Abstract: This study aimed to determine the grain size distribution trend of the Quaternary aged fluvio-lacustrinesediments of the Moralli Stream system, which is located within the borders of Van province to the east of Lake Van ,and to reveal the effects of hydrodynamic energy on the erosion, transportation ,and depositional processes. Moreover,it aimed to define the sedimentary facies ,and geomorphological characteristics of the depositional environment. Inorder to determine the grain size distribution trends of the sediments, a grain size analysis was carried out by usinga total of 42 sediment samples taken from the source area of the Moralli Stream in the area where it spills in Lake Van. It was determined that the grain size of the sediments taken from the stream source area was dominantlybetween 0.42-0.25 mm, and the grain size of the sediments taken from the downstream parts of the channel bed wasbetween 0.125-0.037 mm. The mean grain size of the fluvial sediments is fine-very fine, the sorting parameter isgenerally in the poorly sorted class, the skewness degree is strongly negatively skewed, and the degree of kurtosiswas determined as platykurtic. The grain size of the sediment samples taken from the delta plain is 4.75-025 mm,the grain size of the sediment samples taken from the delta channels is 1.00-0.25 mm, and the grain size of sedimentsamples taken from the delta front ranged between 0.074-0.037 mm. The delta plain sediments is a coarse sand withpoor sorting, positively skewed and mesokurtic, the delta channel sediments is medium-fine sand with poor sorting,positively skewed and platykurtic, and the delta front sediments is very fine sand with moderate sorting, negativelyskewed and leptokurtic. The grain size of the beach sediments was determined in the range of 1-0.42 mm. Whilesediments contained fine grains of 0.074-0.037 mm in size in the coastal area, especially in the low-energy foreshorearea, it was determined that sediments containing >4.75 mm in size pebbles were deposited in the rip channel andshore break areas. The mean grain size of the beach sediments of Lake Van is very coarse sand and coarse-mediumsand size, and is represented with moderate, moderate-well sorting, symmetric-positive skewness and leptokurticparameters. With the grain size and statistical parameter data obtained as a result of the sedimentological analysescarried out within the scope of this study, the hydrodynamic energy of Moralli Stream has the potential to carry sandsilt-clay size grains, and the wave energy of Lake Van has the potential to transport gravel and sand-size grains.Moreover, it was found that the sediment’s grain size decreased downstream in the drainage basin. In addition, thefacies of sediments deposited in fluvial, delta and lacustrine environments formed by the rise and fall of the waterlevel of Lake Van due to climatic changes were defined and geomorphological structures (coastal arrows, coastalspits, beach barriers, lake and river terraces) emerging from tectonic and hydrodynamic activity were determined