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diluted. Significant differences are not apparent in <br />phosphorus or nitrogen levels between Cibola and <br />DR3-above and below the Palo Verde Irrigation <br />Drain. <br />It appears that the Palo Verde Irrigation Drain <br />contributes POM to the river system. The POM at <br />DR3 was 1.87 g/m3-only slightly greater than the <br />above drain station (Cibola, 1.71 g/m3). It appears <br />that the Palo Verde Irrigation Drain had a continuous <br />low-level effect on conditions in the main channel. <br />Intuitively, this effect is greatest when river flow is <br />low and the Palo Verde Irrigation Drain discharge <br />volume is high. Conversely, as river flow increases, <br />contributions from the Palo Verde Irrigation Drain <br />are diluted and its effects are lessened. <br />Sediment Sluicing <br />Phase 1 results showed that the amount of <br />particulate matter in the drift greatly increased <br />between Imperial Dam and Yuma. Gross observa- <br />tions suggested that the major physical changes <br />occurring between these stations was the diversion <br />of water and the sluicing activities at the diversion <br />points. The average river flow (for 1988) at Imperial <br />Dam was 340 cubic meters per second but, <br />downstream at Laguna Dam the river flow averaged <br />only 66 cubic meters per second-less than 20 <br />percent of the upstream flow. At Imperial Dam, about <br />70 percent flow is diverted into the All-American <br />Canal on the California side of the river, and 10 <br />percent is diverted into the Gila Gravity Main Canal <br />on the Arizona side of the river. At water diversion <br />structures, sluicing operations is a common practice <br />to clear the headworks of silt, sand, and debris. <br />Generally, the practice involves closing the radial <br />gates to divert water into the canal; then one or more <br />of the gates (on the dam) nearest to the canal intake <br />is fully opened to allow a slug of water to flush the <br />intake area free of sediment and debris. The material <br />is passed downstream. Sluicing occurs on an <br />irregular basis but frequently during the peak <br />growing season when the demand for water is great. <br />At theAll-American Canal heading (at Imperial Dam), <br />a special desilting works was built into the structure <br />to remove sediment. From the upstream side of the <br />dam, water enters the canal and passes through <br />shallow, baffle-like channels which slow the water. <br />This allows heavier particles to settle. Each channel <br />can be closed off and its contents (water and <br />sediment) flushed-by way of a drain system-back <br />into a constructed channel below the dam. From <br />there, water flows into Laguna settling basin. The <br />settling basin is just that, a place for sluiced <br />sediments to settle and accumulate. Accumulated <br />material is periodically dredged from the basin onto <br />dry land. Since this structure operates most of the <br />time, sample stations were added (during phase 2) <br />to isolate the structure for determining how such <br />operations affected drift. One station was selected <br />in the canal proper, downstream of the desilting <br />works, and a second station was located in the <br />Laguna settling basin downstream of Imperial Dam. <br />At Imperial Dam the phase 1 station served as a <br />11 presluicing" station. <br />River water below the Imperial Dam was warmer, <br />more saline, less turbid, and generally more <br />productive (higher TP and chlorophyll a) than water <br />above the Imperial Dam (table 17). These changes <br />relate directly to reduced flow rates and desilting: <br />• Settling of large particles results in reduced <br />turbidity <br />• Warmer, slower water results in greater evap- <br />oration and concentration of dissolved materials (i.e., <br />yields higher specific conductance) <br />• Better light conditions because of lower turbidity <br />and slower water velocities result in high chlorophyll <br />a production <br />The bottom one-half of the table 17 presents <br />concentration of particulate matter and relative <br />composition regarding the proportions of fine and <br />coarse material. Total dry weight of particulate drift, <br />at both the canal and river below Imperial Dam, had <br />lower mean values than the amount measured above <br />the dam. The desilting works and settling basin <br />functioned as designed. The total amount of <br />Table 17.-Mean values for various parameters measured in the <br />lower Colorado River both above and below Imperial Dam <br />(Laguna) and within the All-American Canal. <br /> <br />Parameter Above <br />Imperial Dam <br />In-Canal <br />(AAC) Below <br />Imperal Dam <br />(Laguna) <br />Temperature, °C 20.1. 20.3 21.2 <br />Oxygen, mg/L 9.9 10.0 9.9 <br />Conductance, µS/cm 927 926 988 <br />Turbidity, NTU 21.1 12.3 18.9 <br />Total inorganic N, mg/L 0.316 0.374 0.316 <br />Total P, mg/L 0.011 0.011 0.013 <br />Chlorophyll a, mg/m3 2.64 2.41 3.99 <br />(maximum) (4.20) (3.30) (6.62) <br />Mean flow "(m3/s)(1988) 340 241 64 <br />Particulate Matter <br />Total dry weight, g/m3 26.28 19.64 15.56 <br />%>25 µm 38 47 30 <br />%<25 µm 62 53 70 <br />Total inorganic, g/m3 23.60 16.65 13.71 <br />%>25 µm 39 54 32 <br />%<25 µm 61 46 68 <br />Total organic, g/m3 (POM) 2.68 2.99 1.85 <br />%>25 µm 29 12 15 <br />%<25 µm 71 88 85 <br />35 cubic meters per second diverted into Gila Gravity Canal <br />and 0.3 cubic meter per second diverted to Mittry Lake inflow <br />channel. <br />22