Is Louisiana sinking?

Sharks in the Mississippi are not common, but they show up occasionally. says geologist Roy Dokka, Louisiana State University, Baton Rouge. What is a salt water fish doing in the fresh waters of the Mississippi? The answer is that the river is not always fresh.

"Most of the time, the force of the river keeps salt water out, but when the river is low, its force is diminished, and salt water, which is heavier and denser than fresh water, flows underneath the fresh water and up the river." This happens because the bottom of the Mississippi is below sea level.

Sharks in the river aren't the only effect of low water. "Many small towns along the lower Mississippi get their water from shallow wells. When salt water flows up the river, it seeps into their aquifers and contaminates the wells."

Over time. a riverbed moves like a snake, explains Dokka. As the force of the current wears away the outside banks of a river's loops, it deposits silt and sand on the inside banks. The loops gradually move downstream, and the mouth of the river moves back and forth across the land, like the end of a hose that has been turned loose, pouring out its sediments and adding to the coastline. In fact, Louisiana was created by sediments washed from 32 states. Two million years ago, the coastline began at the southern Arkansas border.

"When sediments are laid down, approximately 50% of their volume is water. When layer after layer of...