By specifying an extra boundary condition in terms of hydraulic gradient at the stream-aquifer interface, and using the Green’s function of the linearized Boussinesq equation, analytical solutions have been developed to determine aquifer diffusivity explicitly. Four different types of Dirichlet boundary condition describing stream gage hydrographs have been employed. They are: (1)Linearly rising water level in the stream; (2)exponentially rising water level in the stream; (3)water level represented by a sinusoid; and (4)hydrographs approximated by cubic splines. The last one is quite general and is capable of analyzing different shapes of smooth stream hydrographs expected to occur in nature. Hydraulic gradient at the interface was computed using splines. The solutions were validated by analyzing the stream-aquifer data observed in a sand tank model. They yielded consistent values of the aquifer diffusivity at different times, and therefore are useful in determining the parameter of the aquifers contiguous to streams.