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3. WATER CHEMISTRY <br />3.1 METHODS <br />Water quality parameters measured included discharge, <br />temperature, Water clarity, conductivity, major ion and nutrient <br />(nitrogen, phosphorus, and silica) concentrations, pH, and <br />dissolved oxygen levels. Mainstream discharge measurements were <br />taken from the USGS continuous stage recorder at Lee's Ferry, or <br />were estimated from the BOR Stream Simulation and Reservoir <br />Regulation (SSARR) model (See Graf et al. 1986). Tributary <br />discharges were calculated from depth-velocity profiles made with <br />a meter stick and Marsh-McBirney Model 201D current meter. Water <br />temperature, conductivity, pH, and dissolved oxygen were measured <br />in the field with a multiple-probe Hydrolab Model 9000 Surveyor <br />II meter. Major ions and nutrients were analyzed at the BOR <br />water quality laboratory in Salt Lake City, Utah. Instruments <br />utilized for these analyses are given in Table 3.1. Appendix 3.1 <br />provides a monthly schedule of locations where discharge, <br />Hydrolab, major ion, and nutrient samples were taken. Because <br />too many samples did not have coincident discharge and <br />conductivity or ionic concentration measurements, values <br />appearing in this report are not flow-weighted. <br />3.2 RESULTS <br />Discharge <br />Instantaneous discharge of the Colorado River at Lee's Ferry <br />varied between 1,160 cfs and 56,600 cfs during the period April <br />1984-May 1986 (USGS, Water Resources Data for Arizona, <br />provisional data). Mean daily flows ranged from 3,487 cfs to <br />51,570 cfs, and the mean, median, and mode of mean daily flows <br />were 26,233 cfs, 24,988 cfs and 25,000 cfs, respectively. The <br />cumulative frequency hydrograph of mean daily flows for this <br />period is roughly sigmoidal in shape with a lower plateau <br />beginning near 14,000 cfs and an upper plateau at around 31,500 <br />cfs (Figure 3.1). A secondary mode occurs at 42,000 cfs. <br />The cumulative frequency polygon o daily flow coefficient <br />of variation (CV-SD/x, half-hour i ervals) during the same <br />period rises precipitously in the ange of 0% to 3% at which <br />point slightly more than 50% the observations have been <br />included (Figure 3.1). Approximately 80% of all observations lie <br />-18-