The introduction of a sedimentary balance sheet in a lake environment

摘要

The flood of a granitic relief in Limousin formed a reservoir with irregular outlines: the lake of Saint-Pardoux consists of five basins separated by straits. The average dimensions of this lake (324.21 ha, volume of 21.97 Mm(3), 16.70 m of maximal depth) make a natural laboratory of it, a model for experiments of limnological geography. An experimental protocol, based on observations and measures but also on the use of materials as sediments traps, gives significative data on the morphological evolution of the lake. It allows a quantitative study of sedimentological balance. Studies about erosion give the following results: 61.3% of the coast are in recession. The average rates are located between 0.2 and 27 cm.yr(-1). The period of the most active erosion takes place in the first ten years of the lake. The volume of eroded sediments was estimated at 4270 m(3). The study of the sedimentation shows a sedimentation's gradient which decreases from upstream to downstream. It results from various parameters to which belong the impact of hydrodynamics, the surface and the morphology of the lake. Biogenous sediments represent the majority of deposits. The average rate of sedimentation of the lake is of 2.9 mm.an(-1), which classifies Saint-Pardoux in the global norm of oligotrophic lakes. This study allows to establish the first real sedimentary balance which concludes, in terms of quantity, in the superiority of autochtonous contributions (72.4%) compared with allochtonous contributions. The lake, producer of sediments, exists itself and must not be considered any more as disturbing element of a river system.