Peralada is bathed by the rivers and tributaries of the Llobregat, the Anyet, the Orlina, the Muga and the Mill irrigation canal. They hide in its waters, secrets that can reveal the health of this river. In a participatory workshop we can discover the animals that hide under the water and between the stones in a very simple and practical way. We can recognize how they are and what they are, how they live and what they do on the river. Finally, and classifying with very practical identification sheets we can determine the good quality of the rivers of Peralada.