South Ledge is a navigational hazard in Wrangell Narrows, a water passage that extends generally south for about 22 miles (35 km) between Mitkof Island to the east and Kupreanof and Woewodski Islands to the west, about 25 miles (40 km) northwest of Wrangell and 12 miles (19 km) south of Petersburg, Alaska. The passage was named ‘Proliv Vrangelya’ in 1838 by the Russian navigator Captain G. Lindenberg, in honor of Ferdinand von Wrangel, who served as governor of Russian-America from 1829 to 1834. Lindenberg commanded the Prince Menshikof, a Russian-American Company ship. Wrangell Narrows has about 60 navigational aids—including range lines, lights, and buoys—because of the many hazards such as islets, reefs, and shoals. The channel is narrow and intricate, especially between dangerous ledges, and tidal currents are strong. Several sections tend to fill with sediment, so considerable maintenance dredging is required. Tidal currents enter Wrangell Narrows from both ends during the flood and meet near the midpoint. Current velocity varies considerably. Near Petersburg, at the northern end, it is about 3.5 knots (6.5 kph). The strongest currents occur off Turn Point, Spike Rock, and South Ledge, reaching 4–5 knots (7–9 kph). During spring tides, velocities of 6–7 knots (11–13 kph) may occur. Part of the Inside Passage, Wrangell Narrows is used by fishing boats and Alaska Marine Highway ferries. It is one of the few safe passages through the Alexander Archipelago at this latitude but is too shallow and narrow for large freighters or 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 furs from the interior plateau. In 1792, Captain George Vancouver explored the area but did not chart the narrows, concluding it was only an embayment and not a through passage. In 1834, the Russian-American 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. 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 lost. Following the Alaska Purchase in 1867, Captain Richard W. Meade, aboard the USS Saginaw, was tasked with correcting errors on the Russian chart and made the first transit of the passage by a U.S. Navy vessel. In 1895, SS City of Topeka was stranded in the narrows and recovered; in 1899, it struck another rock there but was salvaged and repaired. In 1901, the 983-ton barge Colorado left Juneau for Tacoma, Washington, carrying sulfur (a by-product of gold mining). Under tow by the British steam tug Pilot, it hit a reef in Wrangell Narrows opposite Anchor Point, about 14 miles (23 km) south of Petersburg. Colorado Reef and a nearby creek both take their names from that wreck. Archived newspaper accounts show that in 1928 work was done to dynamite the reef and improve the channel. By then, the swift currents and hazards of the passage were well known to vessel captains. More recently there has been interest in harnessing its 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 used a storage pond filled by the incoming tide through a sluice, then emptied it during the outgoing tide through a water wheel that provided mechanical power to mill grain. From the 1960s to 2012, only five projects were developed commercially. However, new technologies have advanced considerably in recent years, and several full-scale demonstration projects are under way. Although generating electricity from tides is not yet widely used, it has near-term potential. Two forms of energy come from tides. Tidal range—or the height of the rise and fall—creates potential energy. Technologies to capture tidal range energy exploit the head-pressure difference between high and low water levels. Suitable sites exist where geological and hydrological conditions funnel large volumes of water into enclosed areas such as bays and estuaries. Tidal range energy is predictable, depending on the relative motion of the sun, Earth, and moon rather than on weather. The other form of energy is tidal stream energy, produced by currents during flood and ebb cycles. Tidal stream technologies that convert this kinetic energy into usable power have progressed significantly toward commercialization in the past five to seven years, and they fall into two main categories. The first comprises horizontal-axis and vertical-axis cross-flow turbines, which have blades set either parallel or perpendicular to the current. The second involves reciprocating devices that have blades called hydrofoils – shaped like airplane wings – that move up and down as the tidal stream flows on either side of the blade. Three sites have been identified in Wrangell Narrows for potential tidal stream energy —Turn Point, South Ledge, and Spike Rock. Read more here and here. Explore more of South Ledge and Wrangell Narrows here: