Water-quality trends in the nation’s rivers and streams, 1972–2012—Data preparation, statistical methods, and trend results

Scientific Investigations Report 2017-5006
By: , and 



Since passage of the Clean Water Act in 1972, Federal, State, and local governments have invested billions of dollars to reduce pollution entering rivers and streams. To understand the return on these investments and to effectively manage and protect the Nation’s water resources in the future, we need to know how and why water quality has been changing over time. As part of the National Water-Quality Assessment Project, of the U.S. Geological Survey’s National Water-Quality Program, data from the U.S. Geological Survey, along with multiple other Federal, State, Tribal, regional, and local agencies, have been used to support the most comprehensive assessment conducted to date of surface-water-quality trends in the United States. This report documents the methods used to determine trends in water quality and ecology because these methods are vital to ensuring the quality of the results. Specific objectives are to document (1) the data compilation and processing steps used to identify river and stream sites throughout the Nation suitable for water-quality, pesticide, and ecology trend analysis, (2) the statistical methods used to determine trends in target parameters, (3) considerations for water-quality, pesticide, and ecology data and streamflow data when modeling trends, (4) sensitivity analyses for selecting data and interpreting trend results with the Weighted Regressions on Time, Discharge, and Season method, and (5) the final trend results at each site. The scope of this study includes trends in water-quality concentrations and loads (nutrient, sediment, major ion, salinity, and carbon), pesticide concentrations and loads, and metrics for aquatic ecology (fish, invertebrates, and algae) for four time periods: (1) 1972–2012, (2) 1982–2012, (3) 1992–2012, and (4) 2002–12. In total, nearly 12,000 trends in concentration, load, and ecology metrics were evaluated in this study; there were 11,893 combinations of sites, parameters, and trend periods. The final trend results are presented with examples of how to interpret the results from each trend model. Interpretation of the trend results, such as causal analysis, is not included.

Suggested Citation

Oelsner, G.P., Sprague, L.A., Murphy, J.C., Zuellig, R.E., Johnson, H.M., Ryberg, K.R., Falcone, J.A., Stets, E.G., Vec-chia, A.V., Riskin, M.L., De Cicco, L.A., Mills, T.J., and Farmer, W.H., 2017, Water-quality trends in the Nation’s rivers and streams, 1972–2012—Data preparation, statistical methods, and trend results (ver. 2.0, October 2017): U.S. Geological Survey Scientific Investigations Report 2017–5006, 136 p., https://doi.org/10.3133/sir20175006.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Foreword
  • Abstract
  • Introduction
  • Objectives and Scope
  • Methods
  • Trend Results
  • Summary
  • Acknowledgements
  • References
  • Appendix 1. Streamflow, Water–Quality, and Ecology Sites Included in Trend Analysis
  • Appendix 2. Variations in Parameter Reporting for Selected Parameters
  • Appendix 3. Laboratory Method and Change Timeline
  • Appendix 4. Step-Trend Analysis of Changes in Laboratory Analysis and Sample Collection Methods
  • Appendix 5. Laboratory Performance Bias Evaluation Using Percent Recovery in U.S. Geological Survey Branch of Quality Systems Double-Blind Reference Samples over Time
  • Appendix 6. Analysis of Trends in Annual Streamflow
  • Appendix 7. Trend Results
  • Appendix 8. Comparison of Trends Determined Using the Seasonal Kendall Test and the Weighted Regressions on Time, Discharge, and Season Model
Publication type Report
Publication Subtype USGS Numbered Series
Title Water-quality trends in the nation’s rivers and streams, 1972–2012—Data preparation, statistical methods, and trend results
Series title Scientific Investigations Report
Series number 2017-5006
DOI 10.3133/sir20175006
Edition Version 1.0: Originally posted April 4, 2017; Version 2.0: November 1, 2017
Year Published 2017
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) New Mexico Water Science Center
Description Report: xv, 136 p.; 8 Appendixes; 5 Data Releases; Project Site; Version History
Country United States
Online Only (Y/N) Y
Additional Online Files (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details