Pacific salmon positively influence the abundance of stream organisms by delivering marine-derived nutrients to the stream ecosystems where they spawn and die.  Salmon also negatively influence stream biota by physically disturbing sediments during migration and spawning.  We conducted two complementary field experiments to quantify the influence of these two contrasting ecological roles on stream ecosystems.  To investigate salmon as sources of nutrient enrichment we increased the abundance of salmon carcasses by installing 140, 1.5m-long carcass-retention devices in a 110m length of stream channel.  Within this experiment we installed 2 x 2m salmon exclosures that locally prevented disturbance to stream sediments.  Dissolved concentrations of ammonium, soluble reactive phosphorus, and nitrate increased greater than 15-fold with the arrival of migrating salmon.  Late in the salmon run, retention devices resulted in an approximately five-fold increase in carcass abundance.  Despite this pronounced increase, dissolved nutrients and benthic biofilm increased only slightly in the carcass-retention reach.  In contrast, biofilm abundance in salmon exclosures doubled relative to control plots where salmon disturbed sediments.  Collectively, these results suggest that retained salmon carcasses do not necessarily have a strong positive influence on dissolved nutrient concentrations or biofilm, whereas salmon disturbance can strongly modulate nutrient-enrichment effects on benthic organisms.