Waimakariri River Sediment Plumes

Waimakariri River Sediment Plumes

The Waimakariri River is one of the main rivers in Canterbury on the east coast of New Zealand’s South Island. It flows 151 kilometres southeast from the Southern Alps to the Pacific Ocean. The river enters the ocean north of Christchurch and flows into Pegasus Bay (named after the brig Pegasus).
The Waimakariri River is a gravelly river. Much of this gravel comes from bank erosion and channel degradation and is transported downstream. Over time, depending on the speed and volume of the water, there is a migration of gravel tongues from the mouth into the bay and around the estuary along the bay. The Waimakariri is almost entirely responsible for the sediment that forms the nearshore of the bay.
For the last 11 years (2013-2024), since the launch of the Landsat 8-9 and Sentinel-2 satellites we have monitored sediment discharge from the river into the bay under clear skies  (Figure 1).
During this time, approximately 40-50 sediment plumes of different sizes have been observed, which is 4-5 times a year. They occur after heavy rainfall in the mountains and more frequently in the second half of the year – from July to November.
There are two interesting phenomena that are rarely observed elsewhere.
Double plume. The spread of two sediment plumes from the river mouth in different directions was observed no more than a dozen times (under clear skies) in the 11 years indicated (Figures 2, 3). This situation is most likely due to changes in the relief of the bottom of Pegasus Bay just before the point where the river emerges and flows into the bay. Pebble deposits raise the bottom of the bay opposite the mouth to near sea level, creating a shoal (this is clearly visible from above by the wave roll, Figure 5).
Drains open up from the south and north, carrying water filled with river sediments. These drains form “point” sources of sediment plumes.
Interestingly, double plumes were observed in 2020-2024 and – to a lesser extent – in 2013-2014. They were not observed in the period 2015-2019 (again: satellite observations from under clear sky conditions).
In most of the cases observed, there is no obstruction upstream of the river mouth, so it is a “normal” single sediment plume  (Figure 4).
The second interesting feature of the Waimakariri River sediment plumes is the presence of a pulsating sediment concentration in the river water plume coming out of the estuary. This concentric wave-like change in concentration continues throughout the plume and is clearly visible in both double and single plumes (Figures 3, 4). The dynamics of water flow through narrow drainage channels and the deposition of sediment and gravel may play a role.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

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