Interactions – Rakaia River Catchment
Processes are Tectonic Uplift and Orographic Rainfall
Both processes have elements which interact.
Elements – Relief affects
Precipitation
Elements –Precipitation affects
Relief
The type of relief affects the amount of rainfall received.
In the upper Rakaia River Catchment the relief is high and mountainous e.g. including the main divide of the
Southern Alps and ranges such as the
Mt Hutt Range. The steep relief of the upper catchment receives high
Precipitation affects the relief.
Due to the high precipitation (8,ooo mm/yr) in the upper catchment of the
Rakaia River Catchment there is a high rate of fluvial erosion. The relief of the upper catchment is being shaped by this dominant process. On the slopes fluvial erosion processes precipitation (1000-8000mm/yr) as a result of the process of orographic rainfall. The westerly air flow crossing NZ has to rise over the mountain barrier and the air cools, condensates and precipitation occurs.
Due to the high altitude much of this falls as snow. This results in a summer maximum flow in the Rakaia
River due to snow melt in Spring and
Summer.
As the Rakaia River has a short and steep catchment and many tributaries flowing into the main river channel the river flow rises quickly when a storm event occurs in the upper catchment.
The lower catchment is on the
Canterbury Plains and is a rain shadow area so it has much lower precipitation – under 1000mm/yr. such as rainsplash, sheetwash, rilling and gullying are frequent and have created many rills and gullys. There are numerous V-shaped valleys with single thread streams and interlocking spurs created by fluvial channel erosion processes eroding quickly into the soft sedimentary mudstones and sandstones.
Because of the high precipitation and weak geology a very large load is carried by the river and this is why the Rakaia is a braided river.
Also where the river moves onto gentle slopes – e.g. river terraces of the lower catchment of the
Canterbury Plains the large amount of material carried by the Rakaia River from the upper catchment means there is a high rate of fluvial deposition. This creates deposition features such as the alluvial fan at
Little River.