Cook Inlet 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.The watershed covers about 100,000 km² of southern Alaska, east of the Aleutian Range and south of the Alaska Range, receiving water from its tributaries the Knik River, the Little Susitna River, and the Susitna and Matanuska rivers.