The gyarados evolved during my shift, so you get a video of my pokemon helping contain the situation. I'm the one that arrives at the start of the video.
[A video is uploaded. It was taken by one of the security cameras in Area 2, so the quality isn't that good.]
A gyarados can be seen emerging from the river's water. Several safari zone employees are already there, releasing their pokemon. Another person, presumably Cyan, approaches riding a tauros from the south, near the camera. He exchanges some words with their coworkers while releasing Venom.
The gyarados gets out of the water while the other pokemon set up. A pidgeot uses tailwind, but it doesn't affect all pokemon, so Venom uses quiver dance for himself. Venom uses sleep powder, but it fails. The gyarados uses brine, shooting a stream of water that hits several flying pokemon, making it difficult for them to fly. Luckily, Venom managed to avoid most of it, but he does seem to have trouble with one of his wings.
Someone's persian scratches the gyarados, who reciprocates with an ice fang, freezing the pokemon. Bull runs towards the water pokemon in a zigzag pattern and jumps towards it. Bull used trailblaze, which is not super effective duty to the flying type of gyarados.
Someone's butterfree uses stun spore, which works, paralyzing the wild pokemon. Suddenly, something jumps out of the lake. This catches everyone by surprise, and as they look to the sky a flashing light can be seen coming from above. A second gyarados lands and bites a nearby kangaskhan. Its trainer sends their rhydon to counterattack. A rock tomb hits the second gyarados, damaging it considerably. However, the pokemon uses brine, knocking out the rock/ground type.
A safari zone employee throws what looks like a safari ball at the first gyarados, capturing it. A second one tries to do the same with the other water type, but it escapes. Bull approaches it and uses close combat, which hurts it enough so that a second ball does the job.
A couple of trainer owned chanseys, with the help of some wild ones that got attracted by the fight, help heal the pokemon that participated in the battle. Cyan can be seen petting their pokemon and giving them treats. One man takes the balls with the recently captured pokemons, probably to take them to get healed and calmed down in a controlled environment.