Dirt also kills stream critters when it settles to the bottom and buries them. And it blocks light to underwater plants, so they die too. Foam or suds. Some foam in the stream is natural. But if you see foam that is more than three inches tall, looks like bubblebath and doesn't break apart easily, soap may be in the stream.
Soap can come from people's homes, factories or car washes.
Soap harms stream critters because it breaks the surface tension of the water so insects like water striders sink and drown. A chemical smell can mean harmful chemicals are polluting your stream. A rotten egg smell can mean sewage is getting into the stream from cows, sewage treatment plants or people's homes. Sewage or chemicals can make people and animals sick.