Waterfowl lose their wild instincts when people feed them.  Feeding them disrupts their natural migration behavior and causes them to loose their healthy natural fear of humans.

Feeding causes overcrowding of waterfowl which in turn can spread diseases such as botulism, salmonella, and avian cholera.  These diseases can kill large numbers of birds when they are concentrated in a small area.

Overcrowding leads to an unnaturally high level of fecal material in the water.  This can be a major source of pollution.  

Bread and other processed foods do not have the nutrients the birds need to stay healthy.

There is a greater chance of someone being injured, especially children, when waterfowl are no longer leery of humans.  Canadian geese can become quite aggressive when they are  nesting.