HUMAN AND CLIMATE IMPACTS ON COLUMBIA RIVER HYDROLOGY AND SALMONIDS P. K. NAIK a * and D. A. JAY b a Agricultural Engineering and Water Resources Directorate, Ministry of Municipalities and Agriculture, P.O. Box 31126, Kingdom of Bahrain b Department of Civil and Environmental Engineering, Portland State University, P.O. Box 751, Portland, Oregon 97207, USA ABSTRACT The Columbia River had until recently the world’s largest Chinook salmon runs. Restoration of the system’s severely decimated runs requires understanding changes in the hydrologic variables (e.g. flow and sediment transport) important to salmonids. We describe here methods to distinguish the human and climate-induced contributions to Columbia River hydrologic processes relevant to the crucial seaward spring migration of juveniles through the tidal river and estuary. Flow regulation has caused most of the decrease in peak flow and sediment transport; it has contributed to changes in spring freshet timing. Climate change has reduced peak and average flows and sediment transport, changing spring-freshet timing by several weeks. Irrigation diversion has reduced the annual average flow as much as climate change. A better understanding of historical changes in hydrologic processes entailed in this paper tells us how management and climate have changed the Columbia River system over time. The separation of the climate and anthropogenic influences used here may assist in policy analyses and strategies aimed at restoration of the Columbia River endangered salmonids, and in management of other systems. Copyright # 2010 John Wiley & Sons, Ltd. key words: Columbia River; human impact; flow regulation; climate change; salmon; sediment transport; PDO; ENSO Received 8 December 2008; Revised 9 April 2010; Accepted 20 April 2010 INTRODUCTION This paper investigates methods to distinguish between anthropogenic and climate impacts on the hydrologic processes relevant to juvenile salmonid survival during their migration through the lower Columbia River (CR) and estuary. There is a connection between climate (e.g. the Pacific Decadal Oscillation or PDO) and salmonid survival over the entire Northeast Pacific (Mantua et al., 1997; Petersen and Kitchell, 2001; Crozier et al., 2008; Yates et al., 2008). The latitude of the CR basin (from 418 30 0 to 548 40 0 N.; Figure 1) causes it to exhibit a strong PDO response. Cold PDO phases are correlated with above average river flows and increased salmonid survival. The highest flows tend to occur after La Nin ˜a winters during cold-PDO periods. Warm PDO phases bring lower river flow and poor salmonid survival; warm-PDO, El Nin ˜o winters have on average the lowest flows (Naik and Jay, 1999; Jay and Naik, 2000). The response of Alaskan salmonids to PDO is of the opposite sense (Mantua et al., 1997). The connection between the PDO cycle and salmonids has been attributed to a ‘bottom- up’ response to climate-related changes in water-column stability in open coastal waters (Gargett, 1997). However, ‘top-down’ controls are also likely to be important, including changes in the coastal food web and predation on salmonids associated with changes in coastal water masses (Bi et al., 2007). The climate response of salmonid populations is not merely a response to processes occurring in open coastal waters; fluvial, tidal-fluvial, estuarine and buoyant plume environments are also important to juvenile salmonids (Schtickzelle and Quinn, 2007). This broad spatial sensi- tivity is in part a response to salmonid migration across these environments, but it is also related to their genetic diversity. Some juveniles go to sea very early in their life cycle, while others rear extensively in fluvial and estuarine habitats (Quinn et al., 2007a). Bottom et al. (2005) argue that the resilience of CR salmon to environmental variability is a consequence of ‘diverse salmon genotypes interacting with unique habitat features. These interactions result in a variety of alternative behavioural ‘solutions’ ... by which salmon can navigate through the chain of habitats required to successfully complete their life cycles’. Because of these diverse genome-habitat interactions, the connection between salmonid survival and climate crosses habitat boundaries, but is still system-specific (Walsh and Kilsby, 2007). Thus, high-river flows in the Columbia decrease predation upon juvenile salmonids by increasing turbidity and allowing a rapid salmonid transit through estuary and plume areas of high predation (Pearcy, 1992). On the other hand, some Fraser River stocks deviate from this usual Pacific Northwest RIVER RESEARCH AND APPLICATIONS River Res. Applic. 27: 1270–1276 (2011) Published online 15 June 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.1422 *Correspondence to: P. K. Naik, Agricultural Engineering and Water Resources Directorate, Ministry of Municipalities and Agriculture, PO Box 31126, Kingdom of Bahrain. E-mail: pradeep.naik@water.net.in Copyright # 2010 John Wiley & Sons, Ltd.