High-resolution topographic data reveal that meandering river floodplains can contain complex floodplain-channel networks. However, the influence of this topography on flood initiation and progression is unknown. In this study, we investigate how floodplain-channel networks influence flooding processes in low-gradient river systems. To accomplish this, we conducted a series of numerical modeling experiments using a two-dimensional (2-D) model built in Hydrologic Engineering Center’s River Analysis System (HEC-RAS). First, we simulated floods using 2-D HEC-RAS on the East Fork White River near Seymour, IN, USA, including the current, dense network of floodplain channels. Next, we simulated synthetic versions of this river system where we varied the connectivity among floodplain channels, and also between the floodplain channels and river channel. We found distinct differences in flooding patterns and flood hydraulics among model simulations. Numerical modeling experiments showed that increasing floodplain channel connectivity caused increased spatial variability of wet and dry surfaces across the floodplain, increased residence time of water on the floodplain, and increased floodwave attenuation. Results from this study indicate that topographic connectivity on the floodplain challenges the classic notions of flooding initiation, and potentially increases a floodplains ability to store matter.