The river that serves as the border between the USA and Canada is the Saint Lawrence River. This river is a vital waterway that connects the Great Lakes to the Atlantic Ocean. It plays a crucial role in trade and transportation, facilitating ships' passage between the two nations. The river is also significant for its ecological diversity and scenic beauty, with various parks and recreational activities along its banks. The Saint Lawrence Seaway is a key component for large vessel navigation, making it an integral part of North American maritime commerce.