Dimensions of lava flows can be used to unravel the relative roles of viscosity changes and concurrent formation of levees, stationary margins, and stagnant zones. This approach is applied to data derived from the Mars Orbiter Laser Altimeter (MOLA) experiment for a long lava flow on the plains north of Pavonis Mons, Mars. We obtain a formula for the relative change in viscosity on the basis of a steady state Newtonian flow rate. Our approach features a new length scale that describes the transfer of lava from the active advancing component to passive components. This length scale can be determined from planetary image and topographic data by estimating the volume fraction of lava contained in flow margins relative to the total flow volume. We find only modest changes in viscosity over the distal 175 km of the Pavonis flow. Allowing the flow to also lose volume through degassing (resulting in a density increase) does little to affect the overall viscosity change. Thickening and widening of the flow with distance are as expected for a single coherent, isothermal, viscous flow. This dynamic regime features a balance between the formation of an outer skin and shedding of lava into stationary zones. Requirements for attaining such a regime include a thick flow, shallow slopes over extended distances, and preexisting surface roughness that is small compared to flow thickness. This style of emplacement may explain why many of the long, thick sheet-like flows on the plains of Mars often exhibit an unexpected lack of thickening with distance.