Near a lot of farming communities, there is another problem with water. The nutrients that farmers add in fertilizer are, mostly, water soluble. They run downstream and often end up in lakes, where the extra fertilizer makes algae run wild, until the algae has choked out all other life in the lake. When that algae dies, their rotting corpses suck all the oxygen out of the lake. This is called a Eutrophic lake. An overuse of minerals, which would normally be a good thing, has turned into a bad thing by killing off everything in the lake. The lake is then a smelly pit of half-rotten algae.
I propose that this algae be farmed, with humans or robots removing large portions of it on a regular basis, and using this algae as compost, therefore not requiring quite as much fertilizer. It may also be possible to squeeze oils from this algae, and the oils could make biodeisel to run the tractors.
This is potentially giving me another idea. My finals will be done on Friday, and I can develop the idea then.