April 9, 1947 dam in Morocco is known for its waters intense rate of pollution and its considerable anthropogenic activity. Unfortunately, no former studies were made to investigate its real pollution degree. In this work, we present a method for mapping the vulnerability assessment and the risk of pollution of surface water stored in this dam. For this, we introduced a vulnerability index called IV considering three different parameters which are slope, drainage density and the lithology. The index IV serves to characterize the transfer of a pollutant from the upstream to the downstream of a watershed. This index is the product of these three parameters. The superposition of these parameters through GIS software gave us a map of contamination vulnerability in the watershed. This map shows three vulnerability states: areas of high (72% of basin area), medium (25%) and low vulnerability (3%). Using this map, we can establish a zonation of areas that should be protected to reduce the high rate of contamination in this the dam.