Quercus macrocarpa (Michx.) growing along the Red River, Manitoba, Canada, contain an anatomical signature related to high-magnitude 19th-century floods. Tree-ring samples were collected from 194 Q. macrocarpa over a 100 km transect along the Red River valley. The combined tree-ring record extends from A.D. 1463 to A.D. 1999; sample depth between 1463 and 1650 is limited and made up exclusively of subfossil logs derived from alluvial deposits. Thirteen trees from four sites contain annual rings with reduced earlywood-vessel transverse areas that reflect flooding during the tree's growing season. Flood rings in 1826 are present in 24% of Q. macrocarpa samples and are coincident with the largest flood observed in the Red River valley. Flood rings in 1852 are exhibited in 5.9% of samples and correspond with the second largest Red River flood. These results confirm that Quercus species adapt to prolonged inundation by reducing the transverse area of their earlywood vessels and suggest that anatomical signatures in riverbank trees may be used to identify and delineate high-magnitude paleofloods for low-gradient rivers.