She looks like a Dark Fishing Spider, Dolomedes tenebrosus. She’s carrying her egg sac under her to protect it from predators and parasitic wasps while she hunts. It always looks incredibly difficult for a spider to multitask like that. When the hatchlings are ready to emerge she’ll build a Nursery Web around the egg sac. It will look like a tent. It keeps the babies protected and keeps them from wandering. She’ll continue to guard them until they are ready to go off on their own.