The US rider had led after dressage, but slipped to 4th after cross-country before bouncing back; Canadians won the 4*-S and the 2*-L.
MARS Bromont CCI
The Aussie moved to the top of the leaderboard with both her rides ahead of Canada's Jessica Phoenix and Wabbit following Saturday's cross-country.
After winning the dressage phase of the CCI4*-L the day before, the American celebrated her birthday with another strong test.
Competition got underway with the first day of dressage; the full field of CCI4*-S competitors go up centreline on Friday afternoon.
A total of 77 entries will start competition in the dressage phase on Thursday in six divisions at the iconic Quebec venue.
Entries in the June 5-8 event feature athletes from four countries and include Olympians and the current world number one, Boyd Martin.

Allison Springer and No May Moon take the CCI4*-L, edging Waylon Roberts and OKE Ruby R; new Laframboise Family Legacy Awards.
An influential Saturday has Waylon Roberts and OKE Ruby R on top in the CCI4*-L and Karl Slezak and Hot Bobo leading the CCI4*-S.
The first day of dressage saw Karl Slezak and Chevalier as the top Canadian 4*-L entry in 5th place; competition continues Friday.
Colleen Loach is top Canadian in 4th place with 'Goldie'; Boyd Martin scores a hat-trick in the CCI2*-L, CCI3*-L and CCI4*-S divisions.