[1] 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.
Over the past century, the Red River of the North has been the least stationary river in the continental United States. In Canada, historical and paleoenvironmental evidence indicates severe floods were common during the early 1800s, with the record ce 1826 flood having an estimated peak discharge 50% higher than the second-most severe flood ever observed. Unfortunately, the recorded history of flooding upstream in the United States does not begin until seven decades after this event. If 1826 was an equally exceptional flood on American reach of the river, then current flood-frequency curves for the river underestimate significantly the risks posed by future flooding. Alternatively, if the American stretch did not produce a major flood in 1826, then the recent spate of flooding that has occurred over the past two decades is exceptional within the context of the past 200 years. Communities in the Fargo-Moorhead metropolitan area are building a 58-km long, $2.75 billion (USD) diversion channel that would redirect floodwaters westward around the two cities before returning it to the main channel. Because this and other infrastructure in North Dakota and Minnesota is intended to provide protection against low-probability, high-magnitude floods, new paleoflood investigations in the region would help local, state, and federal policy-makers better understand the true flood threats posed by the Red River of the North.
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