What do all these things have in common? They are all potential sources of water pollution, Obviously, the wastes from factories, bathrooms, farms and streets are not the same. However, they all add things to the environment that would not otherwise be there, and that is the definition of pollution. Pollution also includes adding more of a natural substance (e.g. salt) to the environment (air, land, water ....) than would normally be found there. Either of these kinds of pollution can harm organisms. Although pollution to the land, air and water all affect our watersheds, we will focus on water pollution, since that can directly affect salmon.
Many factories discharge chemicals and water to streams, rivers and lakes. Bathroom waste (sewage) goes to a wastewater treatment plant and the treated waste water is discharged to a surface water body. Rain water washes sand, salt, oil, etc.... from streets into a storm water sewer system which eventually dicharges to a surface water body. The pollution from these sources is called point source pollution because the wastes are discharged through pipes that can be seen and pointed to.
There are also sources of pollution that we cannot point to, or tell exactly where they started, because they don't come out of pipes. This type of pollution usually gets into surface water bodies either in storm water runoff (water that flows across the ground surface) which washes it off the land; or through groundwater which eventually discharges to surface water bodies. We might be able to tell what type of place the pollution came from (for example, a farm), but not the exact location. This type of pollution is called non-point source pollution. On a farm fertilizers and pesticides are put on crops to protect them and help them grow. When it rains, or the crops are watered, some of the pesticides and fertilizers get washed off the fields and end up in nearby water bodies (streams, lakes, etc....). Some pesticides and fertilizers will also soak down into the ground with the water, and end up in the ground water. In either case it is non-point source pollution and it impacts our watersheds. Some other sources of non-point source pollution are yards, gardens, parking lots and landfills (dumps).
People used to think that it didn't matter what they put into nearby water bodies because they thought the water could "clean itself." Water does have a good buffering capacity, that is it can break down some pollution and adjust the acid level (pH), but water bodies also have a limit to what they can take. Think of a sponge - it can absorb a lot of water, but once it has absorbed as much as it can hold, can you make it absorb more? (Try it) The same thing happens with rivers and streams. They can only tolerate (hold) so many chemicals, sediments, salts, etc.... before the water quality starts to decrease. The effects might be noticeable, like the water getting cloudy or tasting bad; or the water might look fine but have unknown chemicals in it. Sometimes it takes special tests to find out if the water is okay.