A new methodology is presented for estimating the rate of levee growth for channelized lava flows on the basis of spatial changes in the dimensions of the channels and levees. The rate of levee growth can then be used to describe the decrease in flow rate as a function of distance along the flow and, subsequently, to constrain rheologic behavior along the flow length. One terrestrial (Mauna Loa 1984 1A) and two Martian (one on the plains north of Pavonis Mons, one near the summit of Ascraeus Mons) examples are shown. A linear "levee building rate function" adequately describes levee growth for all three examples, with the overall magnitude of the rate function inversely correlated with the total volume of material in the levees. Theoretical levee widths based on these linear functions are consistent with those measured in the field or from planetary images. Assuming Newtonian rheologies, the viscosity increases for the Mauna Loa and Pavonis Mons lava flows are about an order of magnitude greater than when the levee building process is ignored. Application to the Mauna Loa 1984 1A flow shows that the model provides results more consistent with independent petrologic studies as well as field observations of the levee formation process. Viscosity changes at Ascraeus Mons are similar to those estimated previously (Zimbelman, 1985) because of the constant thickness of the flow.