The evacuation was ordered because of a “hazardous situation” involving the Northern California dam's emergency spillway. The National Weather Service said the auxiliary spillway is expected to fail and could send an “uncontrolled release of flood waters from Lake Oroville.”
via France Info"Le plus haut barrage des Etats-Unis menace de céder, des dizaines de milliers d'évacuations Les intempéries ont fragilisé la structure du barrage d'Oroville en Californie,, haut de 230 m." http://www.francetvinfo.fr/monde/usa/californie-un-barrage-menace-de-ceder-des-dizaines-de-milliers-d-evacuations_2058609.html