Complex sentence: From its connection to Lake Couchiching, the Severn River is the only drainage from the lake to Georgian Bay, part of Lake Huron (Simcoe itself is not a Great Lake), the canal locks of the Trent Severn Waterway, make this connection navigable.
Split into simpler sentences: From its connection to Lake Couchiching, the Severn River is the only drainage from the lake to Georgian Bay, part of Lake Huron (Simcoe itself is not a Great Lake). The canal locks of the Trent Severn Waterway, make this connection navigable.