Estimates of future flood risks are based on the observations of past floods, but instrumental records of basin hydrology are often too short to assess potential changes in the frequency or magnitude of extreme floods over time. In this study, we show that bur oak (Quercus macrocarpa Michx.) growing along the Red River of the North in North Dakota and Minnesota preserve evidence of past floods within their annual growth rings. Rings formed during major floods often displayed (i) marked reductions in the size of their earlywood vessels or (ii) a more diffuse distribution of vessels throughout the increment. Because of the correspondence between major floods and widespread anatomical anomalies within riparian oaks, we suggest that these features can be described as "flood rings." The frequency of flood-ring formation varied substantially along the river, which implies that this evidence can only provide an accurate estimate of flood history when it is obtained from many trees sampled across a dense network of sites. The rate of flood-ring formation is primarily influenced by flood magnitude but is also controlled by the timing and duration of inundation relative to the period of cambial growth. Although flood-affected oaks are imperfect recorders of past floods, this approach offers significant potential as a means to estimate the preinstrumental flood history of the Red River within the United States.