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Lower Gales Creek Enhancement Planning
Geomorphic Assessment • Technical Study
Tualatin River Watershed Council
July 28, 2006
Introduction
1.1 Problem Statement
In March of 2003 the Tualatin River Watershed Council completed work
on the Lower Gales Creek Habitat Enhancement Plan (LGCHEP). The purpose
of the LGCHEP was to outline an anadromous fish habitat enhancement strategy
for a four mile section of Gales Creek that was identified as a priority
restoration area through the Gales Creek Watershed Assessment Project
(Bruener, 1998). From this process, the LGCHEP identified nine potential
restoration projects designed to improve habitat conditions for steelhead
(Oncorhynchus mykiss) and other salmonids. The nine projects included
diverse habitat enhancement measures such as widening riparian buffers,
increasing channel complexity by installing engineered log jams, and
enhancing floodplain and secondary channel dynamics. To move these projects
forward to the design, permitting, and implementation phase, the Bureau
of Reclamation has requested that a geomorphic analysis be conducted
on the study reach to evaluate the following issues:
- How is the channel functioning geomorphically as compared to historic
conditions?
- What is the expected long term stability of the channel?
- Are the recommended projects appropriate within the existing morphology
and sediment transport regime?
- What effects would the proposed projects have on planform and profile
stability and sediment transport conditions?
Are there external factors, such as land-use change or changes in downstream
base level that would affect the future success of the proposed enhancement
activities?
1.2 Study Goals and Objectives
Swanson Hydrology and Geomorphology (SH+G), an environmental consulting
firm located in the Portland area, was hired to address these questions
through a field and modeling-based geomorphic and hydrologic analysis
of the study area. The geographic scope of the study included the approximately
four mile stretch of the mainstem of Gales Creek from the Stringtown
Bridge near the confluence of Prickett Creek upstream to the Gales Creek
Road Bridge near the Iler Creek confluence (Figure 1). Areas outside
of the study area were evaluated, where necessary, to gain a better understanding
of sediment supply to the study reach and overall planform stability.
The primary study objective was to develop an understanding of the
geomorphology of the study reach so as to support and/or identify potential
restoration efforts that will improve habitat conditions for salmonids
in Gales Creek and the Tualatin River watershed. The tasks outlined by
SH+G to meet the study objectives and prepare a technical document describing
the results are as follows:
- Historic Geomorphic Analysis: An evaluation and description of geomorphic
conditions and functions that were present prior
to intensive land uses of the 19th and 20th centuries,
- Existing Conditions Analysis: An evaluation of current channel morphology
and function, probable expected future conditions,
and opportunities and constraints to restoration. Includes a brief analysis
of existing hydrologic and sediment transport to support the analysis,
- Restoration Project Evaluation: Sites and techniques recommended
in the LGCHEP were evaluated in relation to geomorphic conditions to
determine their effectiveness in achieving the stated goals of the restoration
effort,
- Technical Report – Draft and Final
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