As with the English language, in naming rivers there are exceptions to the rules
Naming a river and its tributaries can be a confusing business. The general convention is that where two rivers meet, the larger of the two carries the name downstream. So when the Mathias and Rakaia rivers merge, downstream they become the Rakaia, as that is the larger of the two. Similarly, in Arthur’s Pass National Park where the sizable White River joins the Waimakariri River near Carrington Hut, downstream the name of the larger river holds sway: the Waimakariri. So far, so simple.
The New Zealand Geographic Board, which makes decisions about such matters, corrected the name of the Windon Burn (a tributary of Mararoa River above Southland’s Mavora Lakes) to Winton Burn and at the same time moved the name from the smaller western stem to the larger eastern stem.
But this convention does not always apply. Take the confluence of the Haast and Landsborough rivers on the West Coast. The Landsborough, having flowed for more than 60km just west of the Main Divide, is by far the larger of the two, but downstream it loses its name to the smaller Haast River.
Other anomalies in the backcountry exist where two rivers join, but neither name is carried downstream. Here are some examples.
Oamaru Hut, Kaimanawa Forest Park
The Mohaka River is a prominent Hawke’s Bay watercourse that begins its existence in the Kaimanawa Ranges, before flowing through Kaweka Forest Park and on down to the sea north of Napier. It’s a popular place for rafting, hunting, fishing and tramping. But where is its head? The map shows the Mohaka begins only after the Kaipo, Oamaru and Taharua Rivers join. Oamaru Hut occupies a terrace overlooking this confluence, and can be reached in 1-2 days tramp beginning at Clements Mill Road.
Julia Hut, Taipo Valley, West Coast
The Taipo drains the mountains west of Arthur’s Pass. It’s an attractive valley featuring several huts and good tramping tracks. Two huts occupy a flat in the valley head: the recently restored Old Julia Hut (an ex-deer culling hut), and the newer one called Julia Hut. Both these huts take their name from nearby Julia Creek, which drains Popes Pass on the Campbell Range. The other major tributary in the headwaters is Mary Creek, a well-used tramping route leading to Harman Pass. So where is the Taipo head? It begins downstream only where Mary and Julia creeks merge.
Meins Knob, Canterbury
Meins Knob, 1276m, is a spectacular sub-alpine knoll overlooking the upper Rakaia Valley. Superb campsites exist on its tussock heights, next to tarns, and with views of both the Ramsay and Lyell glaciers. Downstream lie the mighty braids of the Rakaia. So where is the head of the Rakaia? It doesn’t exist. The Rakaia only begins downstream of the confluence of the Ramsay and Lyell rivers.
Smyth Hut, Wanganui River, West Coast
Just over the Main Divide from the Rakaia Valley is the West Coast’s Wanganui River. But where does it start? Vane Stream and the Evans River drain the Divide, and only downstream of their combined confluence with the Smyth River does the name Wanganui River appear on topographical maps. Smyth Hut occupies a small flat near this confluence, and experienced trampers consider it a destination well worth visiting. However, flooding in January 2013 did cause major damage to the Wanganui Valley Track, and also created a new river channel near Smyth Hut, which means it can become marooned as a wee island during high river levels.