Do rivers upstream weirs have lotics or lentics characteristics?

Abstract

In France the multitude of weirs that exist in rivers could affect the quality of aquatic ecosystems. The current context of recovery of the good state of watercourses, dictated by the European Water Framework Directive, requires understanding of their impact, in order to address operations of river restoration. One of the risks connected to this kind of structure is the loss of lotics characteristics of streams, in favour of lentics characteristics. In fact, they cause a reduction in speed flow, favouring the formation of zones with lentic flow. Through the study of thermal stratification, the major discriminant between running and stagnant waters, the aim of this study is to understand the nature of rivers situated upstream weirs, using four study sites. Results show that the presence of a weir is not synonymous of substitution of the lotic characteristics of a watercourse. Lentic behavior of waters has only been observed when certain environmental and hydrological conditions have been met in the reach.