On 11 March 2011, the Tohoku Tsunami overtopped a weir and penetrated 49 km up the Kitakami River, the fourth largest river in Japan. Similarly, the 2010 Chile tsunami propagated at least 15 km up the Maule River. In the Pacific Northwest of the United States, large tsunamis have occurred along the Cascadia subduction zone, most recently the 'orphan tsunami' of 1700 (Atwater et al.). The expected future occurrence of a Cascadia tsunami and its penetration into the Lower Columbia River became the subject of “the Workshop on Tsunami Hydrodynamics in a Large River” held in Corvallis, Oregon, 2011. We found that tsunami penetration into the Columbia River is quite different from a typical river. The tsunami enters the vast river estuary through the relatively narrow river mouth of the Columbia, which damps and diffuses its energy. The tsunami transforms into a long period, small amplitude wave that advances to Portland, 173 km from the ocean. Understanding this unique tsunami behavior is important for preparing a forthcoming Cascadia tsunami event.