Downstream changes in lava rheology due to cooling, crystallization, and vesiculation have a strong influence on the final length and morphology of a lava flow. Three statistics are proposed to estimate the change in lava rheology with distance along the path of an active flow. These statistics correspond to three separate models of the volumetric flow rate dependence on the thickness of the flow. Each statistic is based on flow dimensions and topographic data that are often available from field measurements or remote sensing. One model assumes an elementary laminar Newtonian flow. A second empirical model often used to describe the flow of complex geologic materials, such as lahars, sediment-laden floods, and debris flows, is also investigated for comparison. A new volume-loss model is also proposed to account for stationary components such as levees and stagnant areas. The three statistics derived from these flow rate models are applied and interpreted for two well-defined lobes from episodes 2 and 18 of the 1983--1984 Puu Oo eruption. The power law dependence of the first two models results only in modest differences in the estimates of rheologic change along the flow path. However, the removal of lava from the active flow to construct stationary components produces significant differences in both the magnitude of computed rheologic changes and the ability to discern trends in rheologic changes along the path of the flow. ¿ 1998 American Geophysical Union