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 displayhigh or excessive levels of algal production. Phosphorus was the limitingnutrient during most of the project period. Average water quality valuesshowed Lake Eucha and Spavinaw Lake to be eutrophic. Examination of eachseparate year showed a significant increase of algae growth for bothlakes between 1998 and 1999. The increased algae growth was concurrentwith an increase of phosphorus load. During the two-year study period,there were significant taste and odor events. There was a relationshipbetween particular phytoplankton species present and taste and odor eventsin both years. The presence of specific diatoms and blue-green algaespecies know to produce undesirable taste and odors were associated withthe taste and odor events. After evaluating the currently available managementtechniques in light of the limnological and modeling analyses of thelakes, reduction of the amount of phosphorus coming from the watershedwas recommended.
Detailed lake modeling showed that a 45 percent phosphorus loadreduction to Spavinaw Lake is needed to reduce the current TSI of approximately57 to a TSI of 50. Nutrient budgeting showed 85% of Spavinaw Lake phosphorusto be from Lake Eucha. In order for the phosphorus load to Spavinaw Laketo be reduced by 45 percent, the phosphorus load to Lake Eucha wouldhave to be reduced by 70 percent, independent of any other measures.Implementing these reductions will require a management plan targetingphosphorus. Specific components of the plan should include:
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