Side weirs are structures that are installed along a channel and when the height of water is exceeding the crest, extra flow inters to the subsidiary channel. In this research, a solution for differential equation of water surface profile over trapezoidal broad-crested side weir in rectangular channels based on the Haar wavelet transform method is reported. Also, the water surface profiles for different width, height and side slope of the weir were measured and compared to those calculated by using the Haar wavelet method. The Lowest error percentage in calculating y1 (maximum error) was observed in the side slop of side weir of 1.5, width of 8 and height of 10 cm. When height and side slope of the side weir were increased, the accuracy of water surface profile estimations using the Haar wavelet method was decreased. The Haar wavelet method is also compared with Runge-Kutta Method. The correlation coefficient of Haar wavelet method and experimental data was more than 0.979 which was greater than the calculated one for the Runge-Kutta method. This result indicates that the Haar wavelet method can estimate the water depths with higher accuracy than the Runge-Kutta method.