We designed and conducted experiments in a heterogeneous sand pack where gravity-destabilized nonwetting phase invasion (CO2 and trichloroethylene) could be recorded using high-resolution light transmission methods. The heterogeneity structure was designed to be reminiscent of fluvial channel lag cut-and-fill architecture and to contain a series of capillary barriers. As invasion progressed, nonwetting phase structure developed a series of fingers and pools; behind the growing front we found nonwetting phase saturation to pulsate in certain regions when viscous forces were low. Through a scale analysis we derive a series of length scales that describe finger diameter, pool height and width, and regions where pulsation occurs within a heterogeneous porous medium. In all cases we find that the intrinsic pore-scale nature of the invasion process and resulting structure must be incorporated into our analysis to explain experimental results. We propose a simple macroscale structural growth model that assembles length scales for substructures to delineate nonwetting phase migration from a source into a heterogeneous domain. For such a model applied at the field scale for dense nonaqueous phase liquid migration, we expect capillary and gravity forces within the complex subsurface lithology to play the primary roles with viscous forces forming a perturbation on the inviscid phase structure. ¿ 2000 American Geophysical Union