The alluvial plain of the Grands Plans (Valais'Alps, Switzerland) is located at 2460 m high. It rests on a substratum which contains a variable amount of bases. A great number of small rivers shift their course on this biotope. This is the reason why floods periodically arise. These are the main causes of the vegetation's dynamic. The vegetation is composed of a patchwork of specialized groupments, within them the rare Caricion bicolori-atrofuscae. The alluvial plain's vegetation is well known because of old studies, the oldest one goes back to 1920. The aim of the present work is to analyze the evolution of the vegetation. It shows that this vegetal patchwork is more a continuum than the sum of really distinct vegetation's units. The pieces of this patchwork are moving all the time on the plain and don't really change their composition. The relictual species of the Caricion bicolori-atrofuscae, like Carex bicolor, are increasing their status on the plain. It confirms that they are the best adapted to the local ecological conditions.