> but there is a lot of farm land also that feeds into those rivers.

Depending on what's on the farm, you have more or fewer problems. If the farm has farm animals, you have biological waste - manure and urine - in the water. If it's farmland, you have fertilizer run-off in your water.

Biologically contaminated water can be made drinkable by boiling or by chemical purification (generally iodine or chlorine) or by filtering. I'd suggest googling
water purification
and reading through the search results for a great deal of in depth information. Louise and I bought a ceramic filtration system, and we'll use that and Britta carbon filters for the water we plan on getting from a stream.

Chemical toxins cannot be removed by any of those means. I'd suggest finding the department in your city or county that deals with water quality of the surface water and asking them specifically about your rivers. Often the people who work there chose that job because it's their passion, and they're often very happy to have someone ask about it so they can discuss it. Ask them what is in the water (if they know specifically) and what you should do to treat it. I'll bet they know.