Bank erosion and channel widening on Lester River in Duluth, Minnesota. Kathy Lee (USGS, Minn.) is in the photo for scale. - F. Fitzpatrick, Aug 21, 2003.
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Cooperators: USGS−Minnesota, City of Duluth, and The Nature Conservancy
Project Chief: Faith A. Fitzpatrick
Location: Duluth, Minnesota
Project Number: AYL00
Period of Project: June 30, 2003–December 31, 2004
Problem
The City of Duluth (City) and The Nature Conservancy (TNC) are 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 are not well documented.
Objectives
The major goals of the study are 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.
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 will be developed for Duluth-area streams. Stream segments will 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. A USGS Scientific Investigations Report will be written with results from the study.
Progress (July 2003–June 2004)
During the first year a literature review was conducted,
drainage basin boundaries were delineated, available GIS
data for basin/landscape characteristics were collected, and
longitudinal profiles for the streams based on 7.5-minute topographic maps were constructed. The Nature Conservancy delineated the streams into segments based on six categories of slope and three categories of valley confinement. Field-based reach surveys were conducted. Historical aerial photos were compiled and analyzed for channel changes.
Plans (July 2004–June 2005)
The classification scheme for Duluth-area streams will be finalized. The final report will be written. |
