A unsteady one-dimensional model can be used to study the interaction of two colliding streams for the formation of MHD shocks. If both &bgr; and &thgr; are small, the interaction evolves to a shock system consisting of slow shocks only. At increasing &bgr; and &thgr;, the interaction may evolve to a system consisting of both slow and fast shocks and eventually to a system consisting of fast shocks only. We study the merging rates of neighboring characteristic curves for compressive waves associated with the interaction region. We first study a linearized wave system produced by an initial small perturbation. The results show that at small &bgr; and &thgr;, the merging rate of neighboring characteristic curves for the slow-mode compressive waves is far greater than that for the fast-mode compressive waves. Then we use a nonlinear example to demonstrate that at small &bgr; and &thgr;, the slow-mode characteristic curves merge rapidly to form a pair of slow shocks, while there is no merging of fast-mode characteristic curves. The results explain why &bgr; and &thgr; have an important effect on the nature of shocks. ¿ American Geophysical Union 1995