I just looked this up because I'm curious. There are currently 4 trillion gallons of water in Lake Powell. Source: Lake Powell Water Database
The EPA spilled 3 million gallons of toxic water into the Animas. The contaminated water is literally a drop in a very large bucket - basically a negligible percentage.
Also, 3 million gallons sounds like a lot, but it's not that much water. 3 million gallons is roughly the equivalent of 5 cfs for 24 hours. By comparison the San Juan (where that water is headed) is currently flowing at 2,700 cfs. So while the spill is not great for the watershed - there will be some serious dilution at work when that spill does reach the lake. This spill will have a noticeable effect on the Animas, but the further down the watershed the less of an effect we will see.
More importantly that bucket that the drop of pollution is flowing into is already polluted. There are plenty of side streams that will provide you with better water to work with - use those for water if you have concerns.