Assessing flood hazard on dynamic rivers

Recurrent flooding of the Red River in 1997, North Carolina's Tar River in 1999, and the Mississippi River in 1993, 1995, and 2001 has led to speculation that human activities contribute significantly to these disasters. New analyses of historical hydrologic data from the Mississippi River quantify systematic increases in flood stages for equal discharges. Analyses of cross‐sectional measurements and other evidence suggest that rising stages result from flow retardation by navigational dikes and floodwater confinement by levees. On dynamic rivers, flood‐frequency statistics must be revised periodically for hazard assessments to keep pace with current conditions, but the present reassessment methodology is prohibitively time‐consuming and expensive.