13. Lake Superior

(1) Chart Datum, Lake Superior.-- Depths and vertical clearances under overhead cables and bridges given in this chapter are referred to Low Water Datum, which for Lake Superior is an elevation 601.1 feet (183.2 meters) above mean water level at Rimouski, Quebec, on International Great Lake Datum 1985 (IGLD 1985). (See Chart Datum, Great Lakes System, indexed as such, chapter 1.)

Dimensions, etc.

(2) Length, steamer track, Duluth Ship Canal to Point Iroquois; about 383 miles.
(3) Length (right line in clear), Duluth Ship Canal to Michipicoten Harbour; 350 miles.
(4) Breadth, on about longitude 86 45'W.; 160 miles.
(5) Depth, maximum recorded; 1,333 feet.
(6) Water surface of lake (including St. Marys River above Brush Point); 20,600 square miles (U.S.), 11,100 square miles (Canada).
(7) Entire drainage basin (including St. Marys River above Brush Point); 37,500 square miles (U.S.), 43,500 square miles (Canada).

(8) General description.-Lake Superior, the largest freshwater lake in the world, is the northernmost, westernmost, highest, and deepest of the five Great Lakes. The lake is fed by the waters of many short swift-flowing streams and drains through the St. Marys River into Lake Huron. The shores of the lake are generally high, rocky, and forested. The lake is sparsely populated, especially along the N shore.

(9) The waters of Lake Superior are colder and form more shore ice than do the other lakes. The navigation season, shorter than the other lakes, is generally about 8 months long. The actual length of the season depends primarily on whether tonnage demands justify the expense of ice breaking for earlier or later vessel movements. Commercial fishing operations from harbors around the lake continue throughout the year except where prevented by ice conditions.

(10) Fluctuations of water level.-- The normal elevation of the lake surface varies irregularly from year to year. During the course of each year, the surface is subject to a consistent seasonal rise and fall; the lowest stage is usually reached at about the close of winter and the highest during the late summer.

(11) In addition to the normal seasonal fluctuation, oscillations of irregular amount and duration are also produced by storms. Winds and barometric pressure changes that accompany squalls can produce fluctuations that last at the most a few hours. A storm of this type in 1939 produced fluctuations at Marquette with a maximum range of 7.4 feet. At other times, strong winds of sustained speed and direction can produce fluctuations that last a few hours or a day. These winds drive forward a greater volume of surface water than can be carried off by the lower return currents, thus raising the water level on the lee shore and lowering it on the windward shore. Fluctuations caused by such winds seldom exceed I foot above or below the normal level, but may cause changes up to 2 feet. An unusually severe storm in 1905 temporarily raised the water level in Duluth by 2.3 feet.

(12) Through an agreement between the United States and Canada, the water level of Lake Superior is controlled by means of compensating works in St. Marys River. The dikes and sluice gates in the river are operated so as to maintain the monthly mean level of Lake Superior as nearly as possible between elevations 599.61 feet (182.76 meters) and 603.22 feet (183.86 meters) above the mean water level at Rimouski, Quebec, on International Great Lakes Datum1985 (IGLD 1985).

(13) Weather.-- Strong winds are a threat from fall through spring over the open waters. Late autumn is the worst, when gales blow up to 6 percent of the time. The W part of the lake is least susceptible since it is somewhat sheltered from the strong winds, many of which have a westerly or northerly component. Fall windspeeds of 28 knots or more occur 11 percent of the time in this region compared to 16 to 18 percent elsewhere.

(14) Spring winds are variable, with N through SE winds common in the morning; southwesterlies also appear in the W. Afternoon directions are similar, with the addition of northwesterlies in the E. Gale frequencies drop to 2 percent or less by May; however, some of the highest winds of the year are encountered during this season. Along the shore, it is a volatile time. At many locations, April registers the highest mean windspeed of the year, while speeds of 28 knots or more also reach a peak. At Duluth, they blow up to 3 percent of the time in April, a month in which its highest windspeed of 65 knots (NE) was recorded. Marquette recorded a 79-knot wind during a May thunderstorm. Winds with easterly components are common in the morning; this is most noticeable at Duluth and Sault Ste. Marie. At Marquette norther- lies prevail. Afternoon winds often have a westerly component, but northerlies and southerlies are frequent too.

(15) Summer winds are often out of the S through W; this pattern is intruded upon by afternoon northwesterlies in the E. Windspeeds are most often in the 10- to 20-knot range with gales and near gales uncommon. Strong winds are usually associated with occasional thunderstorms. In fact, the highest recorded wind on the lake was 8 1 knots (northwesterly) in a June thunderstorm. Along the shore the lake-land breeze results in offshore components in the morning reversing themselves during the day. Morning easterlies give way to W through NW winds at Sault Ste. Marie. Marquette's light and variable breezes yield to a N- through-NE flow.

(16) As autumn progresses, winds blow more and more out of the W and N, and windspeeds are on the increase. By October, gales are blowing up to 5 percent of the time in the east and 2 to 4 percent of the time in the W. Onshore, similar changes are occurring. Early autumn onshore-offshore flow gives way to a variety of S through NW winds associated with migratory highs and lows. Speeds of 28 knots or more, while not frequent, are more so than in summer. Sault Ste. Marie recorded a 52-knot northwesterly dur- ing November. Winter winds are stronger still and remain mostly out of the S through NW with an increase in northerlies.

(17) Thunderstorms can occur at any time, but they are most likely from April through October, particularly during June, July, and August. Over the open waters, thunderstorms are encountered 1 to 3 percent of the time during the summer months. These thunderstorrns are by far most likely in the early morning hours between midnight and 0300 e.s.t.; they occur up to 8 percent of the time during these hours. Minimum activity occurs around midday. The W half of the lake is more vulnerabie to thunderstorms than the E half.

(18) Along the shore, thunderstorms occur on 20 to 30 days annually, including 4 to 7 days per month in the summer. Activity is slightly more frequent at Duluth than at other locations. These thunderstorms can occur as isolated single cells or in violent squall lines. They can generate strong gusty winds and hail. On occasion, tornadoes or waterspouts have been associated with these squalls. Winds in thunderstorms have been recorded at around 80 knots; strong winds are most likely in spring and early summer.

(19) The lake is large enough for strong winds from any direction to have sufficient fetch to build up a sea. However, the Keweenaw Peninsula tends to diminish seas generated by easterlies and westerlies in the S part of the lake. Elsewhere, seas of 25 to 30 feet have been encountered.

(20) In spring, seas can be rough but become less so as summer approaches. Waves of 5 feet or more encountered 30 to 40 percent in April drop off 10 to 15 percent by May. Seas of 10 feet or more also crop up less frequently. An even more dramatic calming occurs in June.

(21) Summer seas rarely build to 10 feet or more, 1 percent of the time, and reach 5 to 10 feet about 10 to 15 percent of the time. Thunderstorms can quickly build rough, choppy seas, but the large waves generated when strong winds blow over a long fetch of water are unusual. Wave heights are 2 feet or less about 60 to 70 percent of the time. Rough conditions return in force during autumn.

(22) Seas of 5 feet or more can be expected 20 to 30 percent of the time in September; by November these figures increase by 20 percent. By late fall, seas are running 10 feet or more about 5 to 10 percent of the time, more than double earlier chances. W waters are the least vulnerable, while central and E waters are more susceptible to the strong winds with northerly and westerly components.

(23) Poor visibilities can be encountered during any season. Radiation fog in autumn, ice fog in winter, and advection fog from spring through fall all contribute to restricted visibilities, as do rain and snow.

(24) Over the open waters, April through August and December are the fog-prone months during the navigation season; June and July are the worst. During this 2-month stretch, visibilities of 2 miles or less may be encountered about 10 to 20 percent of the time, while they fall to 0.5 mile or less about 8 to 17 percent of the time. The most vulnerable waters lie between Keweenaw Point and Au Sable Point. While there is a greater tendency towards fog in the early morning hours, it is not as pronounced as it is onshore.

(25) Along the shore, fog is mainly a morning phenomenon, particularly dense fog. While there are seasonal variations, poor visibilities are common throughout the year. They drop to 0.5 mile or less on about 40 to 60 days annually. At a peak these conditions can be expected on about 6 to 7 days per month. This peak occurs during the summer at some locations with advection fog drifting onshore. Marquette experiences this type of fog. When cold air moves across warm water, fog can set in; this happens at Sault Ste. Marie in late summer and autumn. Radiation fog is also a fall problem, but usually lifts by early afternoon. At Duluth, industrial smoke adds to the visibility hazard.

(26) Ice.-- The large heat-storage capacity of Lake Superior plus the strong winds, waves, and currents which create a continuing overturning of relatively warm, deep water inhibit an early ice cover.

(27) Whitefish Bay, at the lower end of Lake Superior, is a bottleneck area. The shallow bay forms ice rapidly, and the prevailing W winds jam and pack the lake ice into the area. The ice reaches an average thickness of 14 inches and an average maximum thickness of 22 inches. Windrows are at least 4 feet thick in most winters, and thicknesses of 8 to 9 feet may be reached.

(28) In the N part of Lake Superior, ice begins to form along the shore in late January and early December, but because of the nature of the shoreline, significant fast ice develops only in Nipigon Bay and Black Bay. Through January and early February the lake remains open, with drifting patches of slush and new ice. By late February these patches may extend 40 to 50 miles into the lake from the lee shores and may reach a 70 to 90 percent coverage of medium thickness ice. Some open water is always present in midlake. The drifting ice decays through March and reaches open water by the end of the month. After reaching a thickness of 30 to 35 inches, the fast ice along the shore breaks up by mid-April.

(29) In the W end of Lake Superior, fast ice about 4 to 6 inches thick builds NE from Duluth as far as the Apostle Islands. In navigation areas, ridging and rafting of the ice occurs, and the refrozen brash ice may reach a thickness of 4 feet. The lake may briefly become covered 80 to 90 percent, hut strong winds and the associated waves generally compact the thinner ice and stir up warm water, thus reducing the ice coverage to 40 to 50 percent, even in a severe winter.

(30) Fast ice forms in practically all harbors, entrance channels, and bays. In Thunder Bay, the ice reaches an average thickness of 20 to 30 inches through January, February, and March. At Duluth-Superior, ice up to 1 foot thick can form in December depending on the severity of the winter; the average range is 3 to 6 inches. Ice in the harbor reaches an average maximum of 27 inches and, depending on the weather, is in a state of deterioration by mid-April. (See Winter Navigation, chapter 3.)

(31) Local magnetic disturbances.-- Local magnetic disturbances are more prevalent on Lake Superior than on the other Great Lakes. Reports from vessel masters show that the strongest disturbances are along the N shore of the lake, that they decrease in intensity as the distance from this shore increases, and that the tendency is for upbound vessels to be drawn toward the N shore. The disturbances are described in this chapter with the discussion of their locale.

(32) The directive force of the earth's magnetism is rather weak in this region as compared with other navigable waters of the world, and this tends to make the compass needle rather sluggish. Vessel masters should given proper attention to the correction of the compass and the determination of the ship's deviation.

(33) Routes.-- The Lake Carriers' Association and the Canadian Shipowners Association have recommended, for vessels enrolled in the associations, the following separation of routes for upbound and downbound traffic in Lake Superior:

(34) Downbound vessels shall lay a course of 063 for 72 miles from Duluth Ship Canal to pass not less than 12 miles off Devils Island Light; then a course of 078 for 123.25 miles to pass not less than 12 miles off Eagle Harbor Light; thence 086 for 14 miles to pass not less than 12 miles off Copper Harbor Light; thence 105 for 18.25 miles to pass not less than 12 miles off Manitou Light; thence 114 for 131.5 miles to pass not less than 2.5 miles off Whitelish Point Light; thence 148 for 14.75 miles to a point 2.5 miles off Ile Parisienne Light; and thence 139 for 9.75 miles to off Gros Cap Reefs Light; Provided, that vessels leaving Superior Harbor shall lay their course of 045 for 19.75 miles before turning on their course for Devils Island.

(35) Downbound vessels from Two Harbors shall lay a course of 068 for 47 miles to a position not less than 12 miles off Devils Island, then join the general downbound course.

(36) Downbound vessels from Taconite Harbor shall steer 088 for 129.5 miles to a point not less than 12 miles N of Eagle Harbor and there join the general downbound course.

(37) Downbound vessels from Silver Bay shall steer 080 for 147 miles to a point 12 miles N of Eagle Harbor and there join the general downbound course.

(38) Downbound vessels from Ashland shall lay a course of 062 for 133.75 miles from South Channel to intersect the downbound course from Duluth 12 miles N of Eagle Harbor Light.

(39) Downbound vessels from Marquette shall take departure from a point 076 , 2 miles from Presque Isle Harbor Breakwater Light and shall lay a course of 076 for 60.25 miles to pass not less than 7 miles off Au Sable Light; then a course of 083 for 48.75 miles to pass not less than 7 miles off the abandoned lighthouse at Crisp Point and continue for 5.25 miles until intersecting the downbound course from Manitou to Whitefish Point. The foregoing may be accomplished from Marquette by steering 034 for 3.75 miles to the above point of departure, then laying the 076 course.

(40) Downbound vessels from Thunder Bay, Kaministiquia River entrance, shall take departure from a point not more than 0.5 mile 047 from Welcome Island Light and shall lay a course of 137 for 9.25 miles to a position not less than 2.5 miles 227 from Thunder Cape.

(41) From Mission River Entrance, vessels shall take a departure from the Entrance Breakwater Light, and shall lay a course of 110 for 2.5 miles; thence 118 for 8.75 miles to a position not less than 2.5 miles 227 from Thunder Cape.

(42) Vessels shall now lay a course of 098 for 26 miles passing not less than 2 miles 187 off Thunder Cape and passing not less than 1.75 miles 187 from Trowbridge Island Light in order to pass not more than 2.5 miles off Blake Point Light abeam; thence 121 for 2 miles to a point not less than 1 mile 211 from Passage Island Light; thence 148 for 15 miles; thence 119 for 175.25 miles to a point not less than 2.5 miles 030 from Whitefish Point; thence joining the downbound course to Ile Parisienne.

(43) Upbound vessels for the S shore and W Lake Superior points shall lay a course from Point Iroquois Shoal Lighted Bell Buoy 45, E of Point Iroquois, 300 for 5 miles; thence 319 for 7 miles to a point 4 miles off Ile Parisienne Light; thence 328 for 14.25 miles to a point 1 mile off Whitefish Point Light; thence lay a course not over 280 for 14.25 miles to a point not more than 4.5 miles off the abandoned lighthouse at Crisp Point; thence 292 for 133.25 miles to pass not more than 4 miles off Manitou Light and not more than 5 miles off Cooper Harbor Light; thence 266 for 14 miles to pass not more than 5 miles off Eagle Harbor Light; thence 258 for 123.5 miles to pass not more than 5 miles off Devils Island Light; and thence to destination as follows:
(440 243 for 67.75 miles to Superior
(45) 248 for 69.25 miles to Duluth
(46) 258 for 45 miles to Two Harbors.
(47) Upbound vessels for Taconite Harbor shall follow the general upbound course to a point 5 miles N of Copper Harbor, then steer 270 for 143.5 miles to destination.

(48) Upbound vessels for Silver Bay shall follow the general upbound course to a point 8 miles N of Outer Island; then steer 279 for 39 miles to destination.

(49) Upbound vessels for Ashland when 5 miles N of Eagle Harbor Light shall lay a course of 244 to pass 6.5 miles off Michigan Island Light; thence 254 for 12 miles to South Channel.

(50) Upbound vessels for Marquette shall lay a course of 263 for 43 miles from a point not over 4.5 miles off the abandoned lighthouse at Crisp Point to pass not more than 4.5 miles off Au Sable Light; thence 256 for 60.5 miles to destination.

(51) Upbound vessels for Thunder Bay shall take departure from a point 0.6 mile 293 from Gros Cap Reefs Light; thence steer 000 for 1.5 miles; thence head on Whitefish Point Light steering 319 for 8.6 miles to a point 1.5 miles 230 from Ile Parisienne Light; thence 340 for 13 miles to a point 6 miles 065 from Whitefish Point Light; thence 300 for 192 miles to a point not more than 0.5 mile 211 from Passage Island Light; thence 310 for 3 miles; thence 277 for 26 miles, passing not more than 0.75 mile 187 from Trowbridge Island Light, not more than 1 mile 187 off Thunder Cape and not more than 0.5 mile 170 off Hare Island Reef Lighted Buoy A2; thence 319 for 8 miles to pass not less than 1.25 miles from Welcome Island Light; thence to destination.

(52) Vessels bound for Mission River entrance, from the position not more than 0.75 mile 187 from Hare Island Reef Lighted Buoy A2, shall lay a course of 292 for 8 miles; thence 290 for 2.5 miles to destination.

(53) Vessels upbound to other points on the N shore of Lake Superior shall follow the courses as laid down from the Gros Cap Reef Light, to the position 6 miles 065 from Whitefish Point Light; thence to destination.

(54) It is understood that masters may exercise discretion in departing from these courses when ice and weather conditions are such as to warrant it. The recommended courses are shown on charts 14960 and 14961, Lake Superior.

(55) Limits of anchorage in Whitefish Bay are recommended as follows: From a point on the Birch Point range 0.5 mile above Gros Cap, 340 for 2.5 miles; thence 314 to a point 2 miles off Ile Parisienne Light. From a point on the Birch Point range 0.5 mile above Gros Cap, 229 for 0.5 mile; thence 300 for 2.7 miles; thence to a point 3.5 miles off Ile Parisienne Light. No downbound vessel to proceed from this anchorage area unless authorized to do so by the U.S. Coast Guard.

(56) Pilotage.-- The waters of Lake Superior are Great Lakes undesignated waters; registered vessels of the United States and foreign vessels are required to have in their service a United States or Canadian registered pilot or other officer qualified for Great Lakes undesignated waters. Registered pilots for Lake Superior are supplied by Western Great Lakes Pilots Association. (See appendix for addresses.) A pilot exchange point is at the head of St. Marys River about 3.5 miles SE of Point Iroquois. The pilot boat, J. P. IX, docks just above the locks at Sault Ste. Marie. (See Pilotage, chapter 3, and 46 CFR 401, chapter 2.)

(57) Principal ports.-- Compared with the other Great Lakes, Lake Superior is fairly well provided with natural harbors that provide refuge for vessels. In addition, several harbors have been improved with breakwaters to provide the necessary protection. The most important harbors in the lake are at Duluth-Superior and at Lakehead Harbor in Thunder Bay. Drydocking facilities for deep-draft vessels are at Superior and Lakehead Harbor.

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