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ze <br />factors affect the toxicity of environmental contaminants to <br />biological organisms. Chemical and physical factors include <br />contaminant type, chemical species or form, Ph, water temperature, <br />dissolved oxygen, hardness, salinity, and multiple-chemical exposure <br />(antagonism and synergism). Duration of exposure, quantity of <br />contaminant, and exposure pathways from the environment to the <br />organism also affect toxicity. Some trace elements are beneficial to <br />organisms at low concentrations but may be toxic at higher <br />concentrations. Biological and physiological factors affecting <br />toxicity include species, age, sex, and health of the organism. <br />Selenium concentrations can be elevated in areas where irrigation <br />occurs on soils which are derived from or which overlie Upper <br />Cretaceous marine sediments. Percolation of irrigation water through <br />these soils and sediments leaches selenium into receiving waters. <br />Other sources of selenium include powerplant fly ash and oil <br />refineries. Water depletions, by reducing dilution effects, have <br />increased the concentrations of selenium and other contaminants. <br />Recent United States Geological Survey data collected in the Colorado <br />River from 1990 to 1996 show that selenium concentrations under <br />existing conditions (current level of development, not section 7 <br />baseline) have been as high as 2 µg/L in the 15-mile reach and as high <br />as 6 µg/L below the 15-mile reach. In 1995, Colorado's Water Quality <br />Control Commission reduced the chronic selenium standard from 17 µg/L <br />to 5 µg/L. The Service recommended the level be lowered to 2 µg/L. <br />Phya; a7 Hahi a <br />Water depletions, by affecting the quantity and timing of flows, have <br />reduced the ability of the river to create and maintain habitats and <br />have reduced the frequency and duration of availability of certain <br />habitats. <br />Habitat Formation <br />The formation of a variety of channel habitats, including <br />gravel/cobble bars and substrates used by Colorado for spawning, is <br />essential to ensure the availability of the range of habitats required <br />by all endangered fish life stages to fulfill daily requirements <br />(foraging, resting, spawning, avoiding predation, etc.) under various <br />flow conditions. The number and distribution of these channel <br />habitats can be described as channel habitat complexity, diversity, or <br />heterogeneity. Osmundson and Kaeding (1991) found that adult Colorado <br />in the Grand Valley prefer river segments with a complex morphometry <br />over those that are simple. <br />Some important habitats, such as inundated floodplain depressions used <br />by razorback suckers for spawning, are located outside the channel. <br />Floodplain depressions are principally derived from abandoned main <br />channels, side-channels, backwaters, and meander cutoffs. • <br />