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Once the trickle becomes a stream and then a proper river, it becomes vulnerable to direct pollution. Few upper river catchments have a lot of industry, so the main pollution comes from farming. Upper river valleys are typically given over to grazing and pasture, rather than cereals, and hence it is livestock farming practices that pose a threat.

Perhaps the worst is the release of slurry into rivers, depleting their oxygen and destroying their biodiversity.⁴ There are two principal ways this can happen. First and most reported is the failure of slurry-holding pits and tanks, usually as a result of poor maintenance. Such incidents are surprisingly common and often devastating. But there is also the spreading of slurry: while the intention is to retain the liquid manure on the fields to promote grass growth, it can nevertheless run off into streams and rivers. This is particularly problematic if the slurry is applied in winter to frozen fields. The frost makes it possible to get tractors and machinery onto the land, but it also forms a barrier to absorption. The lethal (for the fish) combination is slurry-spreading on frozen ground, quickly followed by heavy rain.

Slurry, badly managed, is a serious threat to the natural environment, but it is not the only way in which animal husbandry can adversely impact on rivers. Sheep-dipping to tackle a range of parasites, worms and foot rot is another detrimental element of (non-organic) farming practices, and it involves water. The residual liquids, after the dipping, have to go somewhere, although their disposal is regulated. Too often this has been out of sight in the watercourses, with sometimes devastating results.⁵

All these activities can be mitigated, often at minimal cost. The release of slurry and maintenance of slurry tanks are already subject to regulation. Spills are illegal and the problem arises not from the regulation but rather the inadequate penalties and enforcement. As budgets have been squeezed, the Environment Agency, Natural Resources Wales, and the Scottish Environment Protection Agency (SEPA) have retreated from the effective policing of the rivers. Catchment system management requires catchment system regulation and enforcement. A large number of incidents go unreported, or when they are investigated it is often too late to identify the source. The fines are clearly not a serious deterrent to farmers. Properly resourced policing and significant fines could all but eliminate these sources of river pollution. The use of drones and new advances in digital technologies to detect diffuse pollution will help to transform detection. There would be costs to the Environment Agency and the other bodies, but the balance of the damage versus these costs points towards more enforcement. The polluters may be fairly diffuse, but the pollutees are diffuse too, all down the river. Slurry in rivers can and should be stopped. Diffuse pollution should be limited. Both make good economic sense. The fact that the costs of the damage may well exceed the value of the total economic output of the farm tells us a lot about the perverse economics of much farming practice. Pollution is under-priced; agricultural output is therefore also under-priced.

Once the river gets to its middle stage, the ratio of grassland to arable land usually shifts towards arable. Conventional arable farming adds several layers of pollution and stress to rivers. It uses intensive fertilisers, pesticides and herbicides, and as with maize and the Somerset Levels example, it leaves the soils exposed to run-off and the depositing of silt in the rivers. It may also take water from the river for irrigation.

Farmers operating in these middle river areas are themselves vulnerable to the flooding that their activities can help give rise to, and hence they want to get the water quickly off their land and into the river, so the rate of run-off is artificially increased by ditches that take the chemical cocktails directly to the river much faster and hence in more concentrated forms. This raises flooding risk downstream, exporting the dangers to others. This was a process once managed through water meadows and vegetation cover along the rivers, in part because this made economic sense in a predominantly mixed farm system, but also because ploughing up riverside meadows required heavier and more powerful tractors and machinery. That can now be done. Once flooding was a resource for farmers to exploit in order to enrich their land. Now it is a menace to get rid of as quickly as possible.

The results are economically very inefficient and the economy would be much better off if many of these practices were curtailed or even stopped altogether. The central issue from an economic efficiency perspective, which we keep coming back to, is that some farmers are polluters who do not pay for the damage they cause others. Instead, they are polluters who expect to be paid not to pollute.

As discussed in greater detail in the next chapter, this should be reversed. If farmers paid for the pollution they caused, they would use chemicals in smaller quantities and target them more accurately. In the case of flooding, if farmers paid for the services that the river provides in taking excess water away, the ditches might not be so deep. They could also store more water. Finally, in a polluter-pays model, silt exported to rivers from riverside ploughing and cropped fields would come with a bill, and fewer of these fields would be ploughed.

With a polluter-pays policy, and hence the right relative prices, the rivers would be in a much better state. Biodiversity would go up, abstraction would go down, more ponds and reservoirs would be created, and the land would be wetter, especially in winter. It is all just good economics, and leads to a much more sustainable farming industry.