An analytical procedure is presented to solve the problem of identifying aquifer diffusivity in an unconfined aquifer and stream interaction system. The fluctuation of the aquifer head in response to a flood wave is used as observations. The governing nonlinear partial differential equation is replaced by a system of nonlinear ordinary differential equations for which the technique of quasilinearization is applied. The procedure is straightforward and converges quadratically. It requires neither graphical matching nor trial and error manipulations. Numerical experiments are presented and compared with published experimental data.