Flood and Human Response: Implications for Geomorphic Adjustment and Fish Habitat in the Pacific Northwest

Patricia F. McDowell and Andrew Marcus, PIs
Michael Hughes, Graduate Research Fellow
Department of Geography
University of Oregon

The overall goal of this project is to use evidence from historic large floods to understand the spatial pattern and controls of channel changes resulting from large floods (>25 yr recurrence interval), and to determine whether human response to historic floods has limited the ability of rivers to regenerate fish habitat.  Large floods on alluvial rivers produce geomorphic changes that create and renew fish habitat and help regenerate riparian vegetation.  Floods renew spawning gravels, scour and deepen pools, and create side channels and other off-channel features.  In addition, cottonwoods and other streamside trees important to stream ecology depend on floods for regeneration.  Using historical aerial photos, documentary evidence, and government agency records, we will reconstruct and analyze spatial patterns of geomorphic change on long continuous river reaches, constructing change maps in GIS.  We will also construct GIS coverages of riverside flood protection structures -- levees, revetments, and other human-built structures that restrict channel change. 

The project will focus on four rivers, and on two historic floods in each river – one flood during the 1940s to 1960s when the rivers were less modified by flood protection structures, and one flood during the 1990s after extensive flood protection works.  The differences in geomorphic change between early and late flood events will show whether and where habitat creation has been reduced by flood protection structures.  In addition, historic development of flood protection structures will be reconstructed to see whether or not flood protection structures were built where the greatest flood impacts occurred.  If flood protection structures correspond to sites with the highest potential for geomorphic change, the ability of a river to regenerate fish habitat may be severely limited. 

The project will also contribute to development of ecologically sensitive flood protection policies.  The conflict between flood protection and restoration of aquatic habitat for endangered salmon has become acute in the study region.  Existing flood protection policies are not compatible with the Endangered Species Act and are currently being revised to be more “fish-friendly”.   Scientific input to this policy development will be important.  This project will produce such scientific input by developing ways to identify geomorphic “hotspots” for fish habitat, and by evaluating whether or not past human response to floods has limited fish habitat. 

Our accomplishments to date are:

• Study site selection:  Our first study site is the Umatilla River in northeastern Oregon.   This river flows through National Forest, Indian reservation, and an extensive zone of private lands, so it represents a variety of land uses and flood protection measures.  We have also done reconnaissance on several other rivers in Washington (Yakima, Methow, Wenatchee, Naches) and Oregon (John Day) to select our second site. 
• Methods:  We have developed methods for rectifying and digitizing historical aerial photos, and assembling aerial photos and digital data.  Procedures for rectifying and digitizing are critical because they control the amount of error associated with our digital products and thus our limits to detecting change between aerial photos at different times.  We have developed a classification system for channel and riparian features visible on historical aerial photographs. 
• Mapping channel and riparian features on historical aerial photographs:  We have scanned and georectified aerial photographs from before and after the 1965 flood and the 1996-7 floods on the Umatilla River.  We have digitized detailed geomorphic features on these images.  Results are shown in the talks posted below. 

Sponsorship:

Supported by the National Science Foundation, Geography and Regional Science and Hydrological Sciences Programs, Award 0215291

Photo Gallery

Related Documents:

AAG 2006 presentation

Comparison of geomorphic change in two flood events on the Umatilla River, Oregon
Powerpoint presentation given at the Columbia Basin Agricultural research Station, Pendleton, OR, on August 18, 2004 -- New!

Classification System for River Channel and Riparian Features from Historic Aerial Photographs and
Digitizing Plan  (this is an early version; updated version will be posted soon)

Link to University of California-Santa Barbara web page