more drama than fact

Just the dog & puppies in the movie. I had no idea about Harry having cats in real life.

The Johnston to Jackson change was intentional as too much was changed from the real David Johnston. Johnston was married, not a bachelor as depicted, and didn't fall in love with a women weeks before St Helens erupted. It was meant to be him, but they changed his character enough that pretty much had to change the last name. Some of the people in the movie were based upon real people or at composites of real people for dramatic purposes, names of course would be changed.
Harry Truman was pretty much as depicted other than the fact that he had many cats, and no dog.

The fictional changes made in the movie weren't "mistakes." As has been said, the name of the geologist was changed because of dramatic license being taken with the story. When you use a real person but change too many things, you risk being sued. Changing the name (even to something very close and thus recognizable), eliminates that risk.

As for Harry Truman being shown with dogs instead of cats, I can only assume it was done because cats are notoriously difficult to work with in dramatic situations. They aren't trainable like dogs are. Any cat owner knows they do pretty much what they want, and movie directors don't like that. At least they showed Harry Truman as the animal lover he was, instead of completely eliding that element of his personality.

It's been a while since I've seen it, but I remember liking "St. Helens" a lot. It wasn't the greatest movie ever made, but it was better than a lot of people give it credit for.

it wasn't a "mistake" it was deliberate. They often change the actual names of people in movies, and that way they don't have to be 100% accurate!

My 6th grade class has a boxful of cards under all that mud that we'd made for Harry. I was out fishing when it happened. We heard what we thought was thunder, and I am terrified of electrical storms (we seldom have thunderstorms like the rest of the planet knows them). Pretty soon my mom shows up screaming for us to get in the car. We weren't done fishing, and she screams that the mountain blew and they're re-routing traffic off I-5 down hwy.101, through our way. Being on the coast, we didn't get ashfall, but our relatives east of us did, describing the blackness, the lightning, the choking heat and ash. Our class had JUST been up to camp Cispus in the mountains just weeks before. My mother almost didn't let me go because the volcano was active. Very vivid memories here...east of the mountain you still find large amounts of ash, too.