Steeply sloping mountain areas, exceptionally heavy rainfall, geologic instability in all except the most southerly areas, and the rapid growth of both population and commercial enterprise have combined to create some of the highest soil-erosion rates in the world, rivaling those of the Himalaya region. Consequently, while rivers are usually quite short in length, they carry extraordinarily high sediment loads, which have built up vast swampy plains and deltas, especially along the Sepik, Ramu, Fly, and Purari river systems. Once they leave the Highlands, often through spectacular gorges, such rivers meander slowly across the sediment plains. 

For example, some 510 miles (820 km) from its mouth, the elevation of the Fly River is a mere 60 feet (18 metres) above sea level, an average downhill gradient of only about 1.5 inches per mile (2.4 cm per km). The high deposition rates create major problems for any proposed human use of those rivers, such as transportation or hydroelectricity generation. The northern volcanic fringe contains some of the most fertile soils of the islands