If you're walking down the street and see a plastic bottle or a bag, make sure to pick it up. Why? If it rains, or if someone washes their car and creates a mini-river to the nearest storm drain, that plastic bottle or bag ends up first in the Los Angeles River and then in the ocean.
This sort of pollution is referred to as "non-point source pollution" because it can't be traced back to any specific or particular source beyond our random, collective carelessness. The time you are offered a plastic bag at a grocery store, think twice before accepting it. Unless you are diligent about recycling that bag will likely end up in the river. When the rains come and the current is up, the bags will twist and knot around the trunks of willows, slowly choking them to death.
Below are several audio stories from Alicia Katano explaining the issue of pollution in the Los Angeles along with a slideshow showing various examples of non-point pollution.