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17 <br />dissolved ox}'gen, hardness, salinity, and multiple-chemical exposure (antagonism and <br />synergism). Duration o: exposure, quantity of contaminant, and exposure pathways from the <br />environment to the orsanism also affect toxicin•. Some trace elements aze beneficial to <br />organisms at low concentrations but may be toxic at higher concentrations. Biological and <br />physiological factors affecting toxicity include species, age; sex, and health of the organism. <br />Selenium concentrations can be elevated in azeas where irrigation occurs on soils which are <br />derived from or which overlie Upper Cretaceous marine sedimenu. Percolation of irrigation <br />water through these soils and sediments leaches selenium into receiving waters. Other sources of <br />selenium include powerplant fly ash and oil refineries. Water depletions, by reducing dilution <br />effects, have increased the concentrations of selenium and other contaminants. In 1995, <br />Colorado's Water Quality Control Commission reduced the chronic selenium standazd from <br />17 µg2 to 5µg2. The Service recommended the level be lowered to 2µg2. <br />Physical Habitat <br />Water depletions, by affecting the quantity and timing of flows, have reduced the ability of the <br />river to create and maintain habitats and have reduced the frequency and duration of availability <br />of certain habitats. <br />Habitat Formation <br />The formation of a variety of channel habitats, including graveUcobble bazs and substrates used <br />by Colorado pikeminnow for spawning, is essential to ensure the availability of the range of <br />habitats required by all endangered fish life stages to fulfill daily requirements (foraging, resting, <br />spawning, avoiding predation, etc.) under various flow conditions. The number and distribution <br />of these channel habitats can be described as channel habitat complexity, diversity, or <br />heterogeneity. Osmundson and Kaeding (1991) found that adult Colorado pikeminnow in the <br />Grand Valley prefer river segments with a complex morphometry over those that aze simple. <br />Some important habitats, such as inundated floodplain depressions used by razorback suckers for <br />spawning, are located outside the channel. Floodplain depressions aze principally derived from <br />abandoned main channels, side-channels, backwaters, and meander cutoffs. <br />The creation of complex channel habitat and the formation and eventual abandonment of channel <br />features from which floodplain depressions aze formed occur primarily during spring runoff <br />when flows aze of sufficient size and duration to cause major changes in channel morphology <br />through significant erosion and deposition of bed and bank materials. The reduction in the <br />magni^.rde, duration, and frequency of high sprir.~ flows has slowed the rate at which channel <br />morpl•:ology changes. Consequently, the creation of complex channel habitat and floodplain <br />depressions has slowed. The placement of riprap and other bank stabilization measures and the <br />construction of dikes and levees impede changes in channel morphology and contribute to the <br />slowed creation of complex channel habitat. In addition, the construction of dikes and levees <br />