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analysis were collected in 1988. Water sample analyses were conducted in the Chemistry, Petrography, and Chemical Engineering laboratory of the Bureau of Reclamation, or by contract, using methods in Standard Methods for the Examination of Water and Wastewater (APHA 1985) for major ions, phosphomolybdate method for total phosphorus and orthophosphate, the automated phenate method for ammonia, colorimetric method with sulfanilamide for nitrate and nitrite with hydrazine reduction used to convert nitrate to nitrite and flame and electrothermal atomic absorption spectroscopy for heavy metals. Turbidity, light extinction coefficient and Secchi depth. - Turbidity was measured using a Hach Chemical Co. Model 16800 Turbidimeter, with backwater samples collected prior to any other sampling activity. The calibrated turbidimeter provided a direct readout of turbidity in NTUs. light extinction coefficient is an index or measure of photic depth. light penetration in both the main river and backwaters was calculated from readings obtained from a Kahl Scientific Instrument Corp. Model 268WA310 underwater irradiameter (limnophotometer). A reading was obtained in air, just beneath the water surface (0.01 m), then at 0.1 m depth increments to the bottom or to 1.0 m. When clouds were present, corresponding air readings (ambient) from a deck cell were recorded to adjust underwater readings for changing solar radiation. Secchi depth was also measured at each main river and backwater site. Chlorophyll i. - Water samples were collected and filtered on-site for subsequent laboratory chlorophyll i determination. Replicate 250 to 500 mL water samples from each site were vacuum-filtered through separate 0.45 ~m pore-size fiberglass filters in the field; the folded filters were placed in separate labeled envelopes and placed on dry ice for transport to the 20