A widely recognized phenomenon of modern and ancient river systems is downstream decrease in grain size. Over the past two decades, theoretical formulations, observations of modern rivers, flume studies, and numerical models have significantly increased our understanding of sediment transport in mixed-grain-size river systems. These have established that downstream fining in modern and ancient rivers can be attributed to a combination of selective sorting, abrasion rate, and accumulation rate. However, no detailed multiple-grain-size sediment transport model has been used to address how much subsidence rate, sediment flux, water discharge, hydraulic geometry, and the mechanics of sediment transport influence grain-size distribution and facies belt development in ancient fluvial systems. We have combined the results of recent empirical and theoretical studies with a multiple-grain- size sediment transport model (MIDAS) to test the sensitivity of downstream fining trends to those controlling mechanisms. Our results demonstrate that subsidence and sediment feed rate are the most important mechanisms controlling downstream fining trends in a foreland basin if the evolution of hydraulic geometry is known. To illustrate the application of this result, we have replicated the textural trends of the Mississippian upper Pocono Formation and Burgoon Sandstone of Pennsylvania. This methodology can be applied to constrain plausible values of accumulation and subsidence rate for ancient alluvial deposits and enhances our ability to interpret paleohydraulic conditions from such facies in a foreland basin.