The avalanches and major landslides called huaycos in Peru are analysed as large volumes of mud flows, loaded with boulders, vegetation, etc and as materials nonconsolidated at surface which on saturating acquire great mobility, moving through small valleys of temporary or dry regime that are subject to heavy, intermittent rainfall and have steep slopes and scarce vegetation. The causes why these disorders occur in the weathered rock slopes are established. Some result of measures are mentioned and analyzed in the great landslide that occur in the Mantaro Dam. -from Authors