When I was walking in the countryside in the south of England some years ago I came across a river where half a dozen cows were wading upstream, grazing on the reeds that were growing in the water. I'm an unreconstructed townie and the scene struck me at the time as both bucolic and charming. It was just like a Constable painting. I didn’t consider for a moment the damage that those cows might be doing to the river, or the ways in which they might be polluting the water. That was in another life. Since then I have worked with scientists on the Demonstration Test Catchment projects and learnt something of how cattle poach riverbanks, resulting in sediment entering the stream, affecting the whole ecosystem of the waterway. I know that those cows were also depositing their urine and faeces directly into the river, and that this encourages the growth of some organisms at the expense of others, with effects that cascade through the food web, with serious consquences for fish and mammals. From a human perspective, if water was being abstracted for the public supply, it meant there were added costs for the water company and their customers. So perhaps this wasn’t such a charming sight after all. Should the farmer have allowed the cows to access the river? Wouldn’t it have been more responsible to fence them in? From the point of view of the consumers who have to drink the water, that’s probably true. I don’t much like the thought of water that has been polluted by cattle coming out of the tap in my kitchen, even if it has been purified. But fencing is expensive, and if the cows can’t drink from the river, they will also need a drinking trough. Who is going to pay for that infrastructure? It’s a big investment for any farmer. But, at the moment, the water company is paying to cleaning up the supply and passing the cost on to consumers. Perhaps it would be more efficient and sensible for the consumer to pay for some fencing to keep the cattle out of the river? As with most things in life, somebody somewhere has to pay, and perhaps we need a more high-profile debate on where the costs should fall for this kind of public good.