Koga Earth dam is one of the earth dams in Amhara region. It is a potential source for the Farmers and the region in general, but recently an evidence of early upstream riprap degradation is observed which will be a cause for the failure of the dam. The findings of the study shows, the design size of Koga dam upstream riprap was under estimated so that the riprap stones move one over the other down the slope of the embankment that intern creates colliding and initiating the development of the riprap cracks. The laboratory analysis indicates that the catchment soil and the reservoir water of Koga dam have got chemical reactions forming carbonate and phosphate salts. These salts are produced from the chemical reaction of the divalent elements forming the riprap rock and the carbonate and phosphate ions accumulated in the reservoir water. From which the carbonate salts exist in high abundant than the phosphate salts. When the rock forming divalent elements react with salt forming ions from the reservoir water, the internal bond of the riprap rocks lowers and aggravates the weathering of the riprap rock. In addition to this, the soluble salts created by this reaction (CaCO3, FeCO3 and Ca3 (PO4)2) are dissolved by water from the surface of riprap rocks. This also facilitates in losing the strength of riprap rocks.