Gunnison River
Elevated selenium in the Gunnison River Basin reflects the underlying geology. The Gunnison River Basin sits atop the Mancos Shale which is naturally elevated in selenium. The seleniferous soil the shale produces have come to the subsurface due to weathering of the rock and uplift. Agricultural irrigation of seleniferous soils is one mechanism of selenium loading to the Gunnison River. As farmland in the Gunnison River Basin is abundant, the amount of selenium reaches well over the normal concentration in areas from Grand Junction to the tributaries in the upper Gunnison River. Some of the most concentrated tributaries of the Gunnison, namely the Uncompahgre, load large amounts of selenium to the Gunnison River. Selenium concentrations in the River can be as high as 11 ppm during peak times. Agricultural areas in the Gunnison River Basin are heavily farmed and provide large amounts of sediment during the growing months. Concentrations of selenium in the Gunnison River rise in April until they peak in August due to irrigation drainage. These peaks in selenium concentrations coincide with the reproduction events of the Colorado pikeminnow and the razorback sucker. High amounts of selenium are deposited into the yolk by the mother and juvenile fish eat algae that is highly concentrated with selenium in August. Effects of high selenium concentrations in fish include reduced oxygenation rates and lowered total energy capacity used for movement and reproduction, with fitness consequences for migratory fish such as the Colorado pikeminnow. Elevated selenium exposure can also cause malformations in adult fish.