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August 1988 rainstorm did backwaters play a major role. In general, backwaters contributed little to POM except when flows increased during a storm and the river flooded backwater complexes. Backwaters function as filters during high flows while trapping coarse material and releasing fine POM back into the river system. 7. Agricultural drains yielded constant low-level contributions of both POM and nutrients into the main river channel. The contribution emanating from drains to the river was diluted due to lower flows than in the main channel. • In Palo Verde Irrigation Drain, mean N03-N was 0.614 mg/L. • In the main channel above the drain at Cibola and below the drain at DR3, mean N03-N was 0.399 and 0.450 mg/L, respectively. • In Palo Verde Irrigation Drain mean PO4-P was 0.021 mg/L. • At all other stations, mean P04-P ranged from not detected-at listed detection limit-to 0.006 mg/L, which indicated phosphorus-limited conditions. POM concentrations were greater in Palo Verde Irrigation Drain than at either Cibola or DR3 (above or below the Palo Verde Irrigation Drain); mean value of POM in the drain was more than twice the mean for the two river stations. 8. In August 1988, an increase in POM concen- trations at stations from Cibola to Yuma occurred because of a storm event. The <25-µm size-fraction POM was greater following the storm than before the storm. The <25-um size-fraction POM increased to 5.26 g/m3 at Imperial Dam following the storm; it was the highest value recorded during the entire sampling period. 9. Largest single increase in POM occurred from Palo Verde Diversion Dam to Cibola. Between these stations, a 90-percent increase occurred during phase 1 and a 101-percent increase occurred during phase 2. 10. At Imperial Dam, the stirring action of the sluicing operation increased the <25-µm size- fraction POM concentration in both the All-American Canal water and in the river channel at Laguna Dam. 28