There are 5 of them and they were built in various stages and time frames. the Army Corp of Engineers maintain the locks and the area around them. There are 11,000 ships that come through the locks every year, and they can be closed at times during the winter months-November through March- due to ice. The ships carry 90 million tons of cargo, and most of that is grain and stone, iron ore, and coal. They traverse through all the lakes to get to this point, that ends at St. Lawrence River toward the Atlantic Ocean.
The locks on the US side are from 1914 through 1943 when the Poe lock was expanded to 110 feet wide and 1200 feet in length to accomodate longer ships. The other locks are only 80 feet wide. There is a 21 feet drop from Superior to Huron lakes at this point. The St. Mary's river still runs through a lot of rapids close to Canada side called St. Mary's Falls.
One of the ways to view the Soo locks is the boat tour. It is basically a 2 hour ferry ride that goes through the locks twice. It start East of the locks in Michigan goes through the soo locks once and past and underneath the Sault ste. Marie International Bridge.
Then it continues on a river view of a steel production facility (Algoma). It returns past the bridge and through the soo lock again and then to the dock. They have 2 docks. Make sure you go to the correct one.
The Saint Mary's River connects Lake Superior to the other Great Lakes at Sault Ste Marie. This stretch of river had a natural set of 21 foot rapids preventing boat traffic, and forcing cargo to be unloaded in town, hauled down Portage Street, then reloaded on ships on the other side. The first locks at this location were constructed on the Canadian side of the river in 1797, but they were destroyed in the War of 1812. The first locks on the US side were completed in 1855, and as compensation, the construction company received 750,000 acres of land in Michigan! The original construction consisted of two 350 foot locks which were operated by the state of Michigan. In 1881 the US Army Corps of Engineers took over the site, and they built the four present day locks: Davis Lock (1914), Sabin Lock (1919-now decommissioned), MacArthur Lock (1943), and Poe Lock (re-built in 1968).
10,000 ships pass through the Soo Locks during the nine months of the year they are open. The locks close in winter when the lakes freeze over.