ABSTRACT
The hyporheic environment is made up of the interstices of alluvia deposited on the river bed. It is a more stable environment than the superficial horizon of the substrate: the deep layers are reshaped only during exceptional floods. The surface water penetrates the hyporheic environment by infiltration and, at the thresholds, by a sub-flow. Stygobious fauna of subterranean origin consists of species that can colonize the hyporheic environment and the deep ground water, and that have characteristics of life in darkness. Two essential factors determine the distribution of fauna in the hyporheic environment: the origin of waters and granulometry of the substrate. There is a close correlation between the distribution of hyporheic fauna, its diversity, and the granulometry of the substrate. The granulo-metry determines porosity, that is, the percentage of spaces occupied by water in relation to the total volume of sediments.
