SUMMARY OF MAJOR ISSUES AND FINDINGS
ISSUE: Do anthropogenic activities adversely affect water quality in the upper Snake River Basin?
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Water quality in the upper Snake River Basin is degraded by a variety of nonpoint and point sources of pollutants. Basinwide, nonpoint sources account for about 98 and 99 percent of the total nitrogen and phosphorus, respectively, introduced annually to the upper Snake River Basin. Nonpoint sources account for most of the sediment reaching streams in the basin. Primary nonpoint sources of pollutants in the basin are agricultural activities, confined-animal feeding operations, rangeland grazing, recreational activities, logging, and atmospheric deposition.
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Stream-water and ground-water quality in agricultural areas is affected by pesticide and fertilizer applications and by crop rotation and tillage practices. Agricultural areas, which account for 21 percent of the basin area, are concentrated along the main stem of the Snake River in the downstream parts of the basin. Agricultural areas contribute pesticides, nitrogen, and phosphorus to streams and ground water.
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Primary point sources of pollutants in the basin are industrial discharges, municipal wastewater-treatment facilities, and fish farms. Permitted discharges from these point sources during 1985-90 were about 269 billion gallons per year. Although point sources account for only a small percentage of the total nitrogen and phosphorus introduced annually to the upper Snake River Basin, they discharge directly to streams, where nitrogen and phosphorus become immediately available for uptake.
ISSUE: Does water use in the basin affect the quality of the water resources?
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Streamflow regulation and water use for irrigation, hydroelectric power production, and industry affect the water chemistry and biological communities of streams. During the irrigation season, diversions and irrigation returns reduce streamflows and degrade stream-water quality. Storage reservoirs for irrigation, flood control, and hydroelectric power reduce streamflow velocities and stream habitat to the detriment of native biological communities.
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Irrigated agriculture is the primary consumptive use of water in the basin. About 8 million acre-feet, or 53 percent, of the water used for irrigation in the basin in 1990 was diverted from surface-water supplies. About half of these diversions are from five canals at Lake Walcott and Milner Dam. About 7.1 million acre-feet, or 47 percent, of the total irrigation use was withdrawn from ground water. Some water applied to fields for irrigation returns to streams or infiltrates back to the ground water, transporting nutrients, pesticides, and sediment.
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Fish farms and hydroelectric power facilities are the primary nonconsumptive water uses in the basin. Many fish farms, primarily along the Snake River between Milner Dam and King Hill, use ground water discharging from springs as a source of water. Fish farms contribute nitrogen, phosphorus, and organic wastes to streams. Diversions and impoundments for storage and hydroelectric power generation alter streamflows and streamflow velocities and transform some reaches of the Snake River from a high-gradient, coldwater river to a slow-moving river with an abundance of aquatic plants.
ISSUE: Have fish communities in the upper Snake River Basin been affected by water-quality degradation?
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There is a large amount of variability in the health of fish communities in the upper Snake River Basin. In some streams of the basin, cutthroat, rainbow, and brown trout fisheries represent some of the best fishing in the Nation. In areas of the basin degraded by land- and water-use activities, the entire fish community is composed of warmwater-tolerant, nongame species of fish.
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High-elevation headwater streams in the basin have excellent water quality; however, of the streams assessed, headwater streams contained the fewest fish and lowest numbers of fish species. In most of the headwater streams assessed, only two to four different fish species were found, primarily trout and sculpin. This lack of species diversity is typical of high-elevation headwater streams in the Western United States, where abundant shading and limited nutrient availability result in cold, nutrient-poor streams.
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Excellent water-quality conditions, in combination with stocking of gamefish, make the upper reaches of the Henrys Fork and the Snake River upstream from its confluence with the Henrys Fork some of the best trout fishing streams in the Nation. These streams support healthy populations of cutthroat, rainbow, and brown trout, but are dependent on stocking.
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From the confluence of the Snake River and the Henrys Fork downstream to Milner Dam, the Snake River and its tributaries are indicative of trout fisheries affected by loss of habitat and introduction of non-native species. Fish communities in this reach also have been affected by nonpoint source inputs of nutrients and sediment and by reductions of streamflows for irrigation and hydroelectric power production.
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The Snake River from Milner Dam to King Hill contains a variety of fish species. However, the reason this reach contains more fish species than do upper reaches of the basin is that the community composition is dominated by introduced warmwater species, which are tolerant of the slow-moving water and degraded water-quality conditions. Few gamefish exist in this reach of the Snake River.
ISSUE: Is nitrate a concern in streams and ground water in the upper Snake River Basin? page 12
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Nitrate concentrations in streams in the upper Snake River Basin were generally low, and none exceeded the U.S. Environmental Protection Agency (USEPA) drinking-water standard of 10 milligrams per liter as nitrogen. In contrast, some ground-water samples contained nitrate concentrations in excess of 10 milligrams per liter.
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The major sources of nitrate in the upper Snake River Basin are synthetic fertilizers, cattle manure, and nitrogen-fixing legume crops such as alfalfa and beans. These sources account for 93 percent of the nitrate input to the basin.
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Nitrate concentrations were less than 2.0 milligrams per liter in 95 percent of 527 stream samples collected during the NAWQA study. However, concentrations of nitrate in many streams were sufficient, in combination with sufficient phosphorus, to result in excessive growth of aquatic plants. Nitrate concentrations were highest in streams draining agricultural areas and in streams receiving large amounts of ground-water discharge.
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Nitrate concentrations exceeded a national background level of 3.0 milligrams per liter in water from 25 percent of 726 wells (mostly domestic and water supply) sampled basinwide during 1991-95. Water in 3 percent of the wells contained nitrate concentrations in excess of 10 milligrams per liter.
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In localized areas, nitrate in ground water is becoming a concern. In agricultural areas north of the Snake River between Burley and Hagerman, water from 10 percent of 105 domestic and irrigation wells sampled contained nitrate concentrations in excess of 10 milligrams per liter. Water from 24 percent of 29 domestic wells in a shallow alluvial aquifer in the Minidoka Irrigation District north of Burley contained nitrate concentrations in excess of 10 milligrams per liter.
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Areas where ground water is most vulnerable to nitrate contamination are those where urban or irrigated agricultural land uses are predominant, where the depth to water is shallow, and where soils are well drained.
ISSUE: Are pesticides and other organic compounds reaching streams and ground water? Are they persisting in the environment? page 14
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Many different pesticides and other organic compounds are reaching the water resources of the upper Snake River Basin. Some compounds, primarily those which have been banned from use, have accumulated in fish tissue and bed sediment. Although most of the compounds that were detected were at low concentrations, some compounds were detected at concentrations that exceeded either drinking-water standards or national guidelines for the protection of fish-eating wildlife.
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The herbicide EPTC, or Eptam, was detected in 30 of 37 basinwide stream samples collected during May and June 1994. The maximum concentration of EPTC detected was 0.31 microgram per liter. Other pesticides or pesticide breakdown products commonly detected in streams were atrazine, desethylatrazine, metolachlor, and alachlor.
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Pesticides in streams were detected in the greatest numbers and largest concentrations during the spring and early summer following early season crop applications. However, atrazine and desethylatrazine were detected in some streams throughout the year.
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None of the pesticide concentrations detected in stream samples exceeded existing drinking-water standards established by the USEPA. Two samples from irrigation drains, however, contained the insecticides chlorpyrifos and methylazinphos at concentrations exceeding USEPA aquatic-life criteria.
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At least 1 organochlorine compound was detected at 6 of 20 bed-sediment sampling sites and 16 of 20 fish-tissue sampling sites during 1991-94. The most commonly detected organochlorine compounds were DDT, the DDT breakdown products DDD and DDE, and PCBs. Concentrations of organochlorine compounds in tissue of fish from the Portneuf River at Pocatello, Rock Creek near Twin Falls, and Snake River near Twin Falls equaled or exceeded national guidelines for protection of fish-eating wildlife.
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At least 1 semivolatile organic compound (SVOC) was detected in all of the 43 bed-sediment samples collected during the study. The most frequently detected SVOCs in bed sediment were phthalates (98 percent of the samples), phenol (86 percent), and 2,6-dimethylnaphthalene (83 percent). However, none of the SVOC concentrations in the bed-sediment samples exceeded aquatic-life criteria.
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Some pesticides and volatile organic compounds (VOCs) also are reaching ground water. During basinwide sampling in 1994 and 1995, water from 76 of 195 domestic, irrigation, stock, and public supply wells (39 percent) contained at least 1 pesticide, and 11 wells (6 percent) contained at least 1 VOC. Water from 26 of the wells (13 percent) contained 3 or more pesticides. Atrazine and desethylatrazine were detected in 27 percent of the wells.
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Results of a survey of domestic and irrigation wells in agricultural lands in the Twin Falls and Burley areas showed that water from 43 of 105 wells (41 percent) contained 3 or more pesticides. None of the pesticide concentrations and only one VOC (1,2-dichloropropane) concentration detected in ground water exceeded drinking-water standards.
ISSUE: What are the water-quality concerns in the Snake River between Milner Dam and King Hill? What are the primary sources of pollutants? page 18
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The Snake River between Milner Dam and King Hill is degraded because of the cumulative effects of decades of agricultural and industrial activities. The degraded water quality in the river results from a combination of excessive nitrogen, phosphorus, pesticides, and sediment, and reduced streamflows. Excessive aquatic vegetation, low dissolved oxygen, and high water temperatures--all manifestations of a nutrient-rich, eutrophic water body--prevent water in this reach from meeting State water-quality criteria.
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Many aquatic species once native to the Snake River downstream from Milner Dam either have disappeared from the reach or are in danger of doing so because of the degraded water quality and streamflow alteration caused by dams.
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Major sources of nutrients to the Snake River between Milner Dam and King Hill include ground-water discharge, fish farms, and municipal wastewater-treatment facilities. Agriculturally affected tributaries and irrigation return flows are sources of additional nutrients and are the primary sources of pesticides and sediment.
