Scientific Investigations Report 2006–5060
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5060
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Abstract
To meet water quality targets and the municipal and
industrial water needs of a growing population in the Tualatin River Basin in
northwestern Oregon, an expansion of Henry Hagg Lake is under consideration.
Hagg Lake is the basin’s primary storage reservoir and provides water during
western Oregon’s typically dry summers. Potential modifications include raising
the dam height by 6.1 meters (20 feet), 7.6 meters (25 feet), or 12.2 meters
(40 feet); installing additional outlets (possibly including a selective withdrawal
tower); and adding additional inflows to provide greater reliability of filling
the enlarged reservoir. One method of providing additional inflows is to route
water from the upper Tualatin River through a tunnel and into Sain Creek, a
tributary to the lake. Another option is to pump water from the Tualatin River
(downstream of the lake) uphill and into the reservoir during the winter—the
“pump-back” option. A calibrated CE-QUAL-W2 model of Henry Hagg Lake’s hydrodynamics,
temperature, and water quality was used to examine the effect of these proposed
changes on water quality in the lake and downstream. Most model scenarios were
run with the calibrated model for 2002, a typical water year; a few scenarios
were run for 2001, a drought year.
Significant findings:
- The proposed dam raise and associated changes to the lake’s
inflows and releases are likely to produce important and measurable changes
in water quality, both in the lake and in the water released downstream to
Scoggins Creek.
- Compared with the base case (measured conditions), most
modifications considered in these scenarios led to cooler annual average water
temperatures, less hypolimnetic anoxia, and lower annual average concentrations
of ammonia and chlorophyll a.
- The amount of water withdrawn from Hagg Lake has water quality
implications. Model scenarios with low water levels produced warmer lake temperatures,
earlier turnover and higher ammonia concentrations in the hypolimnion compared
with scenarios having higher water levels. In the outflow, low lake levels
led to more frequent exceedance of downstream temperature criteria in the
absence of outlet modifications.
- Either diverting upper Tualatin River water through a Sain
Creek tunnel or pumping back downstream Tualatin River water would fill an
enlarged Hagg Lake [7.6 meter (25 foot) and 12.2 meter (40 foot) dam raises]
in 2002. In a drought year such as 2001, however, pump-back would not necessarily
fill the enlarged lake, as water levels would be low at the start of the year,
and drought conditions would reduce the availability of water in the Tualatin
River downstream.
- Simulations of the wintertime transfer of water into Hagg
Lake from the Tualatin River via pump-back resulted in increased phosphorus
concentrations in the lake, especially in 2002 due in part to high concentrations
of orthophosphate in the pump-back water that year.
- Water temperature criteria in Scoggins Creek downstream
of Scoggins Dam will likely not be met without modifications, such as the
construction of additional lake outlets to allow the blending of warm and
cold water from various depths in the lake.
- Model predictions indicate that blending water from near
the lake’s surface (typically warm in the summer) with water from near the
lake bottom (typically cold) would be sufficient to meet downstream temperature
criteria and restore a more natural seasonal temperature pattern in Scoggins
Creek below the dam. A selective withdrawal tower with sliding gates or multiple
fixed outlets would offer dam operators the necessary flexibility for blending.
- The use of multiple lake outlets and various operational
strategies has important effects on water quality in the lake. Hypolimnetic
anoxia and the subsequent buildup of hypolimnetic ammonia can be minimized
or avoided. Lake surface temperature maxima can be decreased, which may help
to minimize blooms of the blue-green algae Anabaena planctonica.
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For more information about USGS activities in Oregon, visit the USGS
Oregon Water Science Center home page.
U.S.
Department of the Interior | U.S. Geological
Survey
