We present the results of an experimental study of dynamical phenomena in the fluid flow through a symmetric nominally two‐dimensional channel expansion. The particular phenomena of interest are observed as a result of modulating the flow rate through the system. In the absence of modulation, the flow downstream of the expansion is found to be steady at low Reynolds numbers. It then loses stability to a high‐dimensional dynamical state for Reynolds numbers above a critical value. However, when a small periodic modulation is added to the flow rate new low‐dimensional dynamical phenomena emerge. In some parameter ranges this novel temporal behavior arises at Reynolds numbers below those at which the transition to irregular fluid flow occurs in the unforced system. In other regions of parameter space the low‐dimensional dynamics suppress the irregular flow of the unforced system. Moreover, the dynamics are organized by the underlying solution structure and show evidence for a subharmonic resonance, quasiperiodicity, and homoclinicity. ©1995 American Institute of Physics.