The surfaces of large rapidly grown KH2PO4 (KDP) crystals can exhibit the formation of valleys in addition to the well-known growth hillocks and associated intervicinal boundaries between them. Understanding the mechanism of formation of these valleys is extremely important because they are observed to be precursors to the formation of solution inclusions on the crystal surface. Ea situ measurements using a precision coordinate measuring machine (CMM) have been made of the surface topography of rapidly grown KDP crystals. These measurements clearly indicate that these valleys are formed by the "bending'' of macrosteps originating from a single growth hillock. Results from numerical simulations of the hydrodynamics and mass transfer for the conditions used in the rapid growth process show that the surface supersaturation held generated on these crystals is inhomogeneous due to the spatially and temporally varying boundary layer thickness on the rotating crystal surface. These numerical results are used together with a simple model for the propagation of steps in an inhomogeneous surface supersaturation field to simulate the resulting crystal surface features. The qualitative agreement between the surface measurements and the numerical simulations is shown to be quite good. The dependence of the valley formation on parameters such as crystal growth rate, rotation conditions, growth hillock location, and impurity level are studied.