From CNN article about the opening of the ride in 1996
"I had the idea to do a ride even before we shot the movie," producer Spielberg explained. "Universal trusted us and they said OK. And 18 months before the film was in release, the ride was already on the drawing board." In fact, some of the ride's drawing boards were props in scenes in the movie. Since the movie was the biggest ticket seller in film history, making the ride was never a controversial decision. As Phil Hettema of Universal Studios Hollywood's attraction development department said, "Deciding to do 'Jurassic Park: the Ride' was, as they say, a no-brainer. We knew everybody would want to see this come to life.

"But, making it come to life really was the challenge. When Steven Spielberg made the movie, he kind of raised the bar on what people expect."

The reproductions of creatures seen in the movie were only one of the hurdles to creating a theme park that meets Universal criteria. "You have to understand that these are full-size animals," Spielberg said of the park's creatures. The movie was able to cheat a little by using computer-generated images, which gave the animals a fuller range of motion than life-size models.
But, Spielberg said, "These I think -- in my opinion -- are the most realistic animatronic actors I have ever seen perform in any of the theme parks of the world."
Leviathans of another type were also employed on the corner of the Universal Studios Hollywood site, where miles of computer cable, vast water pumping and filtration systems, hydraulic controllers for the dinosaurs, and lots of old-fashioned hard work went into making the ride a reality.
Like the movie, the finished product is more than just a jungle cruise. "You want to take your 3-year-old on the first 30 percent of it, then you want to leave your 15-year- old at home for the rest," Spielberg said.
If parts are scary, other parts are soakers. The climactic water drop at the end of the ride is one of the biggest water drops in the world, an 84-foot plunge at a 54 degree angle. It was specifically engineered to make a beautiful and dramatic splash plume at impact. Especially if you're in the front row, you are going to get wet.
The story line may call for things to go dramatically wrong at "Jurassic Park," but in reality, things could hardly have gone more right on opening day.