Leaching describes the "washing out" from soils any soluble chemicals that are not "bound" to the soil particles. It occurs as excess rain (or flood) waters drain through the soil.
Farmers are frequently at fault by adding excess fertilisers (usually nitrates or ammonium compounds, containing nitrogen, which plants use for making proteins). It is these salts that are most likely to be leached and so pass into streams, rivers, lakes etc...
These salts are normally in short supply and hence act as "limiting factors" restricting the growth of plants in such bodies of water. In the presence of leachate, however, the algae grow in huge numbers causing an "algal bloom" . The salts are soon used up and so most of the algae die and sink to the bottom.
Here they are decayed by saprophytic bacteria (and fungi). The bacteria therefore increase in numbers and, as the decay (digestive) processes are energy requiring, the bacteria use "dissolved oxygen" for their respiration.
This may result in the death of all other oxygen requiring organisms, such as fish, water fleas etc... Also, even large flowering plants growing in the water may die.
Thus the bodies of water become lifeless except for the bacteria which can survive the anaerobic conditions. The whole sequence of events caused by the extra salts leading to death of organisms is called "Eutrophication".
Eutrophication may also be caused by sewage / slurry etc... which gets into the water supply because these too contain nitrate rich chemicals. (Washing powders containing phosphates can have the same effect as phosphorus, also a limiting factor).