Wendy Ward, who was tied with Koch after 54 holes, turned in a 6-over 78 Sunday to tie for third with Catriona Matthew at 286. First-round leader Natalie Gulbis was at 287 along with Dorothy Delasin and Soo Young Moon.
Koch and Ward were five shots ahead of the field after Saturday's third round.
"I am so excited," said Koch, who won for the first time since the 2001 LPGA Corning Classic. "You get your first win and that is great, but it is so nice to know you can do it again."
Icher is a 26-year-old Frenchwoman who needed her top 10 finish Sunday because of her conditional status on the LPGA Tour.