The fatal dose of botulism toxin is one-onehundredbillionth of a gram. Chlorine won't 'kill the toxin', as it's a toxin, not a live baceria. It's easy to produce. If a couple dozen pounds of it were dropped in a reservoir right at the point where the intake pipes are, it would be sucked right in with the current and go into the water system, instead of being diluted throughout the entire reservoir. We can't just bythely dismiss such an attack.
Another botulism toxin article:
Botulism article
All we need is something that will destroy the chemical. Chlorination MIGHT do that; hypoclorite ion (bleach) doesn't oxidize directly but causes the formation of free oxygen radicals which in turn oxidize many organic substances. Another possibility though more expensive is ozonation. ChemistCat?