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In cooperation with the U.S. Environmental Protection Agency

Estimated Loads of Suspended Sediment and Selected Trace Elements Transported Through the Clark Fork Basin, Montana, in Selected Periods Before and After the Breach of Milltown Dam (Water Years 1985–2009)

By Steven K. Sando and John H. Lambing

Milltown Reservoir is a National Priorities List Superfund site in the upper Clark Fork basin of western Montana where sediments enriched in trace elements from historical mining and ore processing have been deposited since the completion of Milltown Dam in 1908. Milltown Dam was breached on March 28, 2008, as part of Superfund remediation activities to remove the dam and excavate contaminated sediment that had accumulated in Milltown Reservoir. In preparation for the breach of Milltown Dam, permanent drawdown of Milltown Reservoir began on June 1, 2006, and lowered the water-surface elevation by about 10 to 12 feet. After the breach of Milltown Dam, the water-surface elevation was lowered an additional 17 feet. Hydrologic data-collection activities were conducted by the U.S. Geological Survey in cooperation with U.S. Environmental Protection Agency to estimate loads of suspended sediment and trace elements transported through the Clark Fork basin before and after the breach of Milltown Dam. This report presents selected results of the data-collection activities.

First posted April 7, 2011 Report PDF (2.59 MB) For additional information contact: Director, USGS Montana Water Science Center 3162 Bozeman Ave. Helena, MT 59601 http://mt.water.usgs.gov This report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Suggested citation:

Sando, S.K., and Lambing, J.H., 2011, Estimated loads of suspended sediment and selected trace elements transported through the Clark Fork basin, Montana, in selected periods before and after the breach of Milltown Dam (water years 1985–2009): U.S. Geological Survey Scientific Investigations Report 2011–5030, 64 p.

Contents

Abstract

Introduction

Purpose and Scope

Description of the Study Area

Hydrologic Characteristics

Methods of Data Collection and Quality Assurance

Methods for Estimating Constituent Loads

Estimation of Constituent Loads for High-Intensity Stations

Suspended Sediment

Trace Elements

Estimation of Constituent Loads for Low-Intensity Stations

Estimated Loads Transported Through the Milltown Reservoir Project Area

Daily Loads

Suspended Sediment

Trace Elements

Annual Loads and Mass Balance

Estimated Loads Transported Through the Clark Fork Downstream from the Milltown Reservoir Project Area

Summary and Conclusions

Acknowledgments

References Cited

Supplement 1. Methods for Estimating Censored Concentrations for Water Year 2009 and Comparison with Methods Used in Previous Years

Supplement 2. Regression Equations for Estimating Suspended-Sediment and Trace-Element Discharge for the High-Intensity Stations

Supplement 3. Regression Equations for Estimating Suspended-Sediment and Trace-Element Discharge for the Low-Intensity Stations