Complete details of the model can be found in Bond et al. (2000). Essentially, streams were represented as a two-dimensional lattice (depth was ignored) 30 cells wide and 1000 cells long. Cells within the lattice took one of three states: flowing water, obstacles or DWZ. DWZ were always located directly below obstacles, and were also always of equal size. Obstacles represented rocks and other obstructions observed in streams; their size (and that of associated DWZ) were taken from field surveys of several streams. Organisms moved through the landscape according to a biased (downstream) random walk, with a set of simple probabilistic behavioural rules determining their movement patterns around obstacles, and their likelihood of entering DWZ, where settlement occurred (ending drift). These simplified rules provided a practical way of overcoming the otherwise intractable problem of modelling complex flow dynamics (see Carling, 1992).