Garbage is a kind of pollution, but it is not the only kind of pollution. Scientists have defined pol- lution as anything that causes an ecosystem to become out of balance. Everything in an ecosystem depends on every- thing else in the ecosystem. Some living things depend on other living things for food. An example of this is trout and mayflies.
Trout eat mayflies and other macroinvertebrates.
If pollution causes the mayflies to die, then the trout would have less food and they could die too. When the trout die, some other macro invertebrates, like damselfly nymphs, will greatly increase in number because they are usually eaten by trout. Then the dam- selfly nymphs could starve because there is only enough food for a certain number of damselfly nymphs in the river. Pretty soon, if the river stays out of balance, there are only a few kind of animals that can live there. You might be amazed at some of the kinds of things that can pollute a river.