The most distinctive feature of the topography of New Guinea is the towering cordillera that dominates the skyline of the interior of the island practically throughout its length, together with the dissected plateaus that lie within its central regions. This great mountain system--one of the most impressive in the world--is composed of a number of roughly parallel mountain ranges in the broader parts of the island, narrowing to a single line of hills in the "neck" of the birdlike peninsula (Vogelkop) and to a lofty sierra, the Owen Stanleys, in the southeast. Among the higher and more important ranges, in addition to the Owen-Stanley, are the Oranje and Schneegebirge (Snowy) ranges in Dutch New Guinea and the Mt. Hagen, Bismarck, Musgrave, and Kratke ranges in Australian (eastern) New Guinea. The highest known peak in Dutch New Guinea is Carstensz Toppen, whose gleaming, snow-capped summit rises some 16,400 feet above sea level; highest of the eastern chain as far as surveys now show is Mt. Wilhelm in the east-central part of the island near Chimbu, which rises a little over 15,000 feet and is occasionally dusted with snow. All the snowcapped peaks lie within 8° (550 miles) of the equator.
In the main body of the island to the north and south of the inland ranges lie vast, swampy lowlands most of which are densely forested and carry only sparse native populations. Those of the northern area appear to be geosynclines, like the Mamberamo, Idenburg, and Sepik lowlands, although the Markham-Ramu valley may well be a combined geosynclinal-rift valley form. The southern lowlands are essentially delta plains formed by the three major rivers of the southern slope, the Digoel, Fly, and Purari. Elsewhere elongate coastal or delta plains and low river or marine terraces interdict the coastal hills and ranges in places. Among the more extensive of these are the lowlands about MacCleur Gulf, where producing oil wells are now located; the Geelvink Plain fringing Geelvink Bay; the western extensions of the DigoelFly lowland; the crescent-shaped lowland about the head of the Gulf of Papua, and the Buna-Collingwood lowlands southeast of Lae on the northeast coast. Plateaus other than the one mentioned above in the eastern interior include the interior of the Vogelkop or "bird's head," the Bomberai Peninsula southeast of MacCleur Gulf; the volcanic plateau northwest of the head of the Gulf of Papua; and the low Oriomo Plateau inland from the south-central shores of the island.
Along and inland from the north coast of the island rise several linear mountain ranges. These are all composed of rocks and rock structures older than those of the interior ranges or adjacent lowlands, and at one time in the remote geologic past may have been linked with Asia through Japen Island, the northern mountains of the Vogelkop, Celebes, and Borneo. The highest peaks in this group lie in the Saruwaged and Finisterre ranges, where several summits reach to 12,000 feet and a few slightly exceed 13,000 feet.
New Guinea contains the only large rivers in Melanesia, and these are neither very long compared to the Amazon, Mississippi-Missouri, Nile, or Yangtse nor well known outside New Guinea. Nevertheless some of them are mighty rivers in their own right, in terms of navigable length, volume, and hydroelectric potential. The Fly is navigable by steam launch to a distance of 500 miles above its mouth, and the Sepik over 300. The Purari and Markham are bordered in places by floodplains of rather unusual fertility for equatorial lowlands. In their lower extremities the Fly, Digoel, Ramu, Sepik, and Memberamo are wider than the lower parts of the Mississippi River, as deep ill places, and scarcely less impressive. After exceptionally heavy rains in the interior they may flood hundreds of square miles of lowland, form great, shallow, temporary lakes, and color the sea with muddy water to a distance of many miles out from the shoreline.