This paper presents an analysis of water flow and flow velocity in an open channel of rectangular cross section as a function of channel roughness, ie Manning roughness coefficient. Three different cases of finishing the open channel of rectangular cross-section were analyzed, namely the concrete channel, the channel with smoothed cement mortar and the channel in very poor condition with shore erosion overgrown with sedges and large stones at the bottom of the channel. Analyzes were performed on a rectangular channel 3.0 m wide and with water depths in the channel from 0.5 m to 1.5 m with a step of 0.1 m, for steady flow. The longitudinal fall of the bottom of the rectangular channel in all analyzed cases was 1%.u urađene na pravougaonom kanalu širine 3,0 m i sa dubinama vode u kanalu od 0,5 m do 1,5 m sa korakom od 0,1 m, za ustaljeno tečenje. Podužni pad dna pravougaonog kanala u svim analiziranim slučajevima je iznosio 1%. The paper showed that higher Mannig roughness coefficients give lower water flow in the canal. The importance of the paper is reflected in the fact that for each of the three analyzed treatments of the rectangular channel, a quadratic function of water velocity and flow depending on the water depth in the channel was obtained. During the hydraulic calculation, the type of lining of the open rectangular channel should be taken into account in order for the dimensioned channel to be the most favorable in terms of maximum throughput.