ABSTRACT
Since the second half of the 19th century, the growth of towns and cities and industrial development have led to the use of rivers and streams to drain away waste products. The power of a hydroelectric plant being proportionate to the flow and fall height, these two factors are increased by diverting the water, at the foot of the reservoirs, by means of pressure pipelines that restore the water to the river downstream, at a lower altitude. The mineral salts resulting from biodegradation are at the origin of algal and macro-phyte synthesis downstream, particularly nitrogen and phosphorus salts, which are rare or absent in the rocks of the watershed. Pioneer organisms and ecosystems are characterized by the predominance of abiotic factors, which confirms the major role of seeding: recolonization of the upstream sector from eggs laid by insects with flying adults and plankton seeding.
