Observation of water quality in the mixed reach below the confluence of the Sacramento and Feather Rivers, California, August and November 1975

The area near the confluence of the Sacramento and Feather Rivers was selected for study of water-quality changes that occur when two large rivers mix. Water-quality measurements were made during August 13-14 and November 4-5, 1975. Considering the concentrations of most of the properties and constituents measured, the mixing of the Sacramento and Feather Rivers generally occurs as simple dilution. Specific conductance and silica had consistently larger measured Nalues than theoretical values in the mixed reach. Some deviations from the theoretical value for any one characteristic in the mixed water are probably the result of physical forces such as upwelling near the confluence of the two rivers, or the concentrating effect of evaporation as the mixed water in the Sacramento River flows downstream. Other deviations in values of constituent concentrations or other properties may be the result of plant and animal activities in the water. Benthic invertebrates and phytoplanktonic similarity indexes between selected paired sites correspond to the quantity of water common to each site. Paired sites with the least volume of water in common had the lowest similarity indexes.