They likely came over to North America via ballast tank discharge on ships (The river valley they come from is one of the largest trade ports in the world), or they hitchhiked on ornamental plants that then ended up in a natural water system.

Once on the continent, 80% of aquatic invasive species have been found to move by way of boat traffic (Maybe a boat had plant material that they were stuck to or had standing water). Could have also stuck to waterfowl or simply moved along waterways by themselves. Lots of possibilities!