South Ledge is a navigational hazard in Wrangell Narrows, a water passage separating Mitkof Island to the east and Kupreanof and Woewodski Islands to the west that extends generally south for about 22 miles (35 km) from Frederick Sound near Petersburg at the north end to Sumner Strait at the south end, about 22 miles (35 km) west of Wrangell, Alaska. The passage was named ‘Proliv Vrangelya’ in 1838 by the Russian navigator Captain G. Lindenberg after Ferdinand von Wrangel, who was the governor of Russian-America settlements from 1829-1834. Lindenberg was in command of the Prince Menshikof, a Russian-American Company ship. The Russian name translates to ‘Wrangell Strait’ and historically only referred to the narrowest central portion of the channel, but by 1919, the name for the entire passage was changed from Wrangell Strait to Wrangell Narrows on U.S. charts. Wrangell Narrows has about 60 navigational aids including range lines, lights, and buoys to mark the channel because of the many hazards including islets, reefs, and shoals. The channel is narrow and intricate especially between dangerous ledges and the tidal currents are strong. Several sections have a tendency to fill with sediment and considerable maintenance dredging is required. The tidal currents enter Wrangell Narrows from both ends on the flood and meet approximately at the midway point. The velocity of the current in the narrows varies considerably. At the northern end near Petersburg, the velocity of the current is 3.5 knots (6.5 kph). The strongest currents occur off Turn Point, Spike Rock, and South Ledge where the velocity can reach between 4 and 5 knots (7-9 kph), and during spring tides, velocities of 6 to 7 knots (11-13 kph) may occur. The channel is part of the Inside Passage and is used by fishing boats and ferries of the Alaska Marine Highway. Wrangell Narrows is one of the few safe passages through the Alexander Archipelago at this latitude, but it is too shallow and narrow for very large freighters and cruise ships.

The Stikine Tlingit historically inhabited this area and held a monopoly on trade along the Stikine River. They traded with the Tahltan people upriver, exchanging shells, fish, and later, western trade goods for obsidian, berries, and other inland items. With the arrival of the Russian promyshlenniki, the Tlingit continued this arrangement, acting as middlemen for the furs of the interior plateau. In 1792, Captain George Vancouver explored the area but did not chart the narrows, concluding the waterway was only an embayment and not a through passage. In 1834, the Russian-America Company established Redoubt St. Dionysius at an ancient Tlingit village on the northern end of Mitkof Island. In 1838, Lindenberg produced the first hydrographic survey of Wrangell Narrows and named the channel after the famous explorer. Ferdinand von Wrangel was a Baltic German explorer and seaman in the Imperial Russian Navy, an Honorable Member of the Saint Petersburg Academy of Sciences, and a founder of the Russian Geographical Society. He was appointed chief manager of the Russian-American Company in 1829, effectively the governor of all settlements in North America. He traveled to his post by way of Siberia and Kamchatka. He was recalled in 1834, returned by way of the Isthmus of Panama, and went on record opposing the sale of Alaska to the United States. In 1840, a Russian sailing vessel transiting Wrangell Narrows was caught in fast-moving water on an ebb tide, grounded on a shoal, and was a total loss. Following the Alaska Purchase in 1867, Captain Richard W. Meade on the vessel USS Saginaw was tasked with correcting any errors on the Russian chart and made the first transit of the passage by a U.S. Navy vessel. Among the corrections made was where Lindenberg identified Duncan Canal meeting Wrangell Straight in the vicinity of Boulder flats, behind Woody Island. Meade corrected it to the proper location of Keene Island. In 1895, SS City of Topeka was stranded in the narrows and recovered, and in 1899 struck another rock in the narrows and was salvaged and repaired. In 1901, the barge Colorado displacing 983 tons departed Juneau bound for Tacoma, Washington with a shipment of sulphur, a by product of gold mining. The barge was under tow by the British steam tug Pilot when it hit a reef in the Wrangell Narrows opposite Anchor Point, about 14 miles (23 km) south of Petersburg. Colorado Reef and a nearby creek were both named after the wreck. Archived newspaper accounts detail some work to dynamite that reef and improve the channel in 1928. By this time, the swift currents and hazards of the passage were well known to vessel captains and recently there has been interest in exploiting the tidal stream for energy generation.

Tides are the result of gravitational interaction of the sun, earth, and
moon. Generating energy from tides dates to the 8th-century Roman Empire when tide mills consisted of a storage pond filled by the incoming tide through a sluice and emptied during the outgoing tide through a water wheel that produced mechanical power to mill grain. Since the 1960s, only five projects have been developed commercially in the period up to 2012. However, new technologies have advanced considerably over the past few years and there are a number of ongoing full-scale demonstration projects. Generating electricity from tides is not yet widely used but has potential in the near future. There are two forms of energy derived from tides. Tidal range or the height of the rise and fall of the tides creates potential energy. Exploiting tidal range energy involves technologies that capture the potential energy created by the difference in head pressure between the elevation of high and low tides. Sites for tidal range energy exist in locations geological and hydrological conditions cause large volumes of water to flow into compounded areas such as bays and estuaries. Tidal range energy is predictable since the energy production is not influenced by weather conditions, but rather by the relative motion of the moon, sun, and earth. The other form of tidal energy is produced by the tidal stream or the currents generated during the flood and ebb cycle that creates kinetic energy. Tidal stream technologies that convert kinetic energy into useable energy have made enormous strides in development towards commercialization in the past five to seven years and there are now two main categories. Horizontal-axis and vertical-axis cross-flow turbines that use blades positioned either in parallel or perpendicular to the current direction. The turbines are similar to designs used for wind energy, but due to the higher density of water, the blades are smaller and turn more slowly. The other promising technology is reciprocating devices that have blades or foils shaped like airplane wings that move up and down as the tidal stream flows across the surface. The up and down movement of the hydrofoils is subsequently converted into a rotation for power generation. Greater tidal variation and higher tidal current velocities can dramatically increase the potential of a site for tidal electricity generation. Three sites in Wrangell Narrows at Turn Point, South Ledge, and Spike Rock have been identified for tidal stream energy potential. Read more here and here. Explore more of Wrangell Narrows here: