Scientific Investigations Report 2007–5008

Scientific Investigations Report 2007–5008

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Significant Findings

  1. An annual pattern of water temperature exists in Detroit Lake that was similar in all modeled time periods. The reservoir typically is isothermal and cold at the beginning of the year. In spring, the lake surface warms and a thermocline develops by summer, isolating cold, dense water at the reservoir bottom. In autumn, the water surface cools, and eventually the reservoir mixes, or “turns over,” and becomes isothermal again.
  2. Detroit Lake has an important influence on downstream water temperature in the North Santiam River. Reservoir outflow water temperature reaches an annual maximum in autumn, at times exceeding the water temperature criterion. In the absence of Detroit Dam, the annual water temperature maximum would occur in midsummer. Water released from Detroit Lake also has less daily temperature variation compared to what would occur in the absence of the lake.
  3. Model results demonstrated that if a selective withdrawal device were installed at Detroit Dam, water temperatures of the outflow from Detroit Lake could remain less than Oregon’s maximum water temperature criteria for the North Santiam River all year. A more natural seasonal temperature pattern could be produced through most of the year, but in autumn, the lake did not have enough stored cold water to match this hypothetical seasonal temperature pattern downstream of the dam.
  4. Total dissolved solids (TDS) had an annual cycle in Detroit Lake. During spring storms, the inflowing TDS concentrations were relatively low. As the lake was not yet strongly stratified, these inflows acted to decrease TDS throughout the lake. As summer progressed, TDS concentrations in the inflows typically increased. The summer temperature stratification acted to keep summer inflows, with their higher TDS concentrations, in the epilimnion, preventing these inflows from mixing into the colder, denser hypolimnion. With the breakdown of stratification in autumn, waters with higher TDS concentrations in the epilimnion were mixed throughout the lake.
  5. The largest suspended sediment loads entered Detroit Lake during storm events. During the record-breaking precipitation between December 1, 2005, and February 1, 2006, more mass of suspended sediment entered and was deposited in the reservoir than in the entire calendar years of 2002 and 2003 combined. In summer, when storms were few, the inflow of suspended sediment into the lake was small, and resultant lake concentrations also were low.Most of the mass of sediment entering Detroit Lake was in a size class designated “sand and silt.” Sediment in that size class comprised 85 percent of the inflowing mass in calendar year 2002, 83 percent in calendar year 2003, and 92 percent during the modeled 2005–06 storms.
  6. Although the sand and silt component made up most of the mass of suspended sediment entering the reservoir, it comprised only a small portion of the suspended sediment exiting the reservoir. It constituted only 9 percent of the outflowing sediment mass in calendar year 2002, 7 percent in 2003, and 16 percent during the modeled 2005–06 storms. Most of the mass of sediment leaving Detroit Lake was composed of clay-sized particles.Assuming a bulk density of 1.89 g/cm3, 14,300 m3 (11.6 acre-ft) of sediment was deposited in Detroit Lake in 2002, 11,820 m3 (9.6 acre-ft) in 2003 and 34,900 m3 (28.3 acre-ft) in storms from December 1, 2005, to February 1, 2006. Each of these sediment volumes is less than 0.01 percent of Detroit Lake’s full pool volume of 561 million m3 (455,000 acre-ft). The model results indicate that most sediment deposition occurred in the upper reaches of the reservoir, near the inflows of Breitenbush and North Santiam Rivers.
  7. All inflows contributed suspended sediment to the reservoir outflow. The North Santiam River was the largest contributor, followed by Breitenbush River, in calendar year 2003. The North Santiam River was unique in that it contributed sediment to the outflow in October and November, when contributions from other tributaries decreased. Tributaries that entered Detroit Lake closer to the dam were more likely to contribute suspended sediment that was exported to the North Santiam River downstream of the dam.

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