Suitable soil and water resources in the great Karun river basin, extensive areas of natural resources, favorable climate conditions and energy resources have led to the development of agriculture, industry and population growth in the Karun and Dez rivers margins. This study aims to identify, quantitatively and qualitatively review and determine the pollution load of agricultural drainages in the basin of great Karun (Karun and Dez rivers), determine the volume of drainage water and the impact on the quality of production resources. After field study, sampling (N=96) was done during four seasons in 2013-2014 in 24 input points of drainage water to water sources. The EC, pH, TSS, NO3^-, DO, BOD, PO4^-3, COD features, Cations, Anions and discharge were measured. Results showed that agricultural pollutants with a volume of 2,374 million cubic meters per year are causing pollution of types TDS and NO3^- with 11862 and 65.51 tons per day, respectively. Pollution load of organic materials based on BOD and COD is 29.7 and 211 tons per day, respectively. Results also showed that, the Dez river reach has the largest share in terms of volume of drainage water and incoming pollution load. Moreover, the agricultural drainages of Shoeibieh, Haft –Tapeh Sugar cane, Ajirub and Salimeh, Karun (K), Myanab and Kharur within the Dez river reach, the drainages of Sardarabad (N) and Zahuabad within the Shatit reach and the fish-farming wastewaters within the Gargar river reach are among the most important drainages affecting the quality of water resources and agricultural lands in the downstream basin.