This article relies primarily on customhouse records to chart the changing geography of cross-border marine commerce taking place within the Great Lakes borderland during the nineteenth and early twentieth centuries. Early on, cross-border marine trade was much more important in the lower than upper lakes, and much of this traffic was local in nature, although a few centers on both sides developed strong gravitational pulls that translated into wider commerce fields. Further integration occurred during the middle of the century when a number of factors, including the repeal of the Corn Laws and the negotiation of reciprocity, resulted in increased flows of goods between Ontario and the Great Lakes states. It was also during this period that railways began to replace boats as the primary mode of trade transportation. On the upper lakes, cross-border trade between the ports of this region and the lower lakes increased in response to the growing demand for lumber, minerals, and other resources by expanding cities and industries. Increasingly, the asymmetrical character of cross-border trade became apparent as the volume of flows favored the United States. In this context, new corridors developed as older gateways continued to prosper or decline. Specific channels of trade linking geographically proximate communities on both sides of the St. Lawrence and the lakes continued to exist, but for most of these centers, commerce via waterborne transportation faded considerably. Within the larger trade network of the Great Lakes basin, certain arteries of trade became clogged or closed up whereas others were opened.