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Diatoms occurred frequently in the >505-µm size- fraction due to the screen on the collection bucket becoming partially plugged during sampling. Qual- itative examination of the materials showed com- position to be dominated volumetrically by filament- ous green algae (C/adophora sp.), and by large zooplankton such as adult Daphnia sp. (tables 26A through 33A). Macrophyte fragments from pond- weeds of the genus Potamogeton and the genus E/odea were often present and periodically abund- ant. (As noted earlier, the POM collected in this size- fraction was very small and sampling was later discontinued.) All particulate matter and POM data are reported in tables 42A through 50A. RESULTS - PHASE 2 Phase 2 primary objective was to determine the origin of the POM in the system. Phase 1 POM results were used to formulate phase 2 field studies. Figure 5 depicts the quantity, gross composition, and general distribution of particulate matter during phase 1. Obvious upstream todownstream increases were apparent in both POM and inorganic matter. Comparisons of tailwater stations at Davis Dam and Parker Dam, with the next downstream station at Park Moabi and Headgate Rock Dam, respectively, indicated that reservoirs were trapping inorganic matter. Increases in both organic and inorganic materials downstream from Headgate Rock Dam clearly indicate that additions were being made to the lower river system. Perennial tributaries do not exist which could input such materials between Parker Dam and Yuma. The physical features of the system which could possibly account for these additions were: • Backwater complexes along stream boundaries • Agricultural activities such as irrigation return flows through agricultural drains • Sediment sluicing at water diversion facilities • Streambank erosion along the main channel • Storm events which occurred during the actual sampling New sampling stations were added to isolate representatives of these features along the river system during phase 2. Figures 5 and 6 illustrate, during phases 1 and 2, particulate matter was trapped by the reservoirs. However, the upstream to downstream increases in particulate matter gbserved during phase 1 did not occur during phase 2. Percent POM and POM concentrations in the drift are shown on figures 7 and 8, respectively. These figures indicated that both the concentration and composition of particulate matter fluctuated greatly during phase 2. While all parameters measured in phase 1 were repeated in phase 2, only those parameters which appear relevant to POM are discussed. All data collected are presented in the appendix. Impoundments/Reservoirs The introduction in this study pointed outthat a major question was whether or not dams on the lower Colorado River block transport of organic material. Figure 6 shows that at Parker Dam (just below Lake Havasu) mean concentrations of both total and inorganic particulate matter were lower than the two stations above Lake Havasu-Park Moabi and Havasu Delta. Figure 7 infers that the reservoirs (lakes) release a higher percentage of POM than other stations in the river system. The corollary is 45 PHASE 1 40 ,, 35 E ° 30 25 W 3 zo ® INORGANIC MATTER ® POM DAVIS PARK PARKER READGATE PALO VERDE CII9OLA IMPERIAL -A DAM MOABI DAM NOCK DAM DIVERSION DAM DAM Figure 5.-Composition of dry weight during phase 1 at all sampling stations. 45 ao PHASE 2 35 INORGANIC MATTER E ® POM 30 r 25 w 20 } R _J 15 0 to DAVIS PARK NAVASU PARKER PALO CIBOLA PALO DRS IMPERIAL ALL I DAM MOABI DELTA DAM VERDE VERDE DAM AMERICAN DIVERSION IRRIGATION CANAL DAM DRAIN Figure 6.-Composition of dry weight during phase 2 at all sampling stations. 16