38 THE UNITED STATES GEOLOGICAL SUEVEY. [bull. 227. 
as the Yukon Flats, and here, divided into many interlacing channels, 
covers a zone 10 to 15 miles wide. From the vicinity of Rampart to 
the mouth of the Xanana River it is again contracted, and then, for a 
distance of 600 miles, flows in a meandering course through a broad 
valley, often bounded by hills and bluffs on the right, until it loses its 
great volume of water in the many distributary channels leading it 
through the great mud flats of the delta to Bering Sea. 
The main tributaries of the Yukon in Alaska are the Koyukuk, 
the Porcupine, and the Tanana. 
The Koyukuk enters the Yukon about 400 miles above its mouth 
and brings to it the drainage from an area several hundred miles dis- 
tant along the southern slopes of the Rocky Mountains. The Porcu- 
pine pursues a peculiar course. It rises near the boundary on the 
Alaskan side, flows northeast toward the Mackenzie, and then, turning 
abruptly at the base of the Rocky Mountains, flows southwest and enters 
the Yukon about 200 miles below the boundary. The Tanana has its 
source near the boundary, in the St. Elias and Wrangell mountains. 
It flows northwestward in a course parallel with that of the upper 
Yukon and drains the northern slopes of the Pacific Mountain system 
and the large portion of the Yukon Plateau on either side of it. 
The drainage areas embraced by the Selawik, Kowak, and Noatak 
rivers lie partly within the plateau region and partly within the Rock} 7 
Mountain region. These rivers follow a general westerly course, 
parallel with the mountain ranges, to the waters of Kotzebue Sound. 
The northern slopes of the Rocky Mountains and the plains region 
are drained by many unexplored streams, which have their sources far 
within the mountains, flow in canyons across the plains, and enter the 
Arctic Ocean in an indefinite way, after traversing the ill-drained area 
of the Coastal Plains. 
COAST LINE AND ISLANDS. 
The coast line of Alaska is very irregular and, including that of the 
numerous islands, has a length of about 26,000 miles. It differs greatly 
in character at different places. In southeastern Alaska the mountains 
rise abruptly from the water and dip steeply below its surface. The 
precipitous character of the coast is carried westward through the 
Aleutian Islands. The western and northern coasts are generally low, 
and shallows are common. The most important inlets are Prince Wil- 
liam Sound, Cook Inlet, Bristol Bay, Norton Sound, and Kotzebue 
Sound. There is an occasional fringe of coastal plain along the 
southern coast, and still more of it on the western coast, while along 
the Arctic Ocean it attains a width of 80 miles. 
The important islands are those of the Panhandle of the southeast 
coast, Kodiak, south of Cook Inlet, and Nunivak and St. Lawrence, 
in Bering Sea. 
