The formation of channelized lava flows on a wide uniform slope is investigated both theoretically and experimentally. When a lava is released at a constant flow rate from a point source, we predict that it flows both down and across the slope at the same rate in a early time regime before undergoing a transition to a long-time regime where down-slope flow is faster than lateral flow. Eventually, the lateral flow is stopped by the strength of the growing surface crust, and the flow then travels down slope in a channel of constant width. Using scaling analysis, we derive expressions for the final channel width in both flow regimes, as a function of the flow rate, the slope, the density difference driving the flow, the lava viscosity, the thermal diffusivity, and the yield strength of the crust. We also find a dimensionless flow morphology parameter that controls whether the subsequent channel flow occurs in a "mobile crust" regime or in a "tube" regime. These theoretical predictions are in good agreement with laboratory experiments in which polyethylene glycol wax flows down a wide uniform slope under cold water. The theory is also applied to the understanding of the formation of a basaltic sheet flow lobe in Hawaii, which had an estimated crust yield strength of order 6 ¿ 104 Pa.