Scientific Investigations Report 2007–5008
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5008
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Significant Findings
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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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U.S.
Department of the Interior | U.S. Geological
Survey
