Stream fish often occur in tributaries at high densities, and dispersal between tributaries must occur through the intervening river, whose attributes may differ from those of the tributaries. In Trinidad, tributaries of the Guanapo River have high densities of a killifish, Rivulus hartii, but the river also contains a strong piscivore that may affect the quality of the river as a movement corridor linking the adjoining tributaries. We hypothesized that R. hartii in the river, where they are scarce and confined to margins, would show stress as predicted for an animal in transit through a hostile corridor. We predicted that river fish would take in less food, grow more slowly, and contain fewer mature oocytes than tributary fish. We tested these predictions by comparing R. hartii from paired tributary and river sites for food intake, growth, and oocyte counts. We also asked whether R. hartii could spawn successfully in shallow water, such as at the river margins. To determine whether the river would satisfy criteria for its use as a movement corridor (movement along the river and movement in and out of tributaries), we marked 709 R. hartii in a 500-m stretch of river and two adjoining tributaries and recaptured them on seven sampling dates over a 15-mo period. Contrary to our predictions, R. hartii in the river showed no stress in the form of reduced food intake, growth, or suppressed reproductive output. Instead, we detected no difference in food intake of R. hartii sampled from paired tributary–river sites, and river R. hartii displayed a greater growth rate and contained more mature oocytes than did their tributary counterparts. Laboratory and field studies also revealed that R. hartii can spawn viable eggs in shallow water that does not cover their bodies. The movement study confirmed that the river has a conduit function for communication between tributaries, but the river also has a habitat function, as it contains resident individuals that grow and reproduce in the corridor. This means that movement of alleles and recolonization of local extinctions can occur via offspring of dispersers, rather than require successful movement of individuals directly between tributaries.