What controls the location of ice streams?

Ice streams influence ice sheet mass balance and stability but key aspects of their behaviour remain poorly understood. This paper reviews and discusses one very important aspect: what controls their location in an ice sheet? Seven potential controls on ice stream location are identified from the literature: topographic focusing, topographic steps, macro-scale bed roughness, calving margins, subglacial geology, geothermal heat flux and subglacial meltwater routing. For each control, the theoretical basis for its link to rapid ice flow is introduced, followed by discussion of the evidence of its influence on the location of both contemporary and palaeo-ice streams. Based on this new synthesis, topographic focusing, subglacial geology, meltwater routing and calving margins appear to be most commonly associated with fast ice flow. It is clear that rather than a single control, however, there exist a number of potential controls of varying influence. We propose a hierarchy of factors, with those occurring at the top of the hierarchy exerting a stronger influence on ice stream location, and where present beneath an ice sheet, are very likely to be associated with fast flow. Those factors occurring lower down the hierarchy are less commonly associated with ice streaming but appear to be influential in the absence of more common controls. In such a hierarchy topographic focusing in the presence of a calving margin is the primary control. In the absence of this, ice streams will preferentially occur in areas with favourable subglacial meltwater routing and subglacial geology. In the absence of these, bed roughness, geothermal heat flux and topographic steps may promote ice streaming. Significantly, the primary controls on a given ice stream location are likely to influence its spatial and temporal dynamics. Ice streams governed by the presence of meltwater routing and/or calving processes might exhibit more variable behaviour because these controls can vary over relatively short time-scales compared to controls that vary over longer time-scales, e.g. geothermal heat flux, subglacial roughness, geology and topography. Identifying the controls on ice stream location is therefore of paramount importance when understanding ice stream longevity and their past and future activity