Detroit Lake Temperature and Suspended Sediment Model

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The USGS has worked since 1998 to monitor and study sediment and turbidity throughout the North Santiam River watershed. As part of that assessment, a focused effort was undertaken to examine the effect that Detroit Lake has on temperature issues and sediment transport. In particular, developing a model that simulates the transport and fate of suspended sediment and the dynamics of water temperature in Detroit Lake was deemed to provide an important component of understanding how the lake affects suspended sediment and temperature in the North Santiam and Santiam Rivers downstream.

The objectives of this study were to:

  1. Develop a model of Detroit Lake to simulate circulation, water temperature, total dissolved solids, and suspended sediment in both the reservoir and its outflow, 
     
  2. Understand processes affecting suspended sediment, quantify sediment sources and transport to the lake outlet, and quantify sediment deposition in the lake, and 
     
  3. Understand processes controlling water temperature in Detroit Lake and its outflow, and demonstrate the water temperature effects of a hypothetical selective withdrawal device.

To meet these objectives, USGS personnel constructed, calibrated, and tested a model of circulation, water temperature, and suspended sediment in Detroit Lake. The model was constructed to simulate conditions that occurred in the entire calendar years of 2002 and 2003, as well as the period December 1, 2005, through February 1, 2006, in order to simulate some large winter storm events. During January 2006, about 70 centimeters (27.6 inches) of precipitation were recorded at Detroit Dam, making it the wettest January ever recorded and breaking the previous record set in 1970. Processes occurring in Big Cliff reservoir, the reregulating reservoir below Detroit Lake, were not included in this model.

After the model was constructed, calibrated, and tested, it was used to: (a) examine the sources of sediment to Detroit Lake and the lake's ability to trap those sediments, (b) estimate the amount and pattern of sediment deposition, and (c) simulate the in-lake and downstream effects of adding a hypothetical selective withdrawal to Detroit Dam to control release temperatures.

  1. An annual pattern of water temperature exists in Detroit Lake that was similar in all time periods modeled. The reservoir typically begins the year isothermal and cold. In spring, the lake surface warms and a thermocline develops by summer, isolating cold, dense water at the reservoir bottom. In fall, 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 fall, 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, the outflow from Detroit Lake could remain below 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 fall 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 the fall, 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. 
     
  6. 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 2005-2006 storm events. 
     
  7. 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 2005-2006 storms. Most of the mass of sediment leaving Detroit Lake was composed of clay-sized particles. 
     
  8. Assuming a bulk density of 1.89 g/cm3 (grams per cubic centimeter), 14,300 m3 (cubic meters) (11.6 acre-feet) of sediment was deposited in Detroit Lake in 2002, 11,820 m3 (9.6 acre-feet) in 2003 and 34,900 m3 (28.3 acre-feet) 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-feet). The model results indicate that most sediment deposition occurred in the upper reaches of the reservoir, near the inflows of Breitenbush River and the North Santiam River. 
     
  9. 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 situated that fed directly to the lake closer to the dam were more likely to contribute suspended sediment that was exported to the North Santiam River downstream of the dam.