Gravel and its presence in our rivers has been a perennial talking point for decades, involving landowners, engineers, and scientists, with the conversation playing out while standing on riverbanks, in reports and studies, and in the Council Chamber itself.
The major floods of 27 June and 11 July 2025, and the widespread damage and disruption they caused, has amplified calls to restart major gravel extraction in the rivers to provide capacity for flood flows, an activity which we manage under the Soil Conservation and River Control Act and the Resource Management Act (i.e., the Tasman Resource Management Plan).
It is impossible to do this complex topic justice in a short article. What we can do is share some key points on the current state of gravel management to help inform the discussion moving forward.
To start, we have always extracted gravel from the rivers, continuing to this day, with the collaboration of contractors who use the aggregate in a variety of ways. The amount and location of extraction varies depending on where the need is identified from year to year.
We extract gravel to address identified river management issues, which depends on the specific river and the type of management work we do in that river. Most rivers are managed to minimise bank erosion, and in these areas, we do not extract gravel to increase flow capacity or reduce flooding, since that almost always makes bank erosion worse.
As well, we would need to extract an enormous amount of gravel from the river to have any noticeable benefit during the kinds of floods we’ve just experienced, which would lead to deep channels with constantly eroding banks, and would contribute to lowering of the groundwater table as has been seen on the Wai-iti River.
We carry out extraction of river gravels under a global resource consent which on the Motupiko, Motueka, and Waimea Rivers requires us to maintain the riverbed within an average elevation range, which was established based on our historical river cross-section surveys. This prevents over-extraction while also enabling any extraction required if too much gravel accumulates.
We have monitored gravel levels in the rivers historically using cross-section surveys, and more recently with LiDAR (Light Detection and Ranging) surveys which has allowed us to monitor gravel volumes much more accurately.
Through these historical and ongoing surveys, we have seen that our river catchments, on average, do not supply enough gravel to the rivers to keep them stable, and our rivers are generally cutting down rather than building up.
Gravel supply to the rivers is also dependent on floods, landslips, and other major events in the catchment, which are unpredictable and unplanned. This makes it difficult to safely extract a consistent amount of gravel each year, and any extraction needs to be considered against long-term trends.
For example, our ongoing gravel surveys in the Upper Motueka River showed significant loss of riverbed material in recent years. Over the entire managed section of river, from the Wangapeka River confluence to around North Road bridge, there was a loss of 345,000 cubic metres of material from 2022 to 2023, and a further loss of 142,000 cubic metres from 2023 to 2024. The average riverbed elevation was, before the recent floods, at a level that would prevent gravel extraction under our resource consent in many areas.
What we have observed during floods are areas where gravel has built up along with areas where gravel has scoured and moved downstream, and in both cases, there are extreme examples. The faces of the large gravel rivers have significantly changed, with some gravel beaches growing, some disappearing completely, and entirely new beaches appearing along the river.
While rivers are dynamic, constantly changing systems, flooding pushes them into a new state, a new balance, that will continue to adjust and settle for some time.
The long-term recovery work on our rivers will have this new dynamic state front of mind, and we will be looking to river management practices, old and new, including gravel extraction, that will prepare us for the next major flood as best as possible.
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