In 1996, a compound gauging weir was built in Villaveta Township (Navarra, North of Spain) in the river Erro (Ebro River Basin). Before and after its construction, fish and physical and chemical variables were examined to determine weir’s impact on the reach. Fish captured by electrofishing were measured (total length), and population density and length distribution of the species were estimated. Species diversity (Shannon’s diversity index), dominance (Simpson’s dominance index) and evenness were also calculated. The results revealed an increase in fish population density, especially in smaller specimens, but a decrease in the larger fishes. Substrate and shelter alterations could explain the observed variations in the fish community. In order to improve the consolidation of well-developed fish populations, the placement of great boulders and the restoration of the riparian vegetation are suggested.