River deposits, in which dune cross-bedding is widely preserved, reflect temporal and spatial variations in flow and/or sediment transport conditions that are in turn strongly coupled to climate and/or human impact. Understanding these flux variations is critical in reconstructing paleoenvironments and predicting future changes. I introduce a new approach to quantitative interpretation of fluvial deposits by linking two recent theories that relate: (1) the geometry of the preserved dune cross-sets to the formative dune-bed topography, and (2) the active dune-bed topography to sediment-transport rate. On the basis of previously obtained experimental data, I propose a method to reconstruct the probability distribution of dune-bed surface elevation from the cross-strata preserved under net aggrading conditions. This approach is applied to an ancient fluvial-dune deposit, where the curve expressing the predicted morphodynamic conditions fits within results from experimental and modern natural data. The findings are potentially important for a wide variety of geosciences studies.