PRESENT STUDY DEALS WITH THE PROCESS OF FINE PARTICLES ENTRY IN THE MATRIX OF A GRAVEL BED, WHICH IS KNOWN AS COLMATION. THE SPATIAL PATTERN OF DEPOSITION IS STUDIED THROUGH EXPERIMENTAL MEASUREMENT IN AN OPEN CHANNEL USING A VIDEO CAMERA. THE RESULTS SHOW THAT IN GENERAL DEPOSITION OF FINE PARTICLES ARE IN AGREEMENT WITH BED TOPOGRAPHY. SPECIFICALLY, FINE PARTICLES CANNOT SETTLE DOWN IN THE UPSTREAM SIDE OF THE GRAVEL PARTICLES, WHILE DEPOSITION IS COMMON IN THE DOWNSTREAM SIDE OF THE GRAVEL PARTICLES. MOREOVER, THE RESULTS SHOW THAT LONGITUDINAL SAND BARS ARE FORMED IN WHOLE CROSS-SECTION. FORMATION OF SUCH LONGITUDINAL BARS CAN BE EXPLAINED BY SECONDARY CURRENTS. THE COMBINED EFFECTS OF SAND RIBBONS AND BED TOPOGRAPHY LEAD TO THE COMPLEX SPATIAL PATTERN OF DEPOSITION. THIS MEANS THAT SOME AREAS ARE AREA PREFERENTIALLY DEVOTE TO THE DEPOSITION. THIS FACT QUESTION APPLICABILITY OF COMMON TRANSPORT THRESHOLDS, LIKE SHIELDS OR ROUSE NUMBER, WHICH WERE DEVELOPED BASED ON THE BULK PROPERTIES OF THE FLOW LIKE SHEAR VELOCITY.