Channel‐fed lava‐flow systems lack detailed thermal and textural studies through the boundary between channelized and dispersed flow, and out to the flow front. Here chemical, textural, and morphological analyses were made to define cooling and crystallization rates down the entire system, especially through the zone of dispersed flow. We compare two channel systems active during the 2007 eruption of Piton de la Fournaise, one of which was cooling limited and one of which as volume limited. In the cooling‐limited case, rapid changes in rheology occurred across the zone of dispersed flow, where viscosity increased from 1000 to 1600 Pa s over the last 100 m of the channel system. This was due to an increase in cooling rate from 7°C km−1 over the first 500 m of the system, to 42°C km−1 over the last 100 m, and an increase in microcryst content from 13% to 25%. In the volume‐limited case, the exponentially increasing segment of the down‐flow cooling and viscosity trend is absent. Instead, lava arriving at the flow front is still relatively hot (1161°C compared with a near‐vent temperature of 1167°C) and is thus of relatively low viscosity (1125 Pa s). In the volume‐limited case, because the channel was still in extension when supply to the system was cut, the zone of dispersed flow was extremely short. However, because lava behind the stalled flow front was still hot and fluid, breakouts from the static front resulted in a complex flow front morphology.

FLOWGO is a one-dimensional model that tracks the thermorheological evolution of lava flowing down a channel. The model does not spread the lava but, instead, follows a control volume as it descends a line of steepest descent centred on the channel axis. The model basis is the Jeffreys equation for Newtonian flow, modified for a Bingham fluid, and a series of heat loss equations. Adjustable relationships are used to calculate cooling, crystallization and downchannel increases in viscosity and yield strength, as well as the resultant decrease in velocity.Here we provide a guide that allows FLOWGO to be set up in Excel. In doing so, we show how the model can be executed using a slope profile derived from Google TM Earth. Model simplicity and ease of source-term input from Google TM Earth means that this exercise allows (i) easy access to the model, (ii) quick, global application and (iii) use in a teaching role. Output is tested using measurements made for the 2010 eruption of Piton de la Fournaise (La Réunion Island). The model is also set up for rapid syneruptive hazard assessment at Piton de la Fournaise, as we show using the example of the response to the June 2014 eruption. The FLOWGO thermorheological model of Harris & Rowland (2001) allows a one-dimensional (1D) simulation of downflow changes in rheology and velocity of basaltic lava flowing in an already established channel. FLOWGO applies a cooling model, based on the first principles of radiative, convective and conductive heat loss, to cool and crystallize a control volume of lava as it advances down a channel. At each step the temperature and crystallinity conditions of the precedent step are handed to the next step to estimate the viscosity, yield strength and velocity of the control volume, while conserving mass. The philosophy of FLOWGO during its initiation in 1998 was thus to provide a flexible framework within which relationships between lavaflow heat loss, rheology and flow dynamics could be placed and tested.