Rising water levels in the St. Lawrence Seaway could cost the economy more than $1 billion, shippers and port operators say. A new study from the Chamber of Marine Commerce warns that opening the floodgates further at a dam in Cornwall, Ont., would wash away between $1 billion and $1.75 billion in revenue for businesses on both sides of the border. A board of control recently increased the flow at the Moses Saunders Dam — the only control point on the St. Lawrence Seaway, which includes the Great Lakes — to allow 10,400 cubic metres of water per second out of Lake Ontario.