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<br />l <br /> <br />Within the study area, wastewater treatment plant (WWTPJ effluents did not <br /> <br />I\) <br />~ appear to increase downstream nutrient concentrations of the River by measurable <br />en <br />o increments. <br /> <br />Effects of fertilization practices or irrigation return flow on nutrient <br /> <br />concentrations in the River are extremely difficult to measure due to the non-point <br /> <br />nature of the problem and the large flows L~ the Colorado River. Plateau Creek, <br /> <br />a tributary of the Colorado at Cameo, carries measurable amounts of nutrients <br /> <br />into the River. The nutrient concentrations in Plateau Creek increased as tbe <br /> <br />stream flowed through areas where cattle were penned along side the water. It <br /> <br />is probable that cattle penned on the Colorado River banks also ccn:ribute <br /> <br />nutrients to the River's ecosystem. <br /> <br />Natural erosion also contributes nutrients to the Colorado River. As water <br /> <br />level, turbidity, and suspended solids increased during tr~ spring runoff, <br /> <br />nitrogen levels also increased. The nitrogen compounds, natural components of <br /> <br />the soil, were carried into the River with other allocthonous (sedimentary) <br /> <br />material. <br /> <br />Changes of the substrate brought about dram tic changes in t;.l-;e benthic <br /> <br />populations. Aquatic insects associated with rivers such as the Colorado require <br /> <br />fast currents, high dissolved oxygen, and low concentrations of settleable <br /> <br />material. TrJOugr~ut the study area where such conditions existed, a diverse, <br /> <br />abundant bent.~c population was established. Sedi.~ntation did eliminate many <br /> <br />insect groups along tbe eddies and in ~~e pool areas. During September to <br /> <br />November, 1973, the mayfly genus Tricorythodes sp. dominated in the areas where <br /> <br />-)- <br />