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1 these data are useful for evaluating broad spatial patterns of instream load, and comparisons of instream load to inputs, but may not be sufficiently accurate for local-scale evaluations of water quality. Estimates of the mean annual instream load of total nitrogen entering (Chattanooga, Tenn.) and leaving (Paducah, Ky.) the LTEN River Basin were 29,000 and 60,000 tons per year (tons/yr), respectively. These estimates represent a gain of 31,000 tons/yr, on average, across the area (18,930 mi2) between these inlet and outlet sites. The sum of the mean annual instream load from gaged tributaries to the main stem within the study unit was 14,000 tons/yr; however, this number cannot be directly compared with the gain between the inlet and outlet sites because (1) the gaged area represents only 30 percent of the total area and (2) the period of record at many tributary sites did not correspond with the period of record at the inlet or outlet sites. Estimates of mean annual instream load of total phosphorus at the inlet and outlet sites of the LTEN River Basin were 1,300 and 5,000 tons/yr, respectively, representing a gain of 3,700 tons/yr, on average, across the study unit. The sum of the gaged tributary load, representing only 28 percent of the area contributing to the main stem, was 4,300 tons/yr. Although this number cannot be closely compared with the gain throughout the study unit, for the same reasons given for total nitrogen, a general comparison suggests that the main stem of the Tennessee River and the tributary embayments along the main stem function as a sink for total phosphorus, removing a substantial amount from the water column through deposition or assimilation. The estimates of inputs can be compared and correlated with yields (area-normalized instream loads); significant correlations between estimates of inputs and yields might be useful as predictive tools for instream water quality where monitoring data are not available. Yields of nitrogen correlated moderately well with inputs from nonpoint sources, based on 1992 estimates. Nitrogen yield was highest [3.5 (tons/mi2)/yr] for Town Creek, for which the balance of nonpoint-source inputs to agricultural lands (fertilizer application plus nitrogen fixation plus livestock waste minus harvest) was also the highest [15 (tons/mi2)/yr]. Nitrogen yield was low [1.0 (tons/mi2)/yr] for the Buffalo River, for which the balance of agricultural nonpoint-source input was correspondingly low [3.2 (tons/mi2)/yr, the second lowest]. Correlation of wastewater discharge with yield was poor, and contrasted with the significant correlation between wastewater discharge and median nitrogen concentration during low streamflow. The poor correlation between wastewater discharge and annual yield was expected, however, as wastewater discharge is a small fraction compared with annual yield. In contrast with nitrogen, phosphorus yield did not correlate well with any estimated inputs or land-use types for the tributary basins. Phosphorus yield was highest [1.1 and 0.93 (tons/mi2)/yr] at two sites along the Duck River and at Elk River near Prospect [0.89 (ton/mi2)/yr]; however, estimates of inputs at these sites were in the middle of their respective ranges. The influence of the outcrop of phosphatic limestone formations of the brown-phosphate districts in the lower Duck and lower Elk River Basins might be responsible for the poor correlation between estimated inputs and yields of phosphorus. The outcrop pattern of these phosphatic limestones are an important factor to consider as regional boundaries are established for attainable, region-specific water-quality criteria for total phosphorus. Estimates of sediment input from cropland soil erosion in 1992 ranged from 51 to 540 (tons/mi2)/yr among the major hydrologic units in the LTEN River Basin. Information was not available to estimate this input for individual tributaries. Sediment yield estimates ranged from 65 to 263 (tons/mi2)/yr for the three tributary monitoring basins for which instream data were available, and from 17 to 26 (tons/mi2)/yr for the Tennessee River at South Pittsburg and at Pickwick Landing Dam, respectively. Lower sediment yields for the main stem sites compared with the tributary sites is probably due to sediment deposition in the main stem of the Tennessee River and tributary embayments along the main stem. 2 Sources, Instream Transport, and Trends of Nitrogen, Phosphorus, and Sediment in the Lower Tennessee River Basin, 1980-96 Most of the significant trends in nutrient concentrations from about 1985 to about 1995 were decreasing trends, except for total nitrite plus nitrate, which increased at one site on the Elk River. The spatial distribution of decreasing trends of total nitrogen and total ammonia corresponds with the spatial variation among basins in wastewater loading rate. The time period of observed trends corresponds to the period of improvements in municipal treatment, thus decreases in wastewater effluent concentrations of nitrogen might be responsible for the decreasing trend in instream concentrations at these sites. Concentrations of total phosphorus did not decrease during this period at these sites, as might have been expected considering the reductions in wastewater input of phosphorus during this period. INTRODUCTION In 1997, the U.S. Geological Survey (USGS) began an assessment of the lower Tennessee (LTEN) River Basin as part of the National Water-Quality Assessment (NAWQA) Program. The lower Tennessee River Basin corresponds to the lower half of the Tennessee River Valley (fig. 1). Surface-water-quality data collected in the LTEN River Basin from 1980 to 1996 have been compiled, screened, and evaluated to provide a general description of water-quality conditions, to identify trends in selected water-quality constituents, and to assist the design of NAWQA datacollection activities within the LTEN River Basin. Assessment efforts have been focused on nitrogen, phosphorus, and suspended sediment because these water-quality constituents have been identified by Federal, State, and local resource-management and regulatory agencies in the basin as the issues of greatest concern to the quality of surface-water resources. State water-quality regulatory agencies within the LTEN River Basin have documented that although the quality of surface water in the basin is generally good, poor water quality impairs beneficial uses locally in 109 stream segments and 3 lakes (Alabama Department of Environmental Management, 1996; Georgia Department of Natural Resources, 1996; Kentucky Natural Resources and Environmental Protection Cabinet, 1996; Mississippi Department of Environmental Quality, 1996; Tennessee Department of Environment and Conservation, 1996). Nutrient overenrichment is listed as causing impairment in 37 stream segments and 2 lakes. Although nitrogen and phosphorus are essential nutrients for plant and animal growth, nutrient overenrichment of streams and lakes can promote excessive growth of algae and other aquatic plants. The subsequent decay of this growth in organic matter may deplete dissolved oxygen and adversely affect fish and other aquatic life. Excessive growth of algae and other aquatic plants also is accompanied by increased levels of dissolved organic matter that may cause taste and odor problems. These problems increase watertreatment costs and thus impair the use of the water resource as a drinking-water supply. Although nutrient overenrichment is a major cause of impairment, siltation and suspended sediment are the dominant causes of impairment in streams throughout the LTEN River Basin. Siltation and suspended sediment are listed by State water-quality regulatory agencies as causing impairment in 63 of the 109 impaired stream segments in the basin. The increased turbidity associated with elevated suspended-sediment concentrations reduces light penetration and primary productivity of the water bodies. In addition, excessive sediment deposition (siltation) on streambeds degrades habitat for benthic organisms and reduces spawning grounds for fish. Excessive sediment deposition in reservoirs interferes with recreational use (in the tributary embayments), obstructs commercial navigation channels, and reduces storage capacity (D. Meinert, Tennessee Valley Authority, written commun., 1998). Purpose and Scope The purposes of this report are to (1) describe and quantify the major sources of nitrogen and phosphorus to surface waters in the LTEN River Basin; (2) describe the seasonal and spatial patterns and temporal trends of concentrations and loads of nitrogen, phosphorus, and sediment; and (3) relate spatial and temporal patterns of nutrient concentrations and loads to spatial and temporal variation in sources and other environmental factors. The analyses presented in this report are based on historical water-quality data collected at 49 monitoring sites during the period October 1979-September 1996 (water years 1980-96), and on information on nutrient sources for several years during this period. Comparison of nutrient sources with loads is based on data from 1992. The water-quality

Sources, instream transport, and trends of nitrogen, phosphorus, and sediment in the lower Tennessee River basin, 1980-96