Abstract
The movements of invertebrates within a permanent first order stream in southern Ontario are described, including: drift, upstream migration and post-emergence flight. Large numbers of invertebrates drift within one year. Fecundity estimates of Nemoura trispinosa (Plecoptera) indicated that the upstream flight of ovipositing females, in conjunction with the within-water upstream migrations of immatures and the immigration of mated females from other streams, was not enough to account for headwater densities depopulated through drift. However, we submit that drift does not threaten the maintenance of headwater populations to the extent that has been suggested, and that drift acts on a density dependent basis.
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