Maximum Weekly Average Temperature for 316(a) Demonstrations

In determining the effects of thermal discharges on certain species of fish, the concept of Maximum Weekly Average Temperature has received much attention in 316(a) studies. Based on laboratory data, the Maximum Weekly Average Temperatures for growth, reproduction, and winter survival have been determined for eggs, larvae, and adults for a number of fish species. How these criteria should be applied to be receiving stream temperatures to best represent the thermal conditions to which fish are subjected has not been fully considered. Several approaches to calculating the Maximum Weekly Average Temperatures for receiving water bodies are discussed. Examples include a more simplistic approach of using averages of daily mean temperatures of consecutive 7-day increments for each year of the data set; 7-day running averages of daily mean temperatures for the entire data set; or averages based on cumulative distributions of daily average temperatures. These methods and results are compared to determine which of these methods may be the more reasonable and representative approaches. The running average and the weekly average methods (Methods A and C) both yield acceptable and preferred values of Receiving Water Body Maximum Weekly Average Temperatures.