PREDICTING RIVER FLOODPLAIN AND LATERAL CHANNEL MIGRATION FOR SALMON HABITAT CONSERVATION 1 Jason E. Hall, Damon M. Holzer, and Timothy J. Beechie 2 ABSTRACT: In this article, we describe a method for predicting floodplain locations and potential lateral chan- nel migration across 82,900 km (491 km 2 by bankfull area) of streams in the Columbia River basin. Predictions are based on channel confinement, channel slope, bankfull width, and bankfull depth derived from digital eleva- tion and precipitation data. Half of the 367 km 2 (47,900 km by length) of low-gradient channels (£ 4% channel slope) were classified as floodplain channels with a high likelihood of lateral channel migration (182 km 2 , 50%). Classification agreement between modeled and field-measured floodplain confinement was 85% (j = 0.46, p < 0.001) with the largest source of error being the misclassification of unconfined channels as confined (55% omission error). Classification agreement between predicted channel migration and lateral migration determined from aerial photographs was 76% (j = 0.53, p < 0.001) with the largest source of error being the misclassification of laterally migrating channels as non-migrating (35% omission error). On average, more salmon populations were associated with laterally migrating channels and floodplains than with confined or nonmigrating channels. These data are useful for many river basin planning applications, including identification of land use impacts to floodplain habitats and locations with restoration potential for listed salmonids or other species of concern. (KEY TERMS: floodplains; fluvial processes; lateral channel migration; geomorphology; geospatial analysis; rivers ⁄ streams.) Hall, Jason E., Damon M. Holzer, and Timothy J. Beechie, 2007. Predicting River Floodplain and Lateral Channel Migration for Salmon Habitat Conservation. Journal of the American Water Resources Association (JAWRA) 43(3):786-797. DOI: 10.1111 ⁄ j.1752-1688.2007.00063.x INTRODUCTION Floodplain ecosystems are structured by dynamic hydrological, geomorphological, and climatological processes that create a shifting mosaic of habitat pat- ches within the river corridor (Ward and Stanford, 1983; Beechie et al., 2006), and influence terrestrial and aquatic species diversity, abundance, and succes- sional patterns (Hughes, 1997; Amoros and Bornette, 2002; Buijse et al., 2002; Johnson, 2002). However, river channels throughout the world are now discon- nected from their floodplains through flood control, damming, flow regulation, channelization, and other hydromodification activities (Hughes, 1997; Buijse et al., 2002; Johnson, 2002; Richards et al., 2002; Ward et al., 2002). Such impacts have resulted in decreased ecosystem function, and have contributed to the decline of important fish species (Jungwirth et al., 2002) such as salmon in the United States 1 Paper No. J05061 of the Journal of the American Water Resources Association (JAWRA). Received May 12, 2005; accepted September 15, 2006. ª 2007 American Water Resources Association. No claim to original U.S. government works. 2 Respectively, NOAA Fisheries Northwest Fisheries Science Center, 2725 Montlake Blvd E., Seattle, Washington 98112 (E-Mail ⁄ Hall: jason.hall@noaa.gov). JAWRA 786 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 43, No. 3 AMERICAN WATER RESOURCES ASSOCIATION June 2007