One effect of the intensification of farming since the 1980s has been a decline in soil health.
On average, long term arable fields now have half the level of organic matter (soil carbon) of grassland. This has led to a marked increase in the rates (4 to 20 times those of the early 70s) of soil being washed off fields and into streams and rivers.
In September 2012 over 200,000 tonnes of soil left the Lugg in just one flood, the equivalent of 64 acres of top soil. Rivers have always conveyed a certain amount of soil to the sea but the rate at which it happens nowadays is now detrimental to their ecology.
The soils in Herefordshire and Monmouthshire are especially prone to erosion meaning that the Wye and Usk have always looked muddy in a flood. To the naked eye the issue isn’t immediately obvious. 0.5gm of soil in a litre of water will give you a thick brown river. Increase this to 4gm/l per litre and the water looks the same but is much more damaging for aquatic life.
What’s the problem for fish and other aquatic species?
Most of the fish species of the Wye & Usk lay their eggs in gravel. The eggs rely on water flowing freely through the gravel to bring oxygen and take metabolic wastes away. If the gravels are filled with soil, the flow of water to the eggs is reduced and the eggs suffocate and die.
The soil-filled gravel also supports fewer invertebrates. To compound the impact, the soil also brings with it damaging chemicals, especially phosphate and pesticides.