larry larson, starting with you. is it, in fact, correct in terms of its construction and mission to think of this all as one system? >> well, in fact, the lower mississippi river from cairo south was designed as a system, correct. it has a series of levees, thousands of miles of levees, and then along the points when the water gets really high, constrained by those levees it provides relief points through spillways and floodway bypasses. so it is... it's intended and does operate as a system. correct. >> couric: just to fill in the details a little bit more so, the floodgates and the spillways are when the water... they're stuck there, right? and when the water gets a certain height that's when they come into play? >> yeah. typically they would not be open because if you have a small amount of water in the river, you want to keep that water there because, remember, the water is all... the river's also used for navigation. so you want to keep is this t water there. but when the water gets extremely high, it threats t