The genesis of thick accumulations of carbonate lake deposits on the siliciclastic floodplains of perennial rivers situated below the regional spring line requires (1) a significant source for calcium in the watershed and (2) a protected area to accumulate calcite. The Phanerozoic record of fluvial deposits shows that anastomosing rivers with carbonate-rich provenance situated in distal areas below the spring line are the best environments for floodplain accumulation of significant carbonate lake deposits. Anastomosing rivers exist below the regional spring line and have isolated flood basins generally protected from bedload by high levees, receiving suspended and dissolved load during flooding events. Numerical modeling to test the rate of accumulation for these carbonate sediments was performed using stratigraphic data from the Pennsylvanian succession containing floodplain limestones of the Stewart Quadrangle of Athens County in Ohio (USA). Limestone volumes for six limestone units (Upper Pittsburgh, Fishpot, Sewickley, Benwood/Arnoldsburg, Uniontown, and Waynesburg limestones) were calculated with the assumption of overland input of dissolved calcite from carbonate source areas. In comparing the calcium input rate to the amount of calcium calculated from limestone unit volumes, overland transport of calcium ions, not spring input, can supply sufficient ions to accumulate thick carbonates over the short lifetime of floodplain lakes within anastomosing river systems.