Because of the large physical size of typical boiling-water power-reactor cores, there is a possibility of transients in the spatial power distribution. The vertical coolant flow produces a strong undirectional coupling between the power in the lower and upper parts of the core. This situation is qualitatively analyzed by means of a highly simplified two-node reactor model. The additional assumption that the effective delayed-neutron period and fuel-element thermal time constant are equal makes possible a nonlinear graphical solution of the problem by the parametric trajectory method. In the illustrative numerical examples, the spatial power-distribution transients are mild.