The mouth of Bayou St. John, for a cast of officials from the Army Corps of Engineers and Orleans Levee District, represents more than anything a point of vulnerability. Their priority is simple: keep storm surge from Lake Pontchartrain at bay, and prevent the transformation of a natural waterway into a highway for delivering floodwater to New Orleans' heart. But for an increasingly vocal alliance of New Orleanians, the junction of the bayou and the vast lake represents much more than a flood protection issue.