature. 
     
  3. During normal years, dissolved oxygen became depleted in the hypolimnion by late September; during the drought year, this occurred earlier, by late August. Colder temperatures and lake turnover in November reoxygenated the water column and ended hypolimnetic anoxia in each year. Dissolved oxygen levels in Hagg Lake were controlled mainly by water temperature (solubility) and sediment oxygen demand, and to a lesser degree, by algal photosynthesis and respiration. 
     
  4. Ammonia concentrations generally were low throughout Hagg Lake. However, in all years studied, accumulation of ammonia in the hypolimnion occurred once dissolved oxygen was depleted. Ammonia concentrations as high as 0.43 mg/L as N were measured in November of 2000. 
     
  5. Algae were separated into two groups in the model: blue-green algae, and all other algae. The general algae group had its highest abundance in the spring, due in part to inputs of algae from tributaries and possible resuspension of algal cells during storms. The blue-green algae group tended to bloom in late summer (in August, typically). Orthophosphate concentrations, as well as zooplankton grazing, water temperature, and light, controlled the levels and timing of algal blooms in the model. Concentrations of bioavailable phosphorus appeared to limit the size of the annual blue-green algae bloom. 
     
  6. While a community of zooplankton was found in Hagg Lake, its interactions with other zooplankton and with the lake's algal communities appears to be significantly more complex than what was represented in the model. Future work may be required to better describe the influence of zooplankton on the lake's water quality. 
     
  7. The model captured the dominant processes affecting water quality (temperature, dissolved oxygen, nutrients, and algae) in Hagg Lake and simulated the lake's water quality dynamics with sufficient accuracy for the planned purposes of the model. Comparing measured and modeled water temperature profiles for the 4 years simulated, the mean absolute error (MAE) was less than 0.7oC and the root mean square error (RMSE) was less than 1oC. Comparing measured and modeled dissolved oxygen profiles in the 3 years having reliable data (2000-2002), the model errors typically were less than 1 mg/L.