The Duluth Ship Canal was excavated privately in 1871, passing through Minnesota Point and enabled ships to enter the Bay of Duluth on the Duluth side. Not much later, loading and unloading docks were constructed in the shelter of the new inner harbor. To accommodate this growth, the natural harbor depths of six to eight feet were deepened by the Corps to 12 feet in 1867 and 1874. Then to 16 feet in 1881-82, when new locks were built by the Corps at Sault Ste. Marie, Michigan. In the early 1890s, the Duluth Ship Canal saw cargo tonnage surpass 3.5 million tons. Iron ore from the nearby Mesabe Iron Range began to flow through the harbor in 1892, swelling shipments to millions of tons each year. Congress combined the Duluth and Superior ports in 1896, and provided a joint appropriation of $3 million for harbor improvements. During the next ten years, the channels were enlarged and deepened to 20 feet, both the Duluth and Superior entries were rebuilt and enlarged. The Corps kept pace with the growing Twin Ports industry, widening, deepening, and extending channels to new locations as the harbor and vessels expanded to new size and capacity. Channels were deepened from 20 feet to 24 feet in the 1930s. With the development of the St. Lawrence Seaway system during the 1950s, the waterways throughout the lakes, were made to 27 feet deep to accommodate the largest lake ships, as well as the ocean-going foreign vessels.