Duluth streams geomorphic assessment and stream response to the 2012 flood

Science Center Objects

This study identified major geomorphic processes and factors for Duluth-area streams, assessed their sensitivity to geomorphic change, summarized the effects of the 2012 flood. It also identified channel processes, sediment dynamics, and geomorphic setting can be used to assist managers in post-flood reconstruction activities and stream restoration.

Problem

In the early 2000’s, the City of Duluth (City) and The Nature Conservancy (TNC) were concerned about changes to the geomorphic condition of streams in the Duluth, Minnesota area. Understanding the geomorphic processes within a stream network is important because they relate to the overall water and aquatic resource quality of the stream and to the plant and animal communities and habitat adjacent to the stream. The important geomorphic processes (including hydrologic characteristics and sediment loading, transport, and delivery) affecting the physical characteristics of Duluth streams needed documentation.

In 2012, an extreme flood hit the Duluth area. Geomorphic changes caused by the flood were documented at sites first visited during the 2003-04 study.

 

Objectives

The major goals of the 2003-04 study were to: (1) describe geomorphic conditions for Duluth-area streams, (2) identify major geomorphic processes and factors contributing to current conditions, (3) characterize historical changes in channel morphology and planform, and (4) classify streams based on geomorphic processes and sensitivity to geomorphic change.

The major goals of the 2013 study were to: (1) quantify geomorphic responses to the June 2012 flood at reaches previously sampled in 2003–04; 2) summarize 2012 flood related responses in terms of the broader geomorphic setting using the Duluth segment-scale classification; and (3) describe how relations among channel processes, sediment dynamics, and geomorphic setting can be used to assist managers in post-flood reconstruction activities and stream restoration.

 

Figure showing longitudinal profiles for Lester River in the Duluth, Minn., area. (<, less than)

Longitudinal profiles for Lester River in the Duluth, Minn., area. (<, less than)​​​​​​​

Approach

A literature search was conducted and watershed characteristics (geology, land use/land cover, and soil characteristics, and population) were determined through a GIS. Longitudinal profiles of the streams were compiled. Air photos were gathered by TNC and USGS from 1839−1940, 1950−70, 1991, 2001, and 2002 and historical changes in channel morphology and planform were determined.

Streams were partitioned into segments based on channel slope and valley confinement. The segments formed the initial basis for assessing and documenting historical and current channel and riparian conditions as well as evaluating potential future changes in form and function.

Field surveys of 48 stream reaches were conducted to assess key stream channel and riparian characteristics that were useful for interpreting geomorphic condition and response potential. Sites surveyed included a range of segment types, of channel types representative of distinct geology/landforms, disturbed/developed and undisturbed conditions, those likely to be highly responsive to changes in upstream inputs (for example, prone to sediment deposition, scour or bank erosion). Data at 40 sites include photos and qualitative and semi-quantitative information on segment and channel types, geomorphic condition, riparian conditions (geology, land cover, wetlands, lakes), local variations in geologic setting, human modifications, potential sources for sediment, indications of bank erosion, entrenchment, sedimentation, lateral migration, and head-cutting. At 8 of the 48 sites, very detailed reach surveys were conducted and include quantitative data on channel morphology, water-surface and thalweg slope, bankfull stage, substrate characterization, flood-plain sedimentation rates, large woody debris, and pools. Data from channel cross-section surveys will be used to determine channel adjustments (aggradation, degradation, or lateral migration), and movement of streambed or bank material. Permanent markers for one or both endpoints of the cross sections were established. Bankfull stage was determined through field indicators. Abandoned channels were cored for determining historical changes in bed elevation.

Based on the above information, a watershed-specific classification system was developed for Duluth-area streams. Stream segments were be grouped into types (referred to as geomorphic map units or GMUs) that share similar geomorphic processes and historical sensitivity to changes in sediment load, particle size, woody debris, floods, riparian vegetation, and catastrophic disturbance). Using the information and insights summarized above, the potential pathways will be identified for which each GMU is likely to adjust (that is, sensitivity) in response to changes in key channel-forming processes.

For the 2013 post-flood study field geomorphic assessments were conducted at the 2003-04 reaches with some additional new reaches. Data on channel morphology, substrate, exposed bars and soft sediment, large wood, pools, and bank erosion were compared between surveys. Implications for infrastructure repair were summarized. 

 

Related Websites

http://www.lakesuperiorstreams.org/