The ecological state of a river is an expression of the quality of the structure and the behaviour of the aquatic organisms. The ecological state concerns water, flora and fauna.
The degradation of the ecological state of a river is a consequence of human activities, such as industry or agricultural activities. The main results of these alterations are a lost of biodiversity and water pollution.
One of the indications of a bad ecological quality is the presence of toxicants in the water. Here you have more information, as well as the task. Good job!