An analysis is provided for the spatial variation of the strength of secondary circulation in laminar flow through curved channels. The process of interaction between the main and the secondary flow is definitely shown through experiments. A mathematical model for computing the longitudinal variation of secondary circulation is developed reflecting the effect of the interaction. Experimental results support this model. Because of the important effects of secondary circulation on the river flow profile, techniques used in this model will provide usefull improvements in computational methods for river flows.