Managers at Smith and Bybee Wetlands Natural Area (SBW), an 800-ha preserve in Portland, Oregon, recently installed a water control structure to suppress invasive reed canarygrass (Phalaris arundinacea L.) with spring and summer flooding. We hypothesized that greater depth of flooding would decrease Phalaris growth and percent cover. We randomly established 27 transects throughout SBW before completion of the water control structure and measured percent cover of vegetation prior to flooding in autumn 2003 and then after one growing season in autumn 2004. We also monitored phenological characteristics of individual stands of reed canarygrass growing in different depths during 2004. Overall reed canarygrass cover decreased from 43.7% in 2003 to 41.2% in 2004 (McNemar's test; p < 0.001). Where inundation was > 0.85 m, reed canarygrass cover declined 6.1%. Where this deeper inundation coincided with regenerating willow forest, reed canarygrass cover declined 10.7%. Both before and after higher inundation, reed canarygrass cover was negatively correlated with plant species diversity (before: Spearman's rho  =  −0.67, p < 0.001; after: Spearman's rho  =  −0.41, p  =  0.036). Cover of several native taxa (e.g., Polygonum spp., Bidens spp., Salix lucida) increased in 2004. After flooding, reed canarygrass stands grew more slowly and changed their structural growth pattern. These findings suggest that managed flooding might be operationally useful in suppressing reed canarygrass.