Turnagain Arm (Dena'ina: Tutl'uh) is a waterway into the northwestern part of the Gulf of Alaska. It is one of two narrow branches at the north end of Cook Inlet, the other being Knik Arm. Turnagain is subject to climate extremes and large tide ranges. Geography Turnagain extends in an east–west direction, and is between long. It forms part of the northern boundary of Kenai Peninsula, and reaches on the east to within of Passage Canal, a western branch of Prince William Sound. Turnagain is characterized by remarkably large tides of up to which are the largest tides in the United States. The flood tide often begins with a tidal bore especially on large tides with a strong east wind, which has a height of at times, and runs in from the west at a speed of an hour. At low tide, the arm becomes a broad mud flat, cut by the stream channels. Historically small steamers entered and left on high water though the practice is rare at best since most if not all the places that they went are now connected by road. The region adjacent to Turnagain Arm is very rugged. South Suicide Peak is the tallest mountain rising from the north side of Turnagain, and Mount Alpenglow the highest on the south side. Mountains rise precipitously on both sides of the arm and reach altitudes of . Their tops are ragged and bare. The timber rarely reaches higher than . The smaller valleys are narrow and steep, but the larger ones frequently show by their U-shaped cross section the former presence of glaciers. Glaciers may be still seen at a number of places. Chief among them are Portage Glacier, occupying the Portage Valley between the head of Turnagain Arm and Passage Canal, the two neighboring glaciers which form the headwaters of the Glacier River and Twentymile River, Explorer Glacier, and Skookum Glacier. Besides these, there are several smaller ones on tributaries of Glacier Creek. The Seward Highway follows a portion of the southern edge of the Chugach State Park along Turnagain Arm. Turnagain Arm boasts the second highest tides in North America after the Bay of Fundy. These tides, which can reach , come in so quickly that they produce a wave known as a bore tide. Adventurous kayakers and surfers have taken to riding the tide as an extreme sport. Hikers should take care not