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Snake River-Hells Canyon Phosphorus TMDL – Fitting the Pieces Together

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The Snake River-Hells Canyon (SR-HC) nutrient total maximum daily load (TMDL) is a joint effort by the Idaho and Oregon Departments of Environmental Quality with substantial involvement from a large group of stakeholders. The TMDL area includes 221 miles of the Snake River from the Oregon-Idaho border to the confluence with the Salmon River. The area of focus for this project has an approximate 27,000 square mile drainage area, from the Oregon-Idaho border to the Brownlee Reservoir dam (upper 100 miles of the TMDL reach). Of the 100-mile study reach, approximately half is free flowing river and half consists of Brownlee Reservoir the first of three reservoirs created by high dams constructed for power generation in the 1950's. The dams are owned and operated by the Idaho Power Company (IPC) and IPC is currently in the process of relicensing the project through the Federal Energy Regulatory Commission (FERC). The focus area encompasses portions of western Idaho and eastern Oregon and includes five major tributaries with multiple agricultural drains, which discharge to the Snake River above Brownlee Reservoir. Flows within the Snake River system and tributaries are highly regulated for irrigation, flood control, power generation and recreation purposes.

Since construction in 1958, Brownlee Reservoir has experienced significant dissolved oxygen problems. Dissolved oxygen problems in the reservoir is a major factor triggering the TMDL process and water quality considerations in the FERC license renewal process. IPC published reports identifying the Boise River, located approximately 50 miles upstream of Brownlee Reservoir, as the most significant source of nutrients (phosphorus), and wastewater treatment plant (WWTP) discharges as controllable sources. Other sources of nutrients were also identified as potentially impacting Brownlee Reservoir water quality.

Boise City, Idaho, discharges wastewater from two wastewater treatment plants (WWTPs) into the Boise River, approximately 50 miles upstream of the Boise River and Snake River confluence. It was initially estimated that Boise City may need to significantly reduce their phosphorus discharge to meet Brownlee Reservoir DO targets with additional capital costs of 30 to 50 million and additional operation and maintenance costs of 3 to 5 million per year. Additionally, improvements would be required by other municipal and industrial point source discharges and substantial reductions from non-point sources, such as agricultural practices. Boise City, with the help of their consultant team lead by Brown and Caldwell, participated in the TMDL and FERC relicensing process by: (1) collecting data and conducting modeling and analysis to provide a firm scientific basis for decision-making; (2) helping frame the TMDL development and allocation process; (3) providing technical and regulatory assistance; and (4) working with other stakeholders. Boise City, and their consultant team, developed a strategy to address two key stages of the TMDL process: (1) target definition; and (2) allocation methodology.

To help define the scientific basis for the phosphorus and dissolved oxygen targets, and meet water quality standards, Boise City collected extensive data, performed water quality modeling for Snake River and Brownlee Reservoir and participated with other key stakeholders to conduct Use Refinement efforts to address temperature, aquatic life and dissolved oxygen. Specific technical areas of focus included:

Identification of seasonal and variable concentrations and loadings of nutrients and algae in the Snake River above Brownlee Reservoir.

Determining the limiting nutrient within the system – ultimately defined as phosphorus.

Determining the relative contribution of phosphorus from various sources to the Snake River and Brownlee Reservoir.

Evaluation of the impact of various nutrient reduction scenarios on loadings to Brownlee Reservoir and algae effects in the Snake River.

Providing information regarding loadings from the Snake River for linkage with evaluations of factors affecting water quality in Brownlee Reservoir.

Specifically, modeling efforts assisted in defining relationships between low reservoir dissolved oxygen and seasonal nutrient conditions. In addition, reservoir data were used to assess new means of defining dissolved oxygen conditions and standards multi-dimensionally, rather than as a more limiting single-dimensional standard.
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Document Type: Research Article

Publication date: 2002-01-01

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