When we consider household budgets, we think of money that flows in and out. The Great Lakes water budget takes into account all of the water that flows in and out of the basin. The Great Lakes – St. Lawrence River system spans an area of nearly 300,000 square miles (750,000 square km). Freshwater flows from the highest elevation in northwest Ontario, Canada, through the Great Lakes basin, to the lowest elevation in Quebec, Canada, and to the Atlantic Ocean. The components of the water budget between these two ends of the system are monitored at key points. This publication includes a diagram showing the inputs and outputs of the Great Lakes system, including all connecting channels.