Question

3. What would happen to the dissolved oxygen in the Missouri River if large amounts of waste were dumped into it?

Answer 1

After the waste is dumped, because of the huge increase in organic substrate this creates, the plankton start reproducing greatly, this initial bloom uses up and depletes the oxygen dissolved in the water.
For that reason, after a great amount of waste is dumped in a body of water the dissolved oxygen tends to get greatly reduced or depleted.
That is actually a really simplified explanation, but I hope it helps.

Answer 2

"What would happen to the dissolved oxygen in the Mississippi R…" http://openstudy.com/updates/4f96f5aae4b000ae9eccd0a6