Determination of flow path is strongly controlled by both the preexisting topography and rheology of the lava flow. Using the Okmok 1997 eruption as an example lava flow, we perform numerical simulations of Bingham fluids flowing over preexisting topography and find that flow rheology determines whether or not new lobe formation (or bifurcation) occurs from the main flow at a topographic divide or remains confined by local topography. Therefore, lobe (or branch) length may increase for higher viscosity flows, when considering the effect of topographic control. Also, even one order of magnitude difference in the rheological properties allows lava flows to move in completely different paths, which may suggest that flow bifurcation due to topographic control provides insight to the local rheological properties of a lava flow. The effect reported here should be taken into account in risk assessments of future volcanic hazards.