Because it got really full during the storm surge and has been falling ever since. It drains in a small channel around Bayou Savage (which should be/ should have been blown away with a B52 raid, but would have broken some bird nests).
but even at its natural level, isn't the lake higher then the city - I mean, that's the purpose of the levee right? or are you saying that right now, the city has higher water level then the lake?