The Canadians scored three times in a span of 3:30 late in the second period en route to their third straight title.
"They believed in the process," said head coach Laura Schuler. "I think that was critical for us to be able to pull this off — for them to be able to do the little things.
"They executed those little things perfectly. That's exactly what they did. I think that's why we were able to pull off the win."
The teams were scoreless for most of the first two periods before Potomak opened the scoring on the power play.
Picard made it a two-goal game just over a minute later and Samantha Cogan added another goal late in the period.
"We just kept attacking their net and the puck went in for us," Schuler said. "It was an awesome feeling. It was a great period for us."
The Americans ended the shutout bid early in the third when Alexandria Laing scored with a two-man advantage.
"The girls played extremely hard and I'm very proud of our team," said American coach Jeff Kampersal. "We gave it our best but couldn't overcome the second period Canada had or capitalize when we needed to."
Picard and Potomak added insurance goals late in the period to ice the win for Canada.
"I think our biggest thing was just driving the net and getting pucks deep and not fooling around with it on the blue-lines, said Canadian forward Karly Heffernan. "We just kept working hard and had the faith and believed in each other. Our fitness really showed through it."
Canadian goaltender Shea Tiley made 26 saves. Canada outshot the Americans 30-27.
"They just believed," Schuler said of her players. "They believed in their teammates, in the person to the left and to the right of them and to the staff and the support staff.
"They believed and they gave it everything they had."
Earlier in the day, the Czech Republic defeated Russia 1-0 to win the bronze medal.