This is the process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates. These typically promote excessive growth of algae. As the algae die and decompose, high level of organic matter and the decomposing organisms deplete the water of oxygen, causing the death of other organisms such as fish.
Eutrophication is a natural, slow-aging process for a water body but human activity greatly speeds up the process e.g. through much use of agricultural fertilizers that contain nitrogen and phosphorous which get carried away in run off water and in to waterways.