Problems associated with feeding waterfowl include:
Malnutrition
Dependency
Disease
Water pollution
Environmental degradation
Increased hybridization
Delayed migration
Cumulative effects
Costly management efforts
Devaluation of the species

I got these specifics off of this site where they go into detail on each thing, please do not feed me or my relatives bread: http://www.hardeybordercollies.com/id91.html