greatest rivers in terms of flow, and second only to the Waikato River among North Island rivers. Bridges The river is crossed by 18 road, 3 rail bridges and He Ara Kotahi walk/cycleway. The lowest and longest, Whirokino Trestle and Manawatū River Bridge, carry SH1 over the Moutoa Floodway and over the river. It replaced the 1938 and 1942 bridges in February 2020, at a cost of $70m. Geology The Manawatū is unique among New Zealand rivers in that it crosses a mountain range. The river has formed a "water gap" across the mountains because it is older than the Ruahine and Tararua Ranges. Most rivers arise from an already-existing range of mountains or hills, but beginning about 3 million years ago the central North Island mountain ranges began to uplift across the Manawatū's current course. Because it drained a large catchment, the river had sufficient flow to keep pace with and erode the rising mountains, eventually forming the Manawatū Gorge; other rivers were unable to and were diverted into the Manawatū instead. After exiting the Manawatū Gorge, the river carries rock and sediment down from the mountains. During glacial times, with the prevalence of ice, snow, and bare mountains, this erosion increases and forms a stony elevated terrace. During an interglacial, while the mountains are forested, gravel outwash is reduced and the river cuts down into the terrace, forming a gorge. This cycle has created four distinct terraces between the Manawatū Gorge and Palmerston North. The Manawatū Plain was seabed 5 to 6 million years ago, and as it was raised above water by the action of the Australian and Pacific Plates it buckled, forming five long and low ridges (or anticlines) parallel to the mountains, which impede the flow of the Manawatū, Rangitikei, and Oroua Rivers, forcing them to flow southwest rather than directly into the Tasman Sea. Vegetation At the time of human arrival, the Manawatū Plains were covered with forest. Towards the foothills and the Manawatū Gorge grew black beech, turning into tawa forest at lower altitudes. Along the plains and terraces the forest was mixed podocarp and tōtara, changing to mixed tawa, tītoki, and māhoe in the sand dunes. Across the flood plain of the Manawatū and on the low-lying land bordering the river, the predominant vegetation was semi-swamp forest, mostly kahikatea and pukatea. Three major swamps bordered the Manawatū: Moutoa Swamp north of the