Twentymile River rises from a series of small, unnamed glaciers and snowfields in the western Chugach Mountains and flows generally south for 10 miles (16 km), where it joins Glacier River, then continues another five miles (8 km) southwest to Turnagain Arm, about 12 miles (19 km) northwest of Whittier and nine miles (15 km) southeast of Girdwood, Alaska. Glacier River begins at the terminus lake of Twentymile Glacier and flows south for 1.3 miles (2 km) through a wide valley, where it joins the Carmen River. It then turns southwest and flows three miles (5 km) to meet the Twentymile River. The local name was reported in 1898 by Captain Edwin F. Glenn of the U.S. Army.

Edwin Glenn graduated from the United States Military Academy in 1877 and was commissioned into the 25th Infantry Regiment. In 1898 he commanded relief and exploration missions in the District of Alaska, keeping journals of his travels through Upper Cook Inlet that are now held in the University of Alaska Anchorage‘s archives. Alaska’s Glenn Highway and the town of Glennallen were later named for him. In 1900 he became a judge advocate in the Philippines, where he and the soldiers under his command subjected Filipino prisoners to torture, a crime for which he was tried and found guilty. He served in various capacities during the first world war and retired as a brigadier general in December 1919.

In 1898 Brigadier General Nelson A. Miles dispatched an expedition to Alaska under the command of Captain Edwin Glenn to explore and map a transportation corridor for a railroad or wagon road from the ice-free ports of Seward and Portage Bay (now Whittier) to the Yukon and Tanana rivers. Luther S. Kelly was assigned to the expedition as chief scout and tracker, and was teamed with the geologist Walter C. Mendenhall to find a route through the Chugach Mountains. They found two: one ran from Portage Bay across Portage Glacier, up the Twentymile to Winner Creek, then over Crow Pass to Eagle River; the other went up Indian Creek to Ship Creek and then to the Eagle River flats. Read more here and here. Explore more of Twentymile River and Turnagain Arm here: