Geomorphic responses of Duluth-area streams to the June 2012 flood, Minnesota

Scientific Investigations Report 2016-5104
Prepared in cooperation with the Minnesota Pollution Control Agency
By: , and 

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Abstract

In 2013, the U.S. Geological Survey, in cooperation with the Minnesota Pollution Control Agency, completed a geomorphic assessment of 51 Duluth-area stream sites in 20 basins to describe and document the stream geomorphic changes associated with the June 2012 flood. Heavy rainfall caused flood peaks with annual exceedance probabilities of less than 0.002 (flood recurrence interval of greater than 500 years) on large and small streams in and surrounding the Duluth area. A geomorphic segment-scale classification previously developed in 2003–4 by the U.S. Geological Survey for Duluth-area streams was used as a framework to characterize the observed flood-related responses along a longitudinal continuum from headwaters to rivermouths at Lake Superior related to drainage network position, slope, geologic setting, and valley type. Field assessments in 2013 followed and expanded on techniques used in 2003–4 at intensive and rapid sites. A third level of assessment was added in 2013 to increase the amount of quantitative data at a subset of 2003–4 rapid sites. Characteristics of channel morphology, channel bed substrate, exposed bars and soft sediment deposition, large wood, pools, and bank erosion were measured; and repeat photographs were taken. Additional measurements in 2013 included identification of Rosgen Level II stream types. The comparative analyses of field data collected in 2003–4 and again in 2013 indicated notable geomorphic changes, some of them expected and others not. As expected, in headwaters with gently sloping wetland segments, geomorphic changes were negligible (little measured or observed change). Downstream, middle main stems generally had bank and bluff erosion and bar formation as expected. Steep bedrock sites along middle and lower main stems had localized bank and bluff erosion in short sections with intermittent bedrock. Lower main stem and alluvial sites had bank erosion, widening, gravel bar deposition, and aggradation. Bar formation and accumulation of gravel was more widespread than expected among all main stems, especially for sites upstream and downstream from channel constrictions from road crossings, or even steep sites with localized, more gently sloping sections. Decreases in large wood and pools also were observed throughout the longitudinal continuum of main-stem sites, with immediate implications for fish and benthic invertebrate aquatic habitat. Whether or not the geomorphic conditions will return to their preflood condition depends on the location along the longitudinal continuum. The amount of large wood and pools may return after more moderate floods, whereas bars with coarse material may remain in place, locally altering flow direction and causing continued bank erosion. Results from this study can be used by local managers in postflood reconstruction efforts and provide baseline information for continued monitoring of geomorphic responses to the June 2012 flood.

Suggested Citation

Fitzpatrick, F.A., Ellison, C.A., Czuba, C.R., Young, B.M., and McCool, M.M., 2016, Geomorphic responses of Duluth-area streams to the June 2012 flood, Minnesota: U.S. Geological Survey Scientific Investigations Report 2016–5104, 53 p. with appendixes, http://dx.doi.org/10.3133/sir20165104.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Methods for Stream Geomorphic and Habitat Data Collection
  • Stream Geomorphic Responses to the June 2012 Flood
  • Implications for Infrastructure Repair and Future Rehabilitation
  • Summary and Conclusions
  • Acknowledgments
  • References Cited
  • Appendix 1. Summary of Duluth-Area Segment Classification Characteristics at the 51 Study Sites, 2013
  • Appendix 2. Reach Maps of Intensive Sites
  • Appendix 3. Data Associated with Cross-Section and Longitudinal Profiles at Intensive Sites
  • Appendix 4. Summary of Field Assessment Data for Each Site

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Geomorphic responses of Duluth-area streams to the June 2012 flood, Minnesota
Series title Scientific Investigations Report
Series number 2016-5104
DOI 10.3133/sir20165104
Year Published 2016
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Minnesota Water Science Center
Description Report: vi, 53 p.; Appendixes: 1–4
Country United States
State Minnesota
City Duluth
Online Only (Y/N) Y
Additional Online Files (Y/N) Y