In 1984 the water authorities reported 2961 serious cases of freshwater pollution caused by farming, ca 75% by dairy farming. Liquor from insufficiently wilted silage, farmyard runoff and slurry washed off land or escaping from overflowing or leaking lagoons have all been implicated. The effects of these rich organic wastes on life in streams are described below.

Farm wastes can place an enormous demand on the dissolved oxygen (DO) content of the stream water. Cow slurry is 40-80 and silage liquor 200 times stronger than untreated domestic sewage, so that one volume of silage liauor is capable of completely deoxygenating 40 000 volumes of previously air-saturated water. The wastes also (i) increase turbidity, which reduces the effective penetration of light and thus restricts photosynthesis and primary productivity, (ii) smother the stream bed with a layer of decomposing silt or solids resistant to degradation and (iii) are toxic due to ammonia.