The Mississippi is mighty! It is also mighty muddy!
It wasn't always so. Early French explorers write about the pristine nature of the river. Now floating off New Orleans is 'dead zone' the size of New Jersey.
How did that happen? Hypoxia, or lack of oxygen in the water. Plants need oxygen to grow. In the dead zone there is none. When fish swim into the dead zone, they do not leave.
At this point, all the people using the River run for the bunkers. It is someone else's problem. Some want to blame nitrates. There are tens of thousands of farms using nitrate fertilizer in the Mississippi water basin. "Not us", say the farmer, "Look at the chemical and oil companies off shore". "Not us", say the chemical and oil companies, "Look to growing population of people using the river".
Lots and lots of problems using the river. The problems this generation doesn't solve, the next generation will!