sentences:  Running roughly from south to north, the lake's waters empty into the Richelieu River, where waterfalls at Saint-Jean in Quebec mark the northernmost point of navigation.The American strongholds of Fort Crown Point and Fort Ticonderoga near the lake's southern end protected access to uppermost navigable reaches of the Hudson River.Elimination of these defenses required the transportation of troops and supplies from the British-controlled St. Lawrence Valley to the north.