Aim of the study:
The aim of the study was to determine the applicability of the Colebrook-White equation with modifications to its constants K1, K2 and K3, proposed by various authors of the research on water flow resistance in open channels, taking into account the influence of the grain roughness of the mountain stream bottom.

Material and methods:
Based on the field measurements of the water flow velocity, the water flow rate Q was calculated, and then the average Manning roughness coefficient was calculated using the formula developed by Barnes. This coefficient made it possible to calculate the roughness coefficient determined from the transformed Chézy-Maninng equation. The calculated drag coefficient was compared with the results of the calculations from the Colebrook-White equation with the constants K1, K2 and K3.

Results and conclusions:
As a result of the measurements, the calculated roughness coefficient for this stream is 0.044 m-1/3∙s. It was found that from the Colebrook-White equation with the constants K1, K2 and K3, determined by Keulegan and by Rouse, for the absolute roughness value (ks) in the range 0.063 – 0.066 m, which was calculated by the Thompson and Campbell formula and the Irmay formula, it is possible to obtain the value of the resistance coefficient for which the roughness coefficient is the closest to that obtained from the measurements. The roughness coefficient of the tested section of the Racławka stream, calculated using this modification of the Colebrook-White equation, is within the range of 0.040 - 0.048 m-1/3∙s.