Scientific Investigations Report 2006–5060
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5060
With an increasing human population in the area comes increasing demand for water of high quality. Use of the USGS Hagg Lake water quality model has allowed the examination of the water quality effects of potential modifications to storage in Hagg Lake, including a dam raise, installation of additional outlets, and introduction of additional inflows. The model is able to address a number of water quality concerns, including the quality of the lake’s water and the quality of water delivered to downstream users and to lower Scoggins Creek.
The model’s predicted changes in water quality may be evaluated with water quality goals in mind. These goals may vary depending on the intended use of the water. Some changes that generally are considered to be positive by many users include: decreasing the extent of anoxia in the lake; decreasing ammonia concentrations in the lake; reducing blue-green algae blooms in the lake; meeting a downstream temperature target; lowering concentrations of ammonia, blue-green algae, and total organic carbon in the outflow; and increasing concentrations of dissolved oxygen in the outflow.
For some measures, all scenarios showed improvements compared to the base case. For example, all scenarios had a lower annual average lake volume where dissolved oxygen concentrations were less than 1 mg/L, and had fewer days with low dissolved oxygen conditions. All scenarios produced lower annual average concentrations of ammonia. For other measures, such as chlorophyll a in the lake and the outflow, some scenarios showed improvement while others did not. The modifications that generally had an important effect on water quality can be summarized into three main groups: the amount of water in the reservoir; the quality of the augmented inflows; and the number, type, and operation of the lake’s outlets.
The amount of water in the reservoir depended on factors such as the height of the dam, whether any additional inflows entered the lake from a Sain Creek tunnel or a pump-back option, and the amount of water released from the lake. Model scenarios with lower lake levels generally produced warmer annual average lake water temperatures, earlier lake turnover, and more ammonia in the hypolimnion, compared to scenarios with higher water surface elevations. Lower lake levels produced outflows that were more likely to exceed downstream water temperature criteria and had earlier and higher minimum dissolved oxygen concentrations, which was related to earlier lake turnover.
Although the amount of inflow water affects lake level, which can affect water quality processes in Hagg Lake as discussed above, the quality of the inflow water also affects the lake. Separating the effects of the quantity and quality of inflows is not easy. The influence of the new water depends in part on the volume of inflow relative to the existing lake volume. For instance, in 2002, only about 11.5 percent of the full 12.2 m (40 ft) higher lake volume was from extra inflows. In 2001, about 41 percent of the maximum lake volume was added via pump-back.
Addition of upper Tualatin River water via a Sain Creek tunnel in 2002 produced cooler average lake temperatures and higher dissolved oxygen concentrations. It also produced cooler temperatures and higher dissolved oxygen concentrations in the outflow. The pump-back option, with water from the Tualatin River downstream of the dam, produced cooler average lake temperatures and higher average lake ammonia, chlorophyll a, and orthophosphate concentrations. In the outflow, that option produced cooler water temperatures and higher dissolved oxygen, ammonia, chlorophyll a, and orthophosphate concentrations. The pump-back option can import a substantial load of phosphorus into the lake, which may cause larger blooms of blue-green algae. The change in chlorophyll a concentrations may be small, but could become important.
Substantial differences were observed between scenarios using only the original fixed-elevation outlet (scenarios 0, 1, 5, 9, 10) and those that had selective withdrawal capability (scenarios 2–4, 6–8, and 11–15). Use of selective withdrawal to meet a downstream water temperature target led to cooler annual average lake temperatures, warmer overall outflow temperatures, and a shallower thermocline. Scenarios using only the original fixed-elevation outlet produced warmer annual average lake temperatures, cooler summertime outflows, and a thermocline that was deeper overall and progressively deepened through the summer. Selective withdrawal led to less anoxia in the lake, lower annual average lake ammonia concentrations, and shorter blue-green algal blooms.
Simulation of selective withdrawal showed that this technology allowed water released from Hagg Lake to meet downstream temperature criteria, though success varied between the three types of selective withdrawal simulated. The combination of one fixed outlet with one variable-elevation outlet was able to meet the downstream temperature target in 12 of the 14 scenarios where it was used; the 2 which did not always meet the criteria had exceedances of less than 1 degree‑day. The combination of two fixed outlets had unsatisfactory results in scenarios where significant drawdown occurred, because the ability to blend water from different depths ceased when the upper fixed outlet became dry. Selective withdrawal with only one variable-elevation outlet matched the seasonal trend of the temperature target, but produced large temperature variations in the outflow, exceeding the temperature criteria at times. This occurred because the outlet often was situated in the thermocline, which had a sharp temperature gradient. Seiching of the lake due to wind shear or slight variations in the thermocline structure over a day caused the temperature at the depth of the outlet to change, because the simulation only allowed one adjustment of the outlet elevation per day.
Selective withdrawal scenarios produced higher chlorophyll a concentrations and lower orthophosphate concentrations in the outflow compared to scenarios with the original fixed outlet. The fixed-elevation outlet often drew water below the photic zone, where algal activity occurred.
The existing outlet structure results in the efficient aeration of the released water. If some of this water is routed directly into a pipeline, however, this aeration would not occur. Because some of this water may be used for flow augmentation in the Tualatin River downstream, low dissolved oxygen concentrations could be a concern from September to December (figs. 12, 13), unless aeration is engineered into the pipeline.
For more information about USGS activities in Oregon, visit the USGS Oregon Water Science Center home page.
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