We present a geometric, sediment mass-balance model for the interaction of axial and transverse alluvial systems in a subsiding basin. By comparing the model result with a flume experiment that employed a simplified half-graben tectonic geometry with axial and transverse sediment sources, we quantify rates of axial-transverse erosional sediment mixing. In the experiment, the lateral migration rate of the axial-transverse boundaries due to the sediment mixing scales with sediment supplies delivered by transverse drainages, but not with water (or sediment) discharge from the axial channel or with tectonic tilting rate. Using an empirical lateral erosion rate, the model shows how sediment supply partitioning among the axial, hanging-wall, and footwall drainages controls the width and the location of the axial-channel belt. Comparing the modeling results with field cases demonstrates that transverse sediment fluxes could slow the axial-channel migration or even reverse the movement against the tectonic forcing.