How to prevent river pollution?

Question

such as detroit river.

Answer 1

Remediate the existing river (clean it up).  Keep industrial spills from reaching the river.  Keep campers and other visitors from leaving items on the banks, as it gets into the water.  Plant trees so that runoff doesn't take possibly contaminated soil downstream.  Add fish and other species that "clean" the water.  Add water plants that are slow growing and have been shown to clean water (NOT fast-growing ones).

Answer 2

Don't put junk in the river.