I’ve worked on wetland projects that used a bentonite liner. There are companies that package it in large 20′ wide rolls here in the US. If I remember correctly it needs to be placed on a layer of compacted sand. A supplier will probably have installation details. Have you found any bentonite suppliers there in Portugal?
Are you talking about building a dam? Altering the natural hydrology can really have unforseen outcomes. Sediment loading alone can create major changes (Erosion) downstream. The professions most skilled at figuring out these complex interactions may be in the engineering fields: hydrogeologists, for example. Large scale water manipulations require skill and experience beyond what you will be able to get from a forum like this. I would suggest you find a professional who has proven experience.