The Eucha/Spavinaw watershed is a 415 square mile drainage basin in Mayes and Delaware County Oklahoma (70%) and Benton County Arkansas (30%). Eucha Lake and Lake Spavinaw collect and store the water from Spavinaw Creek (the main drainage channel for the basin) and other sources, to supply the Tulsa Metropolitan area and other local water users. The system was studied from April 1998 to March 2000. Samples were collected regularly from Lake Eucha, Spavinaw Lake, and Lake Yahola (the immediate storage lake for the City of Tulsa Mohawk Water Treatment Plant). Over 800 lake samples and 450 tributary samples were used for evaluative purposes. Chemical analyses and water quality sampling followed the procedures and protocols detailed in the Quality Assurance Project Plan. Tributary stormwater runoff and baseflow were estimated from stream water quality and basin land use analyses developed by Oklahoma State University. Comprehensive limnological analyses were used to assess the current trophic status of the lakes. Lake water quality modeling was employed to investigate the effect of changed lake loadings on the trophic status and to estimate acceptable phosphorus loads to the lakes. Potential lake management options were evaluated and specific options were recommended to restore the lakes to acceptable water quality.

Both Lake Eucha and Spavinaw Lake are nutrient-enriched and display high or excessive levels of algal production. Phosphorus was the limiting nutrient during most of the project period. Average water quality values showed Lake Eucha and Spavinaw Lake to be eutrophic. Examination of each separate year showed a significant increase of algae growth for both lakes between 1998 and 1999. The increased algae growth was concurrent with an increase of phosphorus load. During the two-year study period, there were significant taste and odor events. There was a relationship between particular phytoplankton species present and taste and odor events in both years. The presence of specific diatoms and blue-green algae species know to produce undesirable taste and odors were associated with the taste and odor events. After evaluating the currently available management techniques in light of the limnological and modeling analyses of the lakes, reduction of the amount of phosphorus coming from the watershed was recommended.

Detailed lake modeling showed that a 45 percent phosphorus load reduction to Spavinaw Lake is needed to reduce the current TSI of approximately 57 to a TSI of 50. Nutrient budgeting showed 85% of Spavinaw Lake phosphorus to be from Lake Eucha. In order for the phosphorus load to Spavinaw Lake to be reduced by 45 percent, the phosphorus load to Lake Eucha would have to be reduced by 70 percent, independent of any other measures. Implementing these reductions will require a management plan targeting phosphorus. Specific components of the plan should include:

• Curbing phosphorus loads entering the Eucha/Spavinaw lake system from the lake basin will involve developing and implementing a watershed-wide management plan that uses the water quality of Lakes Eucha and Spavinaw as the ultimate measure of success:
• Involve all the stakeholders in the basin (including the Eucha/Spavinaw Watershed Management Team [Tulsawater.com] and others) in creating and implementing a comprehensive and fair management program.
• Identify the amount of and source or sources (non-point and point) of phosphorus coming from each sub-basin and identify the changes that will most cost-effectively achieve the reduction goal.
• Identify and implement the most cost-effective technologies and management tools to reduce the flow of phosphorus from the watershed.
• Identify and obtain the resources necessary to apply those methods in a sufficient fashion to meet the phosphorus loading goals.