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In cooperation with the U.S. Environmental Protection Agency Gulf of Mexico Program, Nutrient Enrichment Issue Committee

Trends in Nutrient Concentration and Load for Streams in the Mississippi River Basin, 1974–94

By D.L. Lurry and D.D. Dunn

U.S. Geological Survey
Water-Resources Investigations Report 97–4223


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Contents

Abstract

Introduction

Purpose and Scope

Previous Studies

Sources and Description of Data

Methods of Analysis

Computation of Flow-Adjusted Nutrient Concentrations

Computation of Nutrient Loads

Detection of Trends

Nutrient Trends

Trends in Nutrient Concentration

Total Nitrogen

Total Phosphorus

Trends in Nutrient Load

Total Nitrogen

Total Phosphorus

Factors Affecting Nutrient Trends

Data Limitations

Influence of Streamflow on Loads

Mississippi River Diversion to Atchafalaya River

Summary and Conclusions

Selected References

Figures

1–5.  Maps showing:
  1.  Locations of streamflow-gaging stations on rivers in the Mississippi Basin
  2.  Trends in flow-adjusted residuals of total nitrogen concentration for rivers in the Mississippi Basin, 1974–94
  3.  Trends in flow-adjusted residuals of total phosphorus concentration for rivers in the Mississippi Basin, 1974–94
  4.  Trends in estimated annual loads of total nitrogen for rivers in the Mississippi Basin, 1974–94
  5.  Trends in estimated annual loads of total phosphorus for rivers in the Mississippi Basin, 1974–94
6–45.  Graphs showing relation of total nitrogen and total phosphorus concentration to daily mean streamflow and temporal trends in annual streamflow, in flow-adjusted residuals of total nitrogen and total phosphorus concentration, and in estimated annual total nitrogen and total phosphorus loads for:
  6.  Mississippi River, La. (MS1) (07373420)
  7.  Mississippi River, Miss. (MS2) (07289000)
  8.  Mississippi River, Ark. (MS3) (07265450)
  9.  Arkansas River, Ark. (ARK) (07250550)
  10.  Canadian River, Okla. (CAN) (07245000)
  11.  Cimarron River, Okla. (CIM) (07157950)
  12.  Mississippi River, Tenn. (MS4) (07032000)
  13.  Mississippi River, Ill. (MS5) (07022000)
  14.  Missouri River, Mo. (MO1) (06934500)
  15.  Kansas River, Kans. (KAN) (06892350)
  16.  Platte River, Nebr. (PLT) (06805500)
  17.  Missouri River, Iowa (MO2) (06486000)
  18.  White River, S. Dak. (WH1) (06452000)
  19.  Cheyenne River, S. Dak. (CHY) (06439300)
  20.  Cannonball River, N. Dak. (CNB) (06354000)
  21.  Missouri River, N. Dak. (MO3) (06338490)
  22.  Yellowstone River, Mont. (YLS) (06329500)
  23.  Kaskaskia River, Ill. (KAS) (05594100)
  24.  Mississippi River, Ill. (MS6) (05587550)
  25.  Illinois River, Ill. (IL1) (05586100)
  26.  Illinois River, Ill. (IL2) (05543500)
  27.  Mississippi River, Iowa (MS7) (05474500)
  28.  Iowa River, Iowa (IOW) (05465500)
  29.  Mississippi River, Iowa (MS8) (05420500)
  30.  Wisconsin River, Wis. (WIS) (05407000)
  31.  Mississippi River, Minn. (MS9) (05378500)
  32.  Chippewa River, Wis. (CHI) (05369500)
  33.  St. Croix River, Wis. (STC) (05340500)
  34.  Minnesota River, Minn. (MIN) (05330000)
  35.  Mississippi River, Minn. (MS10) (05267000)
  36.  Ohio River, Ill. (OH1) (03612500)
  37.  Tennessee River, Ky. (TEN) (03609750)
  38.  Cumberland River, Ky. (CUM) (03438220)
  39.  White River, Ind. (WH2) (03374100)
  40.  Ohio River, Ky. (OH2) (03303280)
  41.  Ohio River, Ky. (OH3) (03277200)
  42.  Scioto River, Ohio (SCI) (03234500)
  43.  Ohio River, Ky. (OH4) (03216600)
  44.  Monongahela River, Pa. (MON) (03085000)
  45.  Allegheny River, Pa. (ALL) (03049625)

Tables

1.  Locations and drainage areas for selected streamflow-gaging stations in the Mississippi Basin
2.  Period of record and number of nutrient analyses available for selected streamflow-gaging stations in the Mississippi Basin
3.  Trends in annual streamflow and total nitrogen concentration and load for selected streamflow-gaging stations in the Mississippi Basin
4.  Trends in annual streamflow and total phosphorus concentration and load for selected streamflow-gaging stations in the Mississippi Basin
5.  Parameter estimates for total nitrogen load model
6.  Parameter estimates for total phosphorus load model
7.  Mean annual streamflow and nutrient inflows for the Mississippi River, Atchafalaya River, Old River Outflow Channel, and Red River, 1978–93

Abstract

Trends in nutrient concentration and load are computed for 40 stations on 24 streams in the Mississippi River Basin. The drainage area of the Mississippi Basin covers about 42 percent of the conterminous United States, and the Mississippi River discharges about 420 billion gallons per day of freshwater to the Gulf of Mexico. The period for which trends are analyzed varies for each station but generally comprises water years 1974–94. Stations included in this analysis are part of the National Stream Quality Accounting Network operated by the U.S. Geological Survey.

LOWESS (LOcally WEighted Scatterplot Smoothing) smooth lines superimposed on graphs of flow-adjusted residuals of concentration versus time and load versus time indicate short-term trends for each station. Kendall-Theil robust lines superimposed on the same graphs indicate long-term trends. Long-term trends were evaluated using the slope of the Kendall-Theil robust line and Kendall’s tau. Annual loads are estimated with regression analysis and corrected for log-transformation bias with the MVUE (Minimum Variance Unbiased Estimator). Trends in annual streamflow are presented to aid in the interpretation of nutrient trends.

Statistically significant long-term increases in flow-adjusted residual concentrations of total nitrogen were detected at 14 stations, decreases were detected at 6 stations, and no significant trends were detected at 20 stations. Statistically significant long-term increases in total nitrogen load were detected at 5 stations, decreases were detected at 4 stations, and no significant trends were detected at 31 stations.

Statistically significant long-term increases in flow-adjusted residual concentrations of total phosphorus were detected at 2 stations, decreases were detected at 25 stations, and no significant trends were detected at 13 stations. Statistically significant long-term increases in total phosphorus load were detected at 1 station, decreases were detected at 13 stations, and no significant trends were detected at 26 stations.

The mean annual yields were computed from the estimated mean annual loads. Mean annual nitrogen yields are largest for three watersheds in the upper Mississippi Basin; estimated yield of each is greater than 5.0 tons per square mile. Nine stations in the Ohio River Basin have estimated mean annual nitrogen yields greater than 2.0 tons per square mile. Estimated mean annual phosphorus yield is greater than 0.250 ton per square mile at 7 stations, 5 in the Ohio Basin and 2 in the upper Mississippi Basin. Statistically significant trends in annual streamflow were detected at seven stations. Annual streamflow influences trends in load, even when the streamflow trends are not statistically significant.




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