Four decades of paleoflood hydrology through wood anatomy
Forty years ago Dr. Tom Yanosky, a research botanist with the US Geological Survey, reported that ash trees growing along the Potomac River contained rings with abnormal wood anatomy caused by flood damage. Dr. Yanosky recommended these rings — which he dubbed “flood rings” — could be used to estimate the date, seasonal timing, and (most importantly) peak stage of past floods. Since that discovery, flood rings have been identified for forested river systems in eastern France, central Canada, northwestern Quebec, and the lower Mississippi basin, and kindred anatomical signals due to riverine flooding have also been reported for tropical rivers in Bangladesh and Colombia. Flooding experiments conducted on juvenile trees have confirmed the unambiguous interpretation of these abnormal features — flood rings only form after stem flooding — but also underscore the pivotal interplay between phenology and hydrology — to be recorded, floods must happen during the period of earlywood formation. Although the oldest reported flood ring dates to BCE 3180 (in late Neolithic timbers from northwestern France), riparian forests near major population centers have often been recently clear-cut and so far it has been rare for flood-ring chronologies to go back more than a few centuries. Despite that limitation, flood-ring evidence has entered into the planning of flood-mitigation infrastructure in Canada, and as of 2019, has been formally incorporated into US government guidelines for quantitive flood risk assessments. Four decades after Dr. Yanosky’s seminal study on the Potomac, we are now positioned to deliver on his promise of a deeper appreciation of floods gleaned from abnormal tree rings.