The amount of sediment deposited through time at a given place is controlled by both tectonic and climatic factors that change over several length and time scales. For these reasons the sedimentary record is commonly exploited to study global change. To understand this record, most studies have relied on assembling data from a few localities within a given reservoir and have extrapolated those data to whole of the reservoir. Here it is shown not only that this approach has very large inherent uncertainties, but also that the use of extrapolating sedimentary records has little physical significance. A much better approach, although much more difficult, is via a full, three-dimensional mass balance calculation.