Cook Inlet

Cook Inlet (Tanaina: Tikahtnu; Sugpiaq: Cungaaciq) stretches 180 miles (290 km) from the Gulf of Alaska to Anchorage in south-central Alaska. Cook Inlet branches into the Knik Arm and Turnagain Arm at its northern end, almost surrounding Anchorage. On its southern end, it merges with the Shelikof Strait, Stevenson Entrance, Kennedy Entrance and Chugach Passage.

The Cook Inlet and both its arms are bodies of brackish water, containing a turbid mix of ocean salt-water and freshwater runoff from the various rivers and streams. The narrow channel of the inlet funnels the tides creating very fast-moving currents, rip tides, and occasional bore tides. Cook Inlet watershed is the most populated watershed in Alaska. The watershed covers about 100,000 km² (39,000 sq mi) of southern Alaska, east of the Aleutian Range, south and east of the Alaska Range, receiving water from its tributaries, which include the Knik River, the Little Susitna River, the Susitna and Matanuska rivers, Eagle River, Ship Creek, Resurrection Creek, Portage Creek, Kenai River, and many others. The watershed includes the drainage areas of Denali and is the collection point for the runoff from many surrounding glaciers, leading to the high turbidity and large silt deposits. Within the watershed there are several national parks and the active volcano Mount Redoubt, along with three other historically active volcanoes. Cook Inlet provides navigable access to the port of Anchorage at the northern end, and to the smaller Homer port further south. Before the growth of Anchorage, Knik was the destination for most marine traffic in upper Cook Inlet. Approximately 400,000 people live within the Cook Inlet watershed.

Cook inlet, along with the Kenai Peninsula, the Kenai Mountains, the Chugach Mountains, and Kodiak Island, formed approximately 65 million years ago, just after the extinction of the dinosaurs. Like most of Alaska's mountains, they consist mainly of sedimentary rock and metamorphic rock that were deposited on the ocean floor during the Cretaceous period. This area, where a corner of the Pacific Plate subducts under the North American Plate, uplifted the ocean floor and compressed it, causing the crust to thicken and scrunch. Tectonic forces lifted the mountains and peninsula out of the water, forming a valley that was cut off from the ocean to the north and the west. The rest of Alaska's mountains were formed in the same manner, in cyclic events, and the mountain ranges become progressively older the further north they lie. Thus, the Cook Inlet and its surrounding land masses are rather young compared to those of the Alaska Range, which formed around 126 million years ago, or the Brooks Range which formed around 256 million years ago. The valley left by this continental compaction remains open to the Gulf of Alaska to the south, forming the inlet as it exists today. The surrounding mountains were host the large ice sheets and glaciers during the Pleistocene epoch (the Ice Ages, around 2 million to 11,000 years ago) which scoured the land and formed vast plains and moraines surrounding much of the upper inlet from glacial till that was deposited.

Cook Inlet exists in a subsidence zone which contains many faults, and is frequently prone to earthquake activity. The primary fault, the Aleutian Fault, is found in the nearby Aleutian Trench in the Gulf of Alaska, where a corner of the Pacific Plate is forced underneath Alaska at a 45-degree angle. Thus, most of the uplift force occurs along a line from Kodiak Island and up the Kenai Mountains to the Chugach mountains. The inlet lies in a region where the crust is wrinkled under this force and the tectonic forces push the ground downward. In 1964, the 1964 Alaska earthquake, a megathrust earthquake, occurred when over 600 miles of the Aleutian fault ruptured, uplifting the Kenai Mountains 60 feet (20 meters) in under 5 minutes. With a magnitude of 9.2, the earthquake was the fourth largest ever recorded. It devastated Anchorage where much of downtown dropped several stories, and the mountains surrounding the Turnagain Arm subsided 8 feet (2.4 meters), submerging the towns of Portage and Girdwood, as well as long stretches of the Seward Highway.

Because it lies along a subduction zone, the Cook Inlet region contains active volcanoes, including Augustine Volcano and Mount Redoubt, and is part of the Pacific Ring of Fire. Hot magma and steam from the subducting ocean floor build up within these volcanoes, which tend to erupt in fairly regular cycles with very explosive force, often spewing volcanic ash tens of thousands of feet (several kilometers) high. Volcanic eruptions in the region have been associated with earthquakes and tsunamis, and debris avalanches have resulted in tsunamis also. There was an earthquake of the magnitude of 7.1 on December 31, 1901 generated by an eruption that caused several tsunamis. In 2009 a lahar from Mt. Redoubt threatened the Drift River oil terminal.

Cook Inlet has the fourth largest tidal range in the world. The shape of the inlet and its orientation with respect to the lunar orbit causes the tide to come in and go out very rapidly. As the inlet narrows, the speed of the water increases, creating very powerful currents with speeds of up to 6 knots (7 miles per hour). While lined with large areas of silt and mudflats, the central and upper inlet is filled with narrow troughs that may be 150 to 300 feet (50 to 100 meters) deep. These, along with the tides, provide a challenge to ships navigating through the waters. The strong tides create powerful rip tides and bore tides which are sometimes among the largest in the world. Tidal bores occur within the inlet, and especially Turnagain Arm, almost daily, but are usually too small to notice. Large bores tend to occur after extreme tidal lows, appearing as a wall of water sometimes over 10 feet high as the tide comes in all at once. Large bores are less frequent, and are typically formed during a new or full moon, especially when the moon is at perigee. Unlike areas around the Bering Sea where weather is largely affected by sea ice, the waters of the inlet are warmed by the Alaska Current in the Gulf of Alaska, part of the North-Pacific Subpolar Gyre, which affects the climate and keeps the temperatures in the Cook Inlet region fairly moderate compared to the extremes found in other parts of the state.

The inlet was first explored and settled by Alutiiq people, tribes of coastal-dwelling Pacific Eskimos, beginning around 6000 years ago. The Chugach arrived around the first century and were the last of the Alutiiq people to settle in the area, but abandoned it after tribes of Dena'ina people, an Athabaskan people from the interior of the state, arrived sometime between 500 and 1600 AD. In the 18th century, Russian fur hunters (promyshlenniki) were among the first European visitors. The Lebedev Lastochkin Company leader Stepan Zaikov established a post at the mouth of the Kenai River, Fort Nikolaevskaia, in 1786. These fur trappers used Siberian Native and Alaska Native people, particularly Aleuts from the Aleutian Islands and Koniag natives from Kodiak, to hunt for sea otters and other marine mammal species for trade with China via Russia's then-exclusive inland port of trade at Kiakhta.

Other Europeans to visit Cook Inlet include the 1778 expedition of James Cook, its namesake, who sailed into it while searching for the Northwest Passage. Cook received maps of Alaska, the Aleutians, and Kamchatka during a visit with Russian fur trader Gerasim Izmailov in Unalaska, and combined these maps with those of his expedition to create the first Mercator projection of the North Pacific. The inlet was named after Cook in 1794 by George Vancouver, who had served under Cook in 1778. Turnagain Arm was named by Cook, "turnagain" being a moniker he had used before, at his annoyance at having to turn around after exploring another dead end.