Synthesis of Monthly and Annual Streamflow Records (Water Years 1950-2003) for Big Sandy, Clear, Peoples, and Beaver Creeks in the Milk River Basin, Montana

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Abstract

To address concerns expressed by the State of Montana about the apportionment of water in the St. Mary and Milk River basins between Canada and the United States, the International Joint Commission requested information from the United States government about water that originates in the United States but does not cross the border into Canada.

In response to this request, the U.S. Geological Survey synthesized monthly and annual streamflow records for Big Sandy, Clear, Peoples, and Beaver Creeks, all of which are in the Milk River basin in Montana, for water years 1950-2003.

This report presents the synthesized values of monthly and annual streamflow for Big Sandy, Clear, Peoples, and Beaver Creeks in Montana. Synthesized values were derived from recorded and estimated streamflows. Statistics, including long-term medians and averages and flows for various exceedance probabilities, were computed from the synthesized data.

Beaver Creek had the largest median annual discharge (19,490 acre-feet), and Clear Creek had the smallest median annual discharge (6,680 acre-feet). Big Sandy Creek, the stream with the largest drainage area, had the second smallest median annual discharge (9,640 acre-feet), whereas Peoples Creek, the stream with the second smallest drainage area, had the second largest median annual discharge (11,700 acre-feet). The combined median annual discharge for the four streams was 45,400 acre-feet. The largest combined median monthly discharge for the four creeks was 6,930 acre-feet in March, and the smallest combined median monthly discharge was 48 acre-feet in January. The combined median monthly values were substantially smaller than the average monthly values.

Overall, synthesized flow records for the four creeks are considered to be reasonable given the prevailing climatic conditions in the region during the 1950-2003 base period. Individual estimates of monthly streamflow may have large errors, however.

Linear regression was used to relate logarithms of combined annual streamflow to water years 1950-2003. The results of the regression analysis indicated a significant downward trend (regression line slope was -0.00977) for combined annual streamflow. A regression analysis using data from 1956-2003 indicated a slight, but not significant, downward trend for combined annual streamflow.

Contents

Abstract

Introduction

Synthesis of Streamflow Records

Existing Data and Method of Synthesis

Synthesized Monthly and Annual Streamflow Records for Big Sandy, Clear, Peoples, and Beaver Creeks

Limitations of the Method of Streamflow Synthesis

Analysis of Trends in Combined Annual Streamflow for Big Sandy, Clear, Peoples, and Beaver Creeks

Summary

References Cited

Data

Figures

1. Map showing location of study area, Milk River and tributary streamflow-gaging stations for which records were synthesized and streamflow-gaging stations used for record synthesis, Milk River basin of Montana, Alberta, and Saskatchewan.

2–8. Graphs showing:

2. Synthesized annual streamflow for various exceedance probabilities for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana, water years 1950-2003.

3. Combined synthesized annual streamflow for various exceedance probabilities for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana, water years 1950-2003.

4. Variation of combined synthesized monthly streamflow for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River Basin, Montana, water years 1950-2003.

5. Variation in monthly streamflow for Big Dry Creek near Van Norman, Montana, for water years 1950-2003.

6. Variation in daily and monthly streamflow in May 1999 for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana.

7. Analysis of trend in combined synthesized annual streamflow for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana,
water years 1950-2003.

8. Analysis of trend in combined synthesized annual streamflow for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana,
water years 1956-2003.

Tables

1. Data for selected streamflow-gaging stations in the Milk River basin and streamflow-gaging stations used as base stations for record synthesis, Montana.

2. Summary of selected information for synthesis of flow records for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana, water years 1950-2003.

3. Recorded and estimated monthly and annual streamflow for Big Sandy Creek, Milk River basin, Montana, water years 1950-2003.

4. Recorded and estimated monthly and annual streamflow for Clear Creek, Milk River basin, Montana, water years 1950-2003.

5. Recorded and estimated monthly and annual streamflow for Peoples Creek, Milk River basin, Montana, water years 1950-2003.

6. Recorded and estimated monthly and annual streamflow for Beaver Creek, Milk River basin, Montana, water years 1950-2003.

7. Combined recorded and estimated monthly and annual streamflows for Big Sandy, Clear, Peoples, and Beaver Creeks, Milk River basin, Montana, water years 1950-2003.