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fW1938 RESULTS AND DISCUSSION The results of various statistical analyses of the stream-quality data for the Yampa River basin are discuss'edin the following sections. The data also. are evaluated in terms of current Colorado water-qual ity standards. One measure of stream qual ity not included in the following discussion is trans- port of suspended and total sediment in streams of the basin. That aspect of the stream qual ity has been studied by Andrews (1977; 1978). Stream Temperature Typical seasonal stream-temperature patterns for the Yampa River near Maybell, Colo. (site Y-17), and the Little Snake River above Lily, Colo., are shown on figure 2. Similar seasonal variations persist from year to year. Because the seasonal pattern is cyclic and approximates a sine function, the annual increments of the periods of record at these two sites were analyzed using a harmonic-analysis procedure as proposed by ~/ard (1963) and ColI ings (1969). This procedure uses a least-squares regression to fit a sine func- tion of the form T(t)=M+A . [sin (bt+C)], where T(t)=stream temperature, in degrees Celsius, on day t; . M=harmonic-mean temperature, in degrees Celsius; A=ampl itude, in degrees Celsius; b=0.0172 radians per day=27ff365 days; and' C=phase angle, in radians.' . The procedure has been documented in a computer program ,by Steele (1974). The results of the harmonic analyses as appl ied to the daily stream- temperature data for the Yampa River,near Maybell (site Y-17)" the Little Snake River above Li ly, and the Little Snake RivernearLi ly (site Y-1) are given in table 2. Instantaneous daily stream temperatures generally are measured randomly during dayl ight hours. Duringtht;! warmer m9nths, stream temperatures undergo a diel fluctuation (fig. 3) such. that temperatures measured during dayl ight hours are biased toward the maximum values. Thus, harmonic coefficients determined from instantaneous daily stream temperatures might be expected to be somewhere between those determined from daily mean temperatures and those determined from daily maximum temperatures. A proce- dure for time-trend analysis used by Steele, Gi 1 roy, .and Hawkinson (1974) was appl ied to the stream-temperature characteristics in table 2. Using a signi- ficance level of 0,01, no changes in harmonic-mean temperature, ampl itude, or phase angle were revealed at either of the two sites for the period of record. 10 (1)