The Fly and the Sepik are both in the top twenty rivers in the world in terms of water discharge, despite their comparatively small catchments. Both rivers drain around one hundred times more water per unit area than the Amazon River. In their lower reaches the rivers follow meandering courses, flanked by numerous grassy back swamps and oxbow lakes. This is because warping of the Fly River basin has decreased the erosive potential of the river, and drowned part of the floodplain to produce lakes such as Lake Murray. It has been suggested that the Fly River used to drain south into the Gulf of Carpentaria, but that upwarping of the southern part of the basin diverted the river into the Gulf of Papua.
Vegetation consists mainly of moist tropical forest with extensive grass swamps close to the rivers.
Frequent earthquakes, accompanying active tectonic uplift of the mountains mean that landslides are common, further increasing the sediment load of the rivers. Looking down the course of the Tungom River after the landslide of 1994 Vegetation zones in the mountains are controlled by altitude, with a succession through a variety of closed forest types to moss forest at higher altitudes and finally alpine grasslands. Open vegetation types such as secondary forest and grassland are the result of landslides and active human intervention through gardening and burning. The geology posed one of the greatest barriers to the progress of the expedition. In geological terms the party was faced with passing through a series of Miocene karst limestone ranges.