FLOODS AND CHANNEL CHANGE ON THE UMATILLA RIVER, OREGON

Large floods on alluvial rivers produce geomorphic changes that create and renew fish habitat. Floods renew spawning gravels, scour and deepen pools, and create side channels and other off-channel features. Floods transport and deposit large woody debris that is important for small-scale habitat features. In addition, cottonwoods and other streamside trees important to stream ecology depend on floods for regeneration. Human actions for flood protection (levees, revetments, etc.) may limit or change the spatial pattern of channel change during flood events. We examine this hypothesis on Umatilla River, a Columbia River tributary in northeastern Oregon that is home to threatened fish (steelhead, Oncorhyncus mykiss).

Using pre-flood and post-flood historical aerial photos, we have reconstructed spatial patterns of geomorphic change on two 20-km long river sections, creating change maps in GIS. Aerial photographs are scanned and georectified to USGS orthophotos using ERDAS Imagine©. We consistently achieve root mean square error of 1.5 to 3 m. A variety of channel and floodplain features, including main channel, various types of secondary channels, bars, and riparian vegetation, are digitized on the georectified images using ESRI ArcMap©. Change in a flood event is documented using change maps, and linear and areal measurements of change.

Floods in winter 1965 and a cluster of flood events in 1995-97 had significant geomorphic effects on the Umatilla River. Bank protection was less extensive during the earlier flood. In general terms, during floods the active channel widens, active bar area increases, riparian vegetation is locally removed, and sinuosity decreases. Channel change includes lateral migration, downstream migration of meanders, meander enlargement, meander cutoff, and minor avulsion. In both flood events, channel change was highly variable spatially. Channel change is minimal in reaches with bank protection, particularly in agricultural lands downstream of Pendleton. In relatively unprotected sections, channel change is concentrated in some reaches, particularly those with higher initial sinuosity.

This research is supported by NSF award BCS-0215291 from the Geography and Regional Science Program.