At the north, the Mackenzie enters the nearly tideless Arctic Ocean, where floating ice almost completely counteracts the tendency of the wind to produce currents, and a great delta is being extended seaward. The river divides on its delta into many _distributaries_ and enters the sea by several mouths. The sea near the mouths of the river is reported to be shallow, and obstructed by many sand-banks and islands. No survey of the Mackenzie delta has as yet been made, and but little definite information concerning it is available. The Yukon on entering the shallow eastern portion of Bering Sea, where the influence of the tides is small and floating ice is present throughout about nine months each year, is also engaged in building a great delta which projects into the sea and gives the coast-line a bold outward curve. The Yukon begins to divide into separate channels, several of which enter the sea as independent distributaries at a distance of about 150 miles from the outer border of its delta. The distance between the outer finger-like division of the stream is about 90 miles. The Yukon is a graded stream--i. e., is able to carry material in suspension, but not to deepen or fill its channel--in the lower portion of its course, and is making an important addition to the land owing to the dropping of its burden of silt as soon as the still water into which it flows is reached. The stream is thus being extended, and in order to enable it to continue its task of transportation and the delivery of its load to the sea, the extended portion of its channel is built up so as to give a slope down which the waters can flow--that is, the beds of the distributaries are raised, and they also shift their positions from time to time and make additions to the entire surface of the delta. This extension of the stream and deposition of silt by its distributaries have added about 1,000 square miles to the land. Although the delta of the Yukon presents an admirable example of the change in a coast-line produced by the sediment dropped by a great river, the partial surveys of it that have been made are not as yet available for study. Fully as characteristic of the modification of coast-lines made by a stream as any in the world is the well-known example of the delta of the Mississippi. This classical instance illustrates not only the manner in which coast-lines are modified, but the behaviour of a large silt-laden stream which has reduced its valley to a low gradient, and throughout hundreds of miles of its lower course is spreading out a wide flood plain. The extension seaward of this flood plain forms the broad delta at the river's mouth. [Illustration: FIG. 12.--Delta of the Mississippi. After United States Coast and Geodetic Survey.]