Cook Inlet, showing Knik and Turnagain Arms

The Cook Inlet or Nuti Inlet is a large inlet of the Gulf of Alaska in south-central Alaska. It stretches for approximately 195 miles or 310 kilometers southwest to northeast, separating the Kenai Peninsula from mainland Alaska. It branches into the Knik Arm and Turnagain Arm at its northern end, on either side of Anchorage.

The inlet was first explored by Europeans in 1778 when James Cook sailed into it while searching for the Northwest Passage. It was named after Cook in 1794 by George Vancouver, who had served under Cook in 1778.

Its watershed covers about 40,000 square miles or 100,000km² of southern Alaska east of the Aleutian Range and south of the Alaska Range, receiving the Susitna and Matanuska rivers. The watershed includes the drainage areas of Mount McKinley. Within the watershed there are four active volcanoes and seven national parks. The inlet provides navigable access to the port of Anchorage at its northern end, and smaller Seward further south. Approximately 400,000 people live within the Cook Inlet watershed.

The Cook inlet Beluga whale is a gentically distict and geographically isolated stock [1]. The population has fallen to less than 300, apparently due to pollution and habitat disturbance, although hunting has also played a role.

Turnagain Arm is one of only about 60 bodies of water worldwide to exhibit a tidal bore. The bore may be more than six feet high and travel at 15 miles per hour on high spring tides . On a typical day, Turnagain Arm sees tides of more than 30 feet, second in North America to Canada's Bay of Fundy. The ocean's natural 12-hour 25-minute tidal cycle is close to Turnagain Arm's natural resonant frequency, which then reinforces the tide similar to water sloshing in a bathtub.

Turnagain Arm and Knik Arm are known for their abundance of silt. At low tide, the arms' silty bottoms are exposed, making marine navigation impossible. These so-called mudflats are also dangerous to walk on, exhibiting quicksand-like characteristics, and have claimed the limbs and lives of several beach explorers who have wandered out on them. For this reason cruise ships dock at Seward or Whittier in Prince William Sound and transport passengers via bus or train to Anchorage. However, over 95% of freight entering Alaska comes through the Port of Anchorage, which is served by major container ship companies and other carriers.