What meme405 said is true. The chemicals in the water are very low in concentration. Quite close to the concentration that is in drinking water. Now that it is released into the river, it becomes even more diluted. Imperial actually had an application which was approved by the ministry and the government to release the water into the river but it was delayed for what ever reason (quite possibly that the First Nations people would not have any of that). So the quality of the water is not the issue here and it certainly will not contaminate the areas that were exposed. Of course with that being said, drinking one glass will not make you keel over and with the alkaline it would taste awfully salty.

The real problem is dealing with the long term effects of what the heavy metals that are settling out will cause. There are biological factors in play that are consuming these heavy metals so how much of an impact will it truly become? We don't know yet. Maybe the biological system can tolerate it. This will be a long and tedious process because they will have to deal with it case by case, section by section at a time. But this is the real issue.

^This.

In the short term the amount of debris and turbidity created by a major release of water like this is cause for concern. Especially with the salmon run so close.

The sediments and heavy metals are not so bad for the fish, unless they end up in the actual spawning grounds, which for now it seems most actual spawning grounds are pretty safe, the concern is that turbidity in the water can be really harmful to fish. It clogs their gills so they basically suffocate, and it kills their vision, it also disrupts their sense of smell, which is what they use to locate and travel back to their spawning gounds.

Within drinking guidelines.

Mount Polley mine spill: Water quality test results within drinking guidelines - British Columbia - CBC News