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Thickness and Other Characteristics of Overbank Sediment Deposited During an Extreme Flood in May 1978 Along the Powder River, Montana

Scientific Investigations Report 2026-5122
By:  and 
https://doi.org/10.3133/sir20265122

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Abstract

An extreme flood on the Powder River in southeastern Montana in May 1978 inundated its valley and deposited sediment on the floodplains and terraces at multiple heights. The recurrence interval for this flood was less than 1 percent in the reach between Moorhead and Broadus, Montana. Peak discharges at the U.S. Geological Survey streamgages at Moorhead and Broadus were 779 and 711 cubic meters per second (m3/s), respectively, the difference reflecting the water and sediment stored on the valley surfaces. Bankfull discharge depended on the height of the bank at the start of the valley transect and varied from 243 to 713 m3/s. Sediment-thickness and particle-size data were collected and analyzed in the autumn of 1978 by U.S. Geological Survey scientists at about 900 sites along 20 valley transects between Moorhead and Broadus, Mont. These transects were approximately orthogonal to the floodflow across the floodplain from near the edge of the channel to the high-water mark. Estimated maximum flood depths along these transects ranged from 0.9 to 4.2 meters.

Contrary to theory and controlled laboratory experiments, the distribution of sediment thickness and particle sizes along valley transects did not decrease systematically with distance from the main channel but were affected by the distribution of vegetation. Additionally, some water and sediment—primarily muds and silts—were conveyed by subsidiary channels (often connected to the main channel downriver from the valley transect) during the early stages of the flood before water overtopped the banks at the start of the valley transect. The vegetation created natural sediment traps in the recirculation and wake zones in the lee of trees and shrubs. Sediment that accumulated in these traps formed dunes and thus an undulating surface with many local maximums and minimums in sediment thicknesses. Sediment in the traps are referred to as lee dunes, which recorded flow conditions and a predominance of coarsening-upward sequence of particle sizes (mud to silt to sands) starting at the preflood surface. These sequences were associated with the rising limb of the hydrograph, and later as the flood began to recede, the lee dunes recorded a fining-upward sequence associated with the falling limb of the hydrograph.

Suggested Citation

Moody, J.A., and Meade, R.H., 2026, Thickness and other characteristics of overbank sediment deposited during an extreme flood in May 1978 along the Powder River, Montana: U.S. Geological Survey Scientific Investigations Report 2026–5122, 171 p., https://doi.org/10.3133/sir20265122.

ISSN: 2328-0328 (online)

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Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Methods
  • Results and Discussion
  • Summary
  • References Cited
  • Appendix 1. Valley Transect Descriptions
Publication type Report
Publication Subtype USGS Numbered Series
Title Thickness and other characteristics of overbank sediment deposited during an extreme flood in May 1978 along the Powder River, Montana
Series title Scientific Investigations Report
Series number 2026-5122
DOI 10.3133/sir20265122
Publication Date April 01, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston VA
Contributing office(s) WMA - Earth System Processes Division
Description Report: vii, 171 p.; 2 Data Releases
Country United States
State Montana
Other Geospatial Powder River
Online Only (Y/N) Y
Additional publication details