The largest city in Lambton County is Sarnia, which is located at the mouth of the St. Clair River. The two Blue Water Bridges cross the river at Sarnia, connecting it to Port Huron, Michigan. The bridges are one of the busiest border crossings between the two countries. The river is also traversed by two passenger ferries further south, and a rail tunnel, also at Sarnia, runs underneath it. The CN rail tunnel accommodates double stacked rail cars. Along with Sarnia, the population centres in Lambton County are: Corunna, Petrolia, Grand Bend, Wyoming, Forest, and Watford.