NEW : Army Corps of Engineers is expected to open the dam once the flows hit a trigger . NEW : Projections indicate the tipping point could come as early as 2018 , EPA says . NEW : The Delta Spillway has not been opened since 1973 . In addition to the mainline levee along the river , starting near Vicksburg and extending northeast for more than 20 miles , a so-called backwater levee offers shelter . Opening the Morganza Spillway would flood the Atchafalaya River Basin .
NEW : Army Corps of Engineers is expected to open the dam once the flows hit a trigger . NEW : Projections indicate the tipping point could come as early as 2018 , EPA says . NEW : The Delta Spillway has not been opened since 1973 . Opening the Morganza Spillway would flood the Atchafalaya River Basin .