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Backwater marshes occur within every river division downstream of Headgate Rock Dam, but the data collected (during this study) did not show a consistent increase in POM due to the presence of backwaters except during high flows. Backwaters along the lower Colorado River are both lentic and lotic- depending on the river stage. During high flows, backwater marshes are more lotic and are flushed, which add materials to the river. At moderate to low river stage, many backwaters become pothole lakes. Minckley (1979 [101) reported that large backwaters associated with the river channel range to more than 4.0 meters in depth and retain water even when the mainstream is almost dewatered under low flow conditions. It is possible, during moderate to low flow, the backwater complexes produce and store POM which then is contributed to the river during high flow. Daily flow fluctuations and proximity to Parker or Davis Dams are both important variables affecting how backwaters function in the river system. As pointed out by Minckley (1979 [10]) some backwaters close to these dams are flushed almost on a daily basis, while the furthermost have little direct exchange with the main channel. In Imperial Division, backwater complexes are separated from the main channel by dense stands of emergent vegetation. This vegetation may be contributing coarse POM for processing both within the main channel and within backwaters of the division. Undoubtedly, substantial production of plant mate- rial occurs within the backwater habitats and some of this production leaves the backwaters as POM; it can be readily observed each year and was observed during this study. A more complete isolation of one or more backwater, with frequent sampling, would be needed before the true role of backwater effect upon POM distribution can be completely understood. Agricultural production is continuous along the lower Colorado River; for crop irrigation water diversions operate year long. The agricultural drains return some water back to the channel; downstream from Parker Dam these drains are almost perennial tributaries. For example: • The average flow through Parker Dam (1988) was 295 cubic meters per second • The average daily irrigation diversion for Head- gate Rock and Palo Verde Diversion Dams (combined) was 59 cubic meters per second. • The agricultural drains below Parker Dam returned a daily average of 32 cubic meters per second (about 50%) of the diversion water back to the river. Palo Verde Irrigation Drain is the largest agricultural drain in the lower Colorado River. Physical, chemical, and biological conditions in the drain were different than those in the main channel. Mean POM concentration was greater than at any other station during this study (4.75 g/m3); it is not surprising that the drain contributed a substantial POM load to the river (3.5 million kilograms per year). The actual effects of the Palo Verde Irrigation Drain were diluted by the sheer volume of water in the main channel. The Palo Verde Irrigation Drain alone could not account for the increased POM in the river downstream of Parker Dam; other drains were not sampled that entered the river. Comparing aquatic habitats within the Parker Division of the lower Colorado River, Hiebert and Grabowski (1987 [50]) found the Poston Drain to be considerably different from the main channel and very productive for macroinvertebrates. They state the drain provides a nutrient source to the river, as well as a thermal refugium for fishes during winter. The Poston Drain joins the main channel in the middle of the Parker Division and had an average discharge of 4 cubic meters per second for 1988. POM showed an average gain of 32 percent from Headgate Rock Dam to Palo Verde Diversion Dam. The Poston Drain probably would not account for all this POM, but in combination with local backwaters and in-channel production it would appear to be important. The largest single increase in POM between any two stations occurred within the Palo Verde Division between Palo Verde Diversion Dam and Cibola. A 90-percent increase occurred for phase 1 and a 101-percent increase for phase 2 between these stations. Two agricultural drains enter the main river just downstream of Palo Verde Diversion Dam; they have a combined average flow of 10.2 cubic meters per second. The drains could contribute POM to this river reach. This division also has numerous backwater habitats. Data from Palo Verde Irrigation Drain indicate that agricultural drains along the lower Colorado River provide a constant source of POM. The othef potential source of POM is from the streambed and streambanks. Wallace et al. (1977 [511) reported that POM is returned to river waters from streambed sediments by increased flows. Maximum POM concentrations are often associated with storm events, such as flash floods (Fisher and Minckley, 1978 [521). During this study, a summer storm increased the POM. The lack of correlation between chlorophyll a and POM suggests that the majority of the POM was non-chlorophyll bearing detritus. Both shifting streambed sediments and stream bank erosion can contribute detritus (Bilby and Likens, 1979 [531). In the lower Colorado River, daily and seasonal fluctuation of flows cause stream bank erosion and shifts in streambed sediment. Even though upstream dams have decreased the amount of sediment moving through the river, Burke (1986 [11 ]) noted that 2 million metric tons of sediment 26