Because the smallest of the Soo Locks is larger than any Seaway lock, salties that can pass through the Seaway may travel anywhere in the Great Lakes.
Similarly, the largest lakers are confined to the Upper Lakes (Superior, Michigan, Huron, Erie) too large to use the Seaway locks, beginning at Welland Canal that bypasses the Niagara River.

Because of their deeper draft and the lower buoyancy of fresh water, salties may accept partial loads on the Great Lakes, "topping off" upon exiting the Seaway.
Lakers are generally bulk carriers; that is, they carry cargoes of rock, ore, salt or grain in large contiguous holds, not packed in containers.
The earlier ships required rail cars unloading on ore docks and unloading machinery at the receiving docks.
Modern Lakers are self unloaders, allowing them to unload faster and in more ports.

The most common cargoes on the Great Lakes are taconite (a type of iron ore), limestone, grain, salt, coal, cement, gypsum, sand, slag and potash.
Much of the cargo supplies the steel mills of the auto industry, centered around the Great Lakes because of the ease of transport.
Other destinations include coal-fired power plants, highway department salt domes and stone docks, where limestone is unloaded for the construction industry.
US-flag freighters carried the largest portion of the trade, accounting for two-thirds of all cargo by weight.
US boats carried most of the iron, limestone and cement while Canadian boats carried most of the potash and almost all of the salt and grain moved on the lakes.

Destination harbors, boat sizes and legal restrictions greatly affect the pattern of haulage.
US boats hauled almost all of the iron ore on the lakes (79%) - from US mines to US mills on large US ships. This reflects the requirement of the Jones Act, as well as the industry using large volumes of material while being concentrated in a few large harbor locations.
Salt and Canadian grain can be hauled to numerous smaller ports of either country on smaller, mostly Canadian, boats which can also enter the St. Lawrence Seaway with the Canadian ports of Montreal and Quebec City.

Iron Ore Dock in Marquette’s Upper Harbor also know as the Presque Isle Dock - 
This is a picture of the Iron Ore Dock in Marquette’s Upper Harbor. The dock was built in 1911 and is still commercially active.
Each year approximately 7 to 8 million tons of iron in the form of taconite pellets are shipped from this dock.
The dock is owned and operated by the Cliffs Natural Resources.
This steel-framed dock juts out almost a quarter of a mile into Lake Superior.

Most of the pellets shipped from the Presque Isle dock go to Algoma Steel in Sault Ste. Marie, Ontario the largest integrated steel mill.
These pellets, which are roughly 70% iron, will be combined with coke and limestone at the mill to make steel.

The pellets come to the dock via railcars and are dumped into steel “pockets” or bins beneath the tracks. To load the boat, the chute is lowered to the open cargo hatch and a door at the bottom of the pocket opens, allowing the pellets to run into the boat shown in the picture. Loading time is variable, depending on the size of the boat and how prepared the dock is to load. Four hours is typical.
Loading is the responsibility of the First Mate. It is important to load the ore in a proper sequence to avoid over stressing the boat unevenly. Each chute (or drop of pellets) is about 20 tons.

Presque Isle Dock –

America can take pride in the U.S.-flag Great Lakes fleet. No other maritime nation has assembled such a modern, productive fleet of self-unloading vessels. The thirteen 1,000-footers flying the U.S. flag on the Lakes are longer than most of the grandest ocean liners. So technologically advanced are these vessels that they can discharge 70,000 tons of iron ore or coal in 10 hours or less without any assistance from shoreside personnel or equipment. The industry’s carbon footprint is the smallest of any of the major transportation modes.

American Integrity (1000′×105′)
American Spirit (1004′×105′)
American Century (1000′×105′)
Edgar B. Speer (1004′×105′)
Edwin H. Gott (1004′×105′)
James R. Barker (1004′×105′) 1st standard construction 1000-footer
Mesabi Miner (1004′×105′)
Paul R. Tregurtha (1013′6″×105′) Largest vessel on Great Lakes
Stewart J. Cort (1000′×105′) First 1000 ft boat on the Lakes
Burns Harbor (1000′×105′)
Indiana Harbor (1000′×105′)
Walter J. McCarthy Jr. (1000′×105′) Highest cargo capacity (78,850 long tons)

Tug/barge combination (ITB) Presque Isle (1000′×104′7″) Only 1000 foot tug/barge unit

The largest vessels on the lake are the 1000-footers (300 m). These vessels are between 1000 and 1013.5 feet (305 and 309 m) long, 105 feet (32 m) wide and of 56 feet (17 m) hull depth.
They can carry 78,850 long tons of bulk cargo although their loading is dependent on lake water levels especially in the channels and ports.

The most powerful of these, the Edwin H. Gott,[6] carries two Enterprise DMRV-16-4 diesel engines driving twin propellers.
Rated at 19,500 brake horsepower, making the Gott the most powerful lake boat on the seaway. (14.5 MW). This allows a top speed of 16.7 mph (27 km/h).

The MV Paul R. Tregurtha is the largest boat on the lakes, at 1013'6" and 68,000 gross ton capacity.

The Stewart J. Cort, which is not only the first 1000-footer to be put into service on the Lakes, but also the only one built in the traditional wheelhouse-forward Great Lakes style (although all accommodations are forward, and the stern deckhouse is occupied by self unloading equipment and the engines), is another notable vessel. The Cort started life in Mississippi, and was sailed as a much smaller vessel consisting of only the bow and stern sections (appropriately nicknamed "Stubby"), to Erie, Pennsylvania, where she was cut in half and an additional 800+ feet of hull were added.

Another interesting 1000-footer is the Presque Isle, an integrated tug and barge combination.  The Presque Isle is the largest tug / barge composite in the world.

The Canadian fleet needs to travel to and from its major cities along the St. Lawrence Seaway, so the standard length for the Canadian vessels is around 730 feet (Seawaymax-size). The reason for this standard length is the Welland Canal, which bypasses Niagara Falls. The locks here are only about 800 feet (240 m) long, and for safety reasons, the vessels must be at most 730 feet (220 m). Because of the Jones Act, Canadian ships cannot carry ore from American mines to American mills. This ore is the usual cargo of the largest ships.

More common are lake boats in the 600 and 700 foot (183 and 213 m) classes, due to the limitations of the Welland Canal. These vessels vary greatly in configuration and cargo capacity, being capable of hauling between 10,000 and 40,000 tons per trip depending on the individual boat. These smaller boats can serve smaller harbors around the Lakes which have irregular need for their services.

The latest major vessel built for bulk cargoes on the lakes is the articulated tug/barge combo Ken Boothe Sr./Lakes Contender. The 740' barge Lakes Contender and the 135' tug Ken Boothe Sr. entered service in 2012.