The River is restocked now with everything pretty much (the Sacramento River) from Canterra loop to Shasta Dam

Why is this? It's because all the lifeforms were still alive above where the 19,000 gallons went into the river so they could drift downstream because the water washed all the Metam Sodium down at least 20 miles into Shasta Lake then. So, over time all the fish and other organisms moved down there. Of Course and trees and fish and mollusks and amphibians were gone from that original stretch of the river. However, the River is healthy again and people likely fish there all the way from Siskiyou Dam to Shasta Dam now all these years later.

I was living in Mt. Shasta then on Oak Street near where Lilly's Restaurant is now and I remember hearing a big bang in the night and sirens going off as rescue vehicles headed that direction all night long. In the morning we finally found out what had happened and met people who got sick from the fumes as far away as Castle Lake as the fumes wafted up there on the winds. Luckily, where we lived the winds were not going in our direction but rather towards Castle Lake and Dunsmuir. Most people that got deathly sick were in Dunsmuir not Mt. Shasta at all.

For this reason since there likely will be many more spills of various things over the years it's important to understand you might get sick from Canterra Loop south to Dunsmuir if you are there at the time that happened. The Loop cannot be changed really because of the steepness right there unless you rerouted the tracks maybe along the Freeway as far as Dunsmuir and I don't see anyone doing this because that might not be practical either. So, for now at least it's good to realize this type of Metam Sodium Spill could happen again.

What causes this problem?

Well. When someone young and inexperienced loads the cars wrong I would say. 

What people need to do is to have all cars loaded. this likely works okay. But, if you have several empty cars and then heavier full cars at the end on a loop like Canterra loop it is quite likely you are going to have a derailment of some kind.

So, when you give the job of the order of loading the cars and attaching them to the engine who doesn't know this you are likely going to have a derailment at canterra loop.