Euthrophication is caused by excess nutrient run off, such as nitrogen or phosphorus. A common chemical that generally cause euthrophication is fertilizers from farm, if there's heavy rainfall or floods, they're easily washed into the rivers and lakes to cause euthrophication.
Euthrophication generally cause a sudden large scale algao bloom, or other plants, and they may cover the whole surface of water. Once these plants die, decomposers such as bacteria decompose them and take in oxygen for respiration, such that animals from the river originally may die due to lack of oxygen.
