Most of the ice leaving the Antarctic Ice Sheet is discharged to the sea by ice streams. An understanding of the dynamics of these ice streams is therefore crucial if we are to predict the future evolution of the ice sheet and its effect on global sea level. The behavior of Ice Streams A to E of the Siple Coast, West Antarctica has attracted much attention with evidence of recent stagnation and numerous relict ice-flow features. Here I present results from a three-dimensional, thermodynamic ice sheet model of the West Antarctic Ice Sheet (WAIS). The model generates variability in the flow of the Siple Coast ice streams reminiscent of that mentioned above. This is caused by competition between several preferred ice flow pathways in the area. Individual ice streams interact through changes in catchment-area size: a reduction in catchment area reduces the flow of ice entering a stream and can cause stagnation through the associated reduction in deformational heating. ¿ 1998 American Geophysical Union