The St-Jean-sur-Richelieu, Que., native swam a time of four minutes, 43.52 seconds for the win, just six seconds off her Canadian record.
"I can go way faster than my personal best," said Rivard, who was named the female Para-swimmer of the year earlier in the day. "I am going to work during the few months we have to see what I can do.
"I need to have more confidence in myself and really go for it and not be scared of anything."
Alec Elliot of Kitchener, Ont., won the male S10 400 with a time of 4:20.12. He also won the S10 100 butterfly in 53.33. Male Para-swimmer of the year Nathan Stein of Maple Ridge, B.C., placed second in the butterfly and led after 50 metres before Elliot made a push at the finish.
"It's pretty frustrating considering this is a practice for upcoming meets and internationals," said Stein. "That shouldn't happen but it's the name of the game. I seized up too early and he got me."
Tammy Cunningham of Red Deer, Alta., won the S4 50 backstroke in 1:13.62.
Calgary's Morgan Bird finished second in the S8 100 in 1:24.10 and fourth in the 400 freestyle in 5:23.66.
Katarina Roxon of Kippens, N.L., finished second in S9 100 fly in 1:19.59 and was third in the S9 400 freestyle with a 5:11.05.
Tess Routliffe of Caledon, Ont., swam a personal best time of 5:58.48 to finish second in the S7 400 freestyle.