HURON, LAKE, the second largest of the Great Lakes of North America, is about 220 m. in length, 101 m. in width and has a maximum measured depth of about 75o feet. Bordering on it are the State of Michigan and the Province of Ontario, Canada. The area of the water surface of the Lake is 23,010 sq.m. of which 9,110 sq. m. are in the United States. Its entire drainage basin comprises 72,42o sq.m., of which 24,85o sq.m. are in United States. The average mean surface of Lakes Huron and Michigan for the 67-year period (1860-1926) was 580.97 ft. above sea-level; 21.25 ft. below the mean surface of Lake Superior and 8.54 ft. above the mean surface of Lake Erie. The level of the lake is sub ject to annual fluctuation ; the greatest difference between the highest and lowest monthly means of any year was 1.94 ft. and the least difference was 0.61 foot. During the 67-year period referred to above, the difference between the highest (583.66) and the low est (577.36) mean monthly stages was 6.3o feet. In addition to the annual fluctuation there are occasional oscillations of irregular amount and duration. These apparently result from the variations in barometric pressure which may produce changes ranging from a few inches to several feet within a few hours. The level is also affected by winds which have been known to cause a local change of elevation of some 5 ft. in a few hours. Any lowering of the level of the lake, either as a result of natural causes or of diversions, reduces the depths available to navigation and causes loss to ship ping, as ships are loaded to the full depth available in the dredged cuts. The average opening and closing dates of its season of navi gation are April and Dec. 15, respectively at its upper end, and April 5 and Dec. 23 at its lower end.
Lake Huron receives its water from Lake Superior via St. Marys river and from Lake Michigan via the Straits of Mackinac. It empties its water into Lake Erie via the St. Clair river, Lake St. Clair and Detroit river. In its natural condition the St. Marys river was obstructed by falls and shoals ; those, however, at Sault Ste. Marie are now overcome by locks. A channel has been dredged through the river with minimum depth of 21 feet. The Straits of Mackinac have channels sufficiently wide and deep to permit navigation by the largest lake vessels. In their natural condition the St. Clair river, Lake St. Clair and Detroit river were obstructed by shoals, but a through navigable channel not less than 20 ft. deep has been dredged by the United States.
The most important traffic on Lake Huron is the through com merce between Lake Erie and Lakes Superior and Michigan. This traffic consists primarily of shipments of iron ore and grain from Lake Superior ports, grain from Lake Michigan ports and coal from Lake Erie ports upbound. The total freight movement in 1926 was 85,979,087 tons through the St. Marys river and 95,003, 604 tons through the Detroit river.
Rockport and Calcite are the only ports in the United States along the west coast of Lake Huron with sufficient depth of water to accommodate the largest vessels for loading and unloading.
Both these harbours have been created by and for interests en gaged in the limestone trade. In 1926, tons of limestone were shipped from Rockport, and 9,041,301 tons from Calcite. On the south-western shore of the lake is the artificial harbour of Harbor Beach, Mich., located some 6o m. west of the head of St. Clair river. The improvement at this locality by the United States was to provide a harbour of refuge and consists of a breakwater inside of which there is a sheltered area of about 65o acres. Other U.S. harbours on Lake Huron are Bay City, at the head of Saginaw bay, Alpena on the north-west shore of Thunder bay, and She boygan near the Straits of Mackinac. The Canadian ports on Georgian bay, Depot Harbor, Port McNicoll, Tiffin, Midland and Collingwood, receive yearly about a million tons of coal from Lake Erie ports, and some 85,000,000 bu. of grain from upper lake ports, most of it coming from Port Arthur and Fort William on Lake Superior. Some of this grain is milled at lake ports, but most of it is transhipped to rail lines and carried to the eastern sea board for consumption or export.