Depositional Zones

A depositional zone is where sediment has been transported by floodwaters following erosion.

Catchments

Worldwide, the accepted concept for managing river flows, erosion and sediment is the catchment, defined as an area where water (from precipitation, ice and snow melt or from beneath the ground) is collected by the natural landscape and flows through rivers, lakes and groundwater to the sea.

Catchments can be divided into three zones in terms of erosion sediment generation, transport and deposition (see image below). New Zealand is a “land uplifted high” and it is the process of geological uplift and then downwards erosion in our steepland headwaters classified as Zone 1 that generates the majority of erosion sediment in our catchments. This sediment is transported by floodwaters down to the “Transfer zone” classified as Zone 2 in our lowland hills. Sediment in floodwaters may be temporarily deposited in Zone 2, but eventually it is picked up and transported through to Zone 3, the “Depositional zone”. Here sediment is laid down by floods and contributes to the slow building of wide, flat valleys and floodplains.

Erosion and the transport of sediment are natural geological processes, driven by uplift and erosion. However, indigenous and plantation forests in Zone 1 can slow down the supply of erosion sediment generated here that would move down into Zone 2 and Zone 3, because the trees protect and reinforce the soil against erosion. Conversely, clearing of indigenous and plantation forests in Zone 1 can lead to increased erosion rates and sediment volumes supplied to the river systems. Landslides (and other associated mass movement processes) in Zone 1 are the predominant process that deliver sediment and woody debris to the river systems, although large gullies are important in the Tairāwhiti region.

Note that the highest density of human population and associated infrastructure is in Zone 3, followed by Zone 2. We need to manage forest clearance in Zone 1 to control the amount of erosion sediment and woody debris that flows down into Zone 2 and Zone 3, in order to minimise their effects on people, infrastructure and settlements.

River Environments

The following diagram shows the three catchment zones.

• Name

  Zone 1

  Type

  Headwaters

  Description

  Headwater streams swiftly flow down steep mountain slopes and cut deep, v-shaped valleys. Waterfalls and rapids occur in this zone.

• Name

  Zone 2

  Type

  Transfer zone

  Description

  Lower-elevation streams merge to flow down gentle slopes. Valleys broaden as coalescing rivers start to meander.

• Name

  Zone 3

  Type

  Depositional zone

  Description

  At the lowest elevations, a river meanders across a broad, nearly flat valley and floodplain. At a river's mouth, it may divide into separate channels as it flows across a delta extending out to sea. The coastal plain and delta are made of river sediments.

Different zones