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concentrations may also result from the introduction of ground water to the main stem of the river along its course. Irrigated agriculture is known to contribute selenium to the river through three potential avenues: 1) concentration of selenium in the irrigation water by evapotranspiration, 2) selenium pickup from the soils that are irrigated, and 3) selenium pickup in the shale beds underlying the irrigated areas. Development of the oil and gas resources of the Basin, as well as other factors such as urban runoff, domestic and industrial sewage effluents, and spillage of petroleum and petroleum products, have contributed to the presence of polycyclic aromatic hydrocarbons in the biota of the San Juan River. Sampling of fish species from the San Juan River in 1990 and 1991 suggested that aquatic organisms are being exposed to high levels of hydrocarbons such as naphthalene, benzo(a)pyrene, and phenanthrene. Studies funded by the Bureau of Land Management to monitor concentrations of hydrocarbons in the San Juan River since 1991 and studies of fish tissues taken from Colorado pikeminnow and razorback sucker captured in the river during the 1990s as part of the Program's research activities indicate that selenium and hydrocarbon concentrations in the San Juan River currently may not be a factor in limiting recovery potential for the two species. The individual effect of other environmental contaminants or their synergistic or antagonistic effects in the presence of naturally occurring or introduced elements or compounds have not been the subject of site or species specific investigations, and it is not known whether water quality is a limiting factor for recovery. .1 Water Development and Depletions Between Navajo Dam and its confluence with Lake Powell, there are many points of water . diversion, including a number of pumps on the San Juan River. Downstream of the dam, water is diverted by the Citizens Ditch, the Hammond Canal, the Farmers Mutual Ditch, the Fruitland Irrigation Canal, the San Juan Generating Station, the Jewett Valley Ditch, the Four Comers Power Plant, the Hogback Canal, and other water users. A portion of the diversion for the Four Comers Power Plant is returned to the San Juan River via Chaco Wash. Additional return flows enter the San Juan River from various irrigation and municipal diversions, the Animas River, the La Plata River, the Mancos River, McElmo Creek, and Montezuma Creek. Irrigation return flow from Dolores River diversions enters the San Juan River via the Mancos River and McElmo Creek, augmenting the natural flows of the San Juan River. There are seven major diversion structures on the mainstem San Juan River in New Mexico, ranging from soil and boulder dikes to concrete and metal weirs over which the entire river flows. The most upstream of these structures are dikes and levees at the heads of the Citizens Ditch and the Hammond Canal, which are upstream of the confluence with the Animas River. The other diversion structures are located downstream of Farmington, and are the Farmers Mutual Ditch, the Fruitland canal, the San Juan Generating Station diversion, the Four Corners Power Plant pump station, and the Hogback canal. Water for the Jewett Valley Ditch is diverted at the San Juan Generating Station weir, and water for the Cudei project has been supplied from the Hogback canal since 2002. These diversions, as well as other diversions of water from the San Juan River and its tributaries, may in the future result in an average annual net depletion of San Juan River flows of up to 854,376 acre-feet per year based on the baseline depletions used by the US Fish and Wildlife . Service in the Endangered Species Act section 7 consultation completed for Navajo Reservoir 6