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and the first 80 kilometers of tailwaters as con- <br />sumers. This must be considered an important <br />reservoir function during late spring and early <br />summer when macroinvertebrate production in the <br />tailwater is high. While such production is not always <br />welcome, it is almost always noticed in these areas <br />(i.e., swarming midges, caddis flies, and black flies <br />below Davis and Parker Dams). <br />Composition of POM from reservoirs was unique. <br />Peaks occurred in the turbidity levels below the <br />reservoirs as a result of increased plankton in the <br />drift. For example (Feb. 1987), an 8.2 NTU reading <br />was recorded below Davis Dam where readings <br />generally were below 2.0 NTU as noted on table 9A. <br />During nine of the ten paired comparisons of <br />chlorophyll between Davis Dam and Parker Dam, and <br />their respective downstream stations, the stations <br />below the reservoirs had higher chlorophyll a <br />concentrations (table 16A). Below Davis Dam, 121 <br />phytoplankton species were collected, which <br />presumably originated, from the reservoir, and 152 <br />species were collected below Parker Dam (table <br />17A). In both cases this was less than the number <br />of species collected at Havasu Delta. Below Davis <br />Dam, the majority of species were in the <25-pm <br />size-fraction, whereas below Parker Dam the <br />majority of species were in the>25-µm size-fraction. <br />This may be the result of differing withdrawal zones <br />from the two reservoirs. Davis Dam is a hypolimnetic <br />release structure and Parker Dam is an epilimnetic <br />withdrawal. <br />The upstream reservoirs had a direct influence on <br />the zooplankton species collected. Zooplankton were <br />more abundant in the tailwaters below the major <br />reservoirs than at downstream stations. Below Davis <br />Dam, 21 zooplankton taxa were collected and 16 <br />zooplankton taxa were collected from Parker Dam. <br />Nauplii and copepodites were collected frequently <br />below Davis Dam and Parker Dam-the result from <br />reservoir flows. These immature forms were seldom <br />collected at downstream stations. Adult calanoids <br />and cyclopoids were often present and became <br />abundant in May 1988 at Davis Dam and Parker <br />Dam, although concentrations diminished down- <br />stream. The number of zooplankton taxa decreased <br />downstream. <br />Collectively, these data indicate lower Colorado River <br />reservoirs do trap inorganic particulate matter. Also, <br />the data indicate that POM is transformed as it <br />passes through the reservoirs-resulting in little net <br />difference in volume but considerable difference in <br />composition. While the total contribution of POM to <br />the river system is small, the autochthonous <br />production of the reservoirs (primarily plankton) is <br />seasonally important. This POM is contributed at a <br />most important time of the year and in quite a usable <br />form (without abrasive and obtrusive inorganic <br />materials). Late spring and early summer releases <br />of POM may be important energy subsidies to <br />organisms living in the tailwater of reservoirs. <br />Backwaters <br />The lower Colorado River is divided into ten <br />operational divisions; all divisions have backwaters <br />of one type or another. Between Davis Dam and <br />Imperial Dam more than 500 backwaters exist with <br />at least one surface acre of open water (Holden et <br />al., 1986 [21]). Stations sampled during phase 2 did <br />not isolate backwaters, but they did isolate the <br />Imperial and Topock Gorge Divisions. Imperial and <br />Topock Gorge Divisions have the greatest density <br />of backwaters-156 and 80 backwaters, respec- <br />tively-many of them occur in large complexes with <br />multiples of three or more backwaters connected <br />together by small channels or separated only by <br />fringes of emergent vegetation. In these reaches, <br />the backwaters fill side canyons and areas between <br />the main channel and high ground with marshes, <br />small lakes and ponds, and similar wetland habitats. <br />The main river channel is well defined as it weaves <br />through these complexes, its boundaries comprise <br />natural levees of sand and emergent vegetation. At <br />high river stage, water runs freely into and through <br />the backwater complexes, while at low water levels <br />many backwaters either become isolated ponds or <br />drain back into the main channel. <br />Sampling stations at Park Moabi (near Needles, <br />Calif.) and at Havasu Delta isolate the Topock Gorge <br />Division. The sampling station below the Palo Verde <br />Irrigation Drain at DR3 is located at Adobe Ruins <br />and marks the beginning of Imperial Division; <br />Imperial Dam is the lower boundary for the division. <br />Because of nonexistant tributaries and lack of <br />agricultural returns, within these divisions, stream <br />bank erosion along the main channel is less <br />pronounced than in the other divisions (Bureau of <br />Reclamation, 1976 [22]). Differences in POM and <br />other variables between samples from above and <br />below the divisions are believed to reflect effects <br />of the backwaters within these divisions. <br />Figure 11 shows concentrations of inorganic matter <br />above and below the backwater complexes. Gener- <br />ally, these data are unremarkable; it appears that <br />the major differences occurred between river <br />reaches (i.e., Imperial Division versus Topock Gorge <br />Division) and not across the reaches (above versus <br />below backwater complexes). <br />Figure 12 depicts concentrations of POM above and <br />below the backwaters. These data also are unre- <br />markable; they tend to infer that a greater difference <br />exists between divisions rather than within divisions. <br />During January and May, POM was less below the <br />18