Glad I was wrong about that, I enjoy your company on this forum.
I don't protect water quality for a living, instead it is a hobby. I do some volunteer work with a wildlife conservation organization, our primary solutions are mutually beneficial to water quality and wildlife habitat.
Your point about water quality being localized is certainly true but only if you ignore the factor of time.
Being from Iowa, I am well aware of the issues regarding agriculture and pollution. The Farm Bureau runs this state, the DNR (one of their roles is as the state equivalent of the EPA), is run by Farm Bureau connected bureaucrats. Between the FB, corporate interests such as Monsanto and greedy farmers(many of whom are corporate now) and with high corn prices (thanks ethanol subsidies), it has been an all out a rape-age of the natural resources in this state in the last several years not seen since we killed off all of the bison(there is a sign not far from where I grew up that commemorates the last bison killed in the state, makes me sick to my stomach) and drained the swamps(farming would not exist in Iowa without tiling).
My Grandmother owns a farm, the same farm I grew up on. She (compelled by the drainage district) and the other farmers in the area spent a lot of money to have the drainage ditch(water eventually ends up in the gulf) that runs through the farm dredged(pull out all the silt [read soil erosion] and make it more efficient at removing tiled water and runoff). The fall after the dredging, I was back home. I noticed that one of the farmers in the district just a mile upstream of the farm had V-Ripped his field so close to the drainage ditch that his last row was falling over into the ditch, as if the practice of tilling the field in the fall wasn't enough (wind erosion) to ensure that the soil ends up in the water canal.
The farmer in the above example is trying to squeeze one more row of corn into his field, but he's either an idiot or only thinking short term. It'll take a long time for that one more row to pay for the next dredge operation, more likely he thinks he is out-competing his neighbors who will also have to pay for his reckless behavior. Every time I go home there is another farmstead and/or grove of trees that has been bulldozed for more tillable acres.
In the short-term the tall grass prairies[envision Dances with wolves] that deposited the topsoil that created the agricultural "heartland", "breadbasket" of America, have been replaced by a sea of GMO crop fields that thanks to the chemicals sprayed on them are less bio-diverse than a desert. Long term, thanks to soil erosion caused by modern farm practices, they'll just be a desert.
It is indisputable that the Midwest of the US feeds the world, but for how long? It could easily be sustainable long term, with minor changes. The "Dead Zone" is only an omen far removed from the source.