The paper deals with the problem of rebuilding the existing water straight drop structure in Brenna on the Brennica river (Polish Carpathian mountains), which was changed into the rapid hydraulic structure. The technical project was set up in 1988 and finished in the same year. The structure was rebuilt in the field in the early autumn of 1990. One of the concepts of applied fluvial geomorphological solution was used to improve the river channel bed condition. In that case it was found that the existing hydraulic structure reducing river slope and stabilizing river bed can be changed without any harm in to semi-natural riffle structure which could be tolerated by river and organisms living in. Artificial roughness of the slope plate of the rapid hydraulic structure was obtained by placing cobbles along all the slope apron of the structure. The diameter of cobbles was calculated applying various methods, and the optimum value for that dimension was chosen. The cobbles, used for rebuilding purposes, were taken directly from the riverbed, so that the structure is environmentally similar to the site. All work was done due to European Framework Directive for Rivers.