Balham was soon on his feet, unhurt, but Riddell remained down near the guardrail as the St John Ambulance paramedics got to him.
He was later seen walking with a sore hand. The TAB result shows Balham tied for the win in a dead heat with Hank Moody, ridden by Hayden Tinsley.
Wairarapa Racing Club president Wayne Thurston said committee members, senior jockeys and stipendiary stewards inspected the track and decided to cancel the other races.
The cruel irony is more rain would have helped.
"It was just after the line when the horse slipped, his leg went out sideways," he said.
"And it was just in that particular area, the rest of the track is okay.
"We could see how the ground had given way. We just didn't have enough rain. If it hadn't been 30 degrees the day before ... because with that extreme heat, that particular place had dried out a little quicker."
The ground had become greasy in the rain, but was dry only a few centimetres down.
"It just didn't grip."
After studying the action-replay on screen, it was decided the meeting would be abandoned.
"It was a no-brainer. We're all going home to our families, no one's injured."
He said this was the first time a meeting had been called off in his lifetime.
The club was already going to be hurting from a "substantial loss" at the gate, he said.
"This [day] is the big one, and it was just like an average day for us.
"This is going to have quite a major effect on the viability of the club for the rest of the season.
"If this had been five years ago, it could have been enough to close us up."
Fortunately the club is in a better financial position, but "we now have a lot of ground to make up. We've just got to hope it's nice and sunny for Waitangi Day weekend -- hopefully the weather gods will be kinder."