Ecologie d'un torrent Pyrénéen de haute montagne I. - Caractéristiques physiques

Abstract

The physical conditions in a high mountain stream, Estaragne and its tributaries, were analysed in relation to the general geographical conditions.The snow cover over the stream lasts 4 to 7 months according to altitude.Flow rates are constant and low under the snow in winter, high during the thaw (with some current velocities up to 2.50 m/s), and show sudden variations after the snow has melted. At the time of floods, the discharge reflects the nature and the force of disturbances as well as the capacities for retention in the catchment area.Temperatures are close to zero in winter. They rise with the melting of the snow and consequently fluctuate daily, with insolation having the greatest influence on these changes. Both temperature and thermal amplitude increase from source to mouth, but are continuously influenced by the addition ot cold water. Over three years, the mean weekly temperatures attained maximum values of 4.2 °C at 2 350 m and 9.9 °C at 1 850 m altitude, with thermal amplitudes of 1.5 °C and 6 °C respectively. Although the annual number of degree-days at the mouth is twice that recorded near the source, is does not generally exceed 1 000.The smallest tributaries are the earliest to be free of snow cover, they show less strongly the occurrence of floods and are warmer than the main stream. © 1974 Masson.