The animals recolonizing an area of denuded stream substrate are thought to come from 4 main sources. These are drift, upstream migration within the water, migration from within the substrate and aerial sources, e.g., oviposition. An experiment in a Canadian stream showed drift to be the most important source of recolonizing animals, contributing 41.4% of the total number that settled on an area of 1800 cm2. This compares with 28.2% from aerial sources while upstream migration and movement up from within the substrate contributed about equally (18.2% and 19.1%, respectively). All 4 directions are important in repopulating bare areas. Many groups of organisms preferred directions from which they recolonize, and this may lead to the establishment of separate and distinct faunal assemblages if the other directions are excluded.