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Chapter III Affected Environment and Environmental Consequences 30 of the sediment delta, dissolved oxygen is quickly depleted and the sediment chemistry changes. Without dissolved oxygen in the interstitial water, bacteria utilize other oxygen sources such as mineralized forms associated with nitrites and nitrates (NO~-N03) and iron oxides. These biochemical interactions result in changes in mineral forms such as iron oxide conversion to iron hydroxide. Since iron hydroxides do not have the same ability as iron oxides to hold (adsorb) phosphorus, it is released into the interstitial water. When the reservoir is filling, the sediment delta forms as the sediment settles out. Then, when the reservoir is drawn down, the river cuts a new channel through sediments of the delta. Thousands of tons of sediment can be moved during this period, releasing dissolved phosphorus into the water column. The iron hydroxide is also reintroduced back into an oxygenated water column and with time is converted back to iron oxide. This iron oxide can readsorb phosphorus again and settle back to a new location further downstream and usually deeper in the water column. This process is not immediate and some dissolved phosphorus escapes into the water column and becomes biologically available. Therefore, drawdown can increase the primary productivity in the reservoir. In Lake Powell, some increase in bioavailability of nutrients may occur in as the inflow density currents matched up with the depth of withdrawal and are drawn across the reservoir. This could spread inflow nutrients over a larger area of Lake Powell during the summer months. However, only a small increase in nutrient availability is expected and improved primary productivity is expected to be relatively minor. Even a small increase would be beneficial to this relatively nutrient-poor system. In Lake Mead, the bioavailability of nutrients in the inflow zone are also greatly influenced by inflow temperature. Summer inflows to Lake Mead do not presently warm up to equilibrium temperatures before entering the lake. The inflow to Lake Mead is colder (denser) than the warm surface layer of the lake and quickly plunge below the surface. Nutrients in the inflow are not readily physically available once they plunge below the euphotic zone where photosynthesis can occur. Turbidity also limits photosynthesis in the inflow. Water quality data taken by Reclamation indicates that in August 1996, the inflow plunged to a depth ofabout 50-70 feet (15-22 meters). The proposed action would increase release temperatures by about +50C from Glen Canyon Dam and this would increase inflow temperatures to Lake Mead. This temperature increase would cause the inflow to stay closer to the surface layer of Lake Mead and would increase the water travel time within the lighted portion of the reservoir (called the euphotic zone). This would increase the physical bioavailability of nutrients in the inflow area of Lake Mead until the phosphorus settles out of the euphotic zone. A slight increase in algae productivity may occur in the inflow zone of Lake Mead. Because the system is so non-productive overall, any increases in algae growth would be beneficial to the rest of the food chain, including the fishes. In the river below Glen Canyon Dam, the proposed action would increase nutrient, algae, detritus released into the tailwater. The results of Reclamation's reservoir modeling studies are shown in figure 10. Releases from the dam due to surface withdrawals would increase detritus concentrations in the river by about 300 percent. Similar increases can also be seen in both phosphorus and nitrogen (ammonia) levels. These increases in the food base of the