In this study, the level of selected organophosphorus and organochlorine pesticide residues from water samples of Gilgle Gibe (I) hydroelectric dam reservoir and its potential tributaries, Jimma zone, Ethiopia, was determined by gas chromatography-electron capture detector (GC-ECD). Low density based dispersive liquid-liquid microextraction (LD-DLLME) using toluene (as extractant) and acetonitrile (as disperser) was used for extraction of pesticide residues from the samples. Calibration curves constructed at six concentration points have good linearity with coefficient of determination (r2) ranging from 0.995 - 0.999. The limits of detection (LOD) and quantification (LOQ) of the method which were determined as 3 and 10 times the signal-to-noise ratio were ranging from 0.0001 - 2.5810 µg/L and 0.0005 - 8.6050 µg/L, respectively. The efficiency of the method was also evaluated using recovery studies by spiking the water samples with known concentrations of the analytes. The obtained recoveries were ranging from 67 - 105% with relative standard deviations of 0.79 - 12.5%. The findings revealed that the studied water samples contain significant amount of the target pesticides, but endrin was not detected in any of the water sample. Methidathion was also detected only in Nada Qalla and Nada Gudda river water samples. The detected residual concentrations of the target pesticides were above the maximum residue limits, except DDT in acute toxic level. The finding indicated that the studied water samples contain considerable amount of the studied residual pesticides that can influence the health of aquatic organisms and other consumers.