Modeling Changes in Water Quality Resulting from Sediment Delta Interactions
* What implications do reservoir drawdowns and the subsequent changes in water quality resulting from sediment delta interactions have for water delivery and power generation?
Need and Benefit
The Bureau of Reclamation (Reclamation) currently monitors water quality at its reservoirs through testing and sampling by Reclamation or other organizations. Monitoring provides information about the physical, chemical, and biological characteristics of the water. Tools such as water quality models use data from water quality monitoring to simulate processes within reservoirs and the effects of reservoir operations on water quality. The need for these tools is growing due to the increasing demands on water supplies in the western U.S., eutrophication of reservoirs, and implementation of total maximum daily load (TMDL). The increased efforts and attention to water quality have also identified processes that are not well understood. One such issue, sediment delta interactions, is the subject of this project.
The interactions of sediment deltas, which may contain relatively high loads of organic materials, affect water quality in reservoirs. The major inflows to most reservoirs carry large sediment loads that settle out, forming sediment deltas. In addition, many reservoirs also experience cycles of drawdown and refilling. As the sediment deltas go through the cycles of deposition followed by drawdowns, resulting in the delta being exposed and reworked by inflows, and inundated organic and inorganic material being distributed through the reservoir basin, additional nutrients and metals becoming available, aquatic habitats changing, plankton growth escalating or being interrupted, and altering the overall water quality.
There is a lack of experience in simulating sediment delta interactions using existing models. The models either do not simulate the processes or they have not been sufficiently tested and calibrated to field data. Modeling tools need to be further tested and developed to simulate sediment delta interactions and its affect on reservoir water quality. Improving this capability will address problems concerning how sediment delta interactions affect municipal water delivery due to taste and odor issues, fisheries in reservoirs and tail waters, and harmful algal blooms.
Two examples of reservoirs in the Upper Colorado Region illustrate both the need and benefit to develop a greater understanding of the dominant processes occurring in sediment delta interactions with modeling tools:
1. Lake Powell is a large reservoir on the Colorado River in Arizona and Utah. From the years 2000 to 2004, the Colorado River Basin went through a severe drought. This caused the pool elevation of Lake Powell to drop nearly 150 feet and reduced the volume to less than half of its capacity. Snowmelt runoff in 2005 was just above average, but was double that of any of the previous five years. This large inflow cut through exposed sediment deltas, some of which were more than 100 feet thick. This caused an increase in oxygen demand and a subsequent metalimnetic minimum of dissolved oxygen. By the fall of 2005, the metalimnetic minimum reached the dam and was discharged through the powerplant to the river below. Concerns for the trout fishery caused operations at the dam to be altered, resulting in lost power revenue and possible cavitation damage to turbines.
2. Deer Creek Reservoir is a medium-sized reservoir on the Provo River near Salt Lake City, Utah. It is primarily used to supply municipal water to the largest communities along the Wasatch Front. In 2007, the reservoir was drawn down in preparation for Safety of Dams work on the spillway. In the late summer, taste and odor problems became a large issue for the water district and treatment plants. The problems were believed to be related to mobilization of the exposed sediment delta. Operations of the reservoir upstream were modified to alleviate the taste and odor issues from water diverted from Deer Creek discharges.
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