Water Quantity Monitoring (Flows and Rainfall)

Monitoring rainfall and river flows helps us understand how much water is available, how rivers respond to weather events, and how catchments change over time.

This information supports:

Flood warning and management, helping predict and respond to flood events
Water resource management, including water supply and environmental flow requirements
Environmental monitoring, understanding ecosystem stress during droughts and floods
Climate trend analysis, tracking long‑term changes in rainfall and river behaviour
Infrastructure planning, informing design, resilience, and maintenance
progress toward Nelson Resource Management Plan Freshwater outcomes

Together, water‑quantity monitoring helps protect people, property, ecosystems, and water resources across Nelson’s catchments.

How Water Quantity Is Measured - The Hydrology Network

Nelson City Council operates a network of river‑flow and rainfall monitoring sites across all five freshwater management Units (FMUs), including automatic gauges that provide near real‑time data, typically updated every 30 minutes.

These sites allow Council to:

track current river and rainfall conditions
detect droughts and high‑flow events
trigger flood alarms during extreme weather
understand how frequently floods and low‑flow conditions occur

To ensure accuracy, Council staff visit monitoring sites regularly to carry out physical river‑flow measurements (gaugings), measured in litres per second (L/s) or cubic metres per second (m³/s). These checks confirm that automated data aligns with long‑term flow records.

Because Nelson City forms part of wider catchments draining into Tasman Bay, Nelson City Council data is presented alongside Tasman District Council monitoring, providing a complete regional picture.

Monitoring Sites in the Nelson Region

Water level/flow and rainfall monitoring sites within Nelson City

These sites help track conditions in key urban and rural catchments.

Additional river‑flow and water‑level data from Tasman District catchments is included to provide a full regional picture, recognising that many rivers cross council boundaries and share downstream environments.

River‑Flow and Water‑Level Data

Nelson River Flow Data by site:

Tasman District River Flow Data by site:

Nelson Rainfall Data

By site:

What We Know – State and Trends

Long‑term monitoring shows that rainfall and river flows in the Nelson region can vary significantly from year to year.

Average Conditions

The average annual rainfall across Nelson’s monitored catchments is around 1,000 mm per year
Historically, February–March tend to be drier months, with October typically wetter

Recent Variability

In recent years, rainfall patterns have become more variable, with:

prolonged dry periods
large rainfall events occurring outside traditionally wetter months

Between 2019 and 2023, rainfall and river‑flow records show frequent extremes, contributing to drought stress, flooding, erosion, and landslides in several catchments.

What We Use Flow Data For

River‑flow and water‑level data is used to calculate a range of statistics that inform freshwater management, including:

mean and median flows
low‑flow statistics, such as the 7‑day Mean Annual Low Flow (7‑day MALF)

7‑day MALF is calculated by identifying the lowest 7‑day average flow for each year and then averaging these values across the full period of record. This statistic is widely used to help:

set and manage minimum flows
assess water availability during dry periods
understand ecological stress during low‑flow conditions

Flow statistics are derived either from long‑term continuous records or, where continuous monitoring is not available, from correlations with similar nearby sites.

Significant Rainfall Event - August 2022 Floods

A Record‑Breaking Event

In August 2022, Nelson experienced an exceptional rainfall event, described as approximately a 1‑in‑120‑year event.

Notable observations included:

Brook at Third House recorded 275.6 mm in 24 hours and 806 mm for the month
Maitai at Forks recorded 695 mm over five days and 749 mm in the month
Monthly totals exceeded 230–250% of the long‑term August average

Rainfall persisted over several days rather than falling in short, intense bursts, leading to sustained high river flows and widespread catchment impacts.

Other major rainfall and flood events occurred:

February 2021 — significant flooding in Whangamoa and Collins catchments
February 2022 — record flooding in the Collins catchment
May 2023 — flooding and landslides in Brook, Maitai, and Wakapuaka catchments

Much of this period coincided with a prolonged La Niña phase (2019–2023), which likely contributed to cumulative impacts.

River Flows and Flood Behaviour

Key observations:

Many rivers experienced multiple flood peaks during the August 2022 event
The Collins catchment recorded one of its largest floods on record, with an estimated 70+ year return period, following an even larger event just six months earlier
In the Wakapuaka catchment, high flows caused substantial erosion and bed‑level change
Smaller catchments, such as Orphanage Creek, experienced more moderate events

Catchment Impacts

Major flood events resulted in:

significant riverbank erosion
riverbed aggradation (sediment build‑up), including up to 0.8 metres of bed‑level rise at the Wakapuaka at Hira monitoring site
damage to monitoring infrastructure requiring repair or replacement

These changes affect river capacity, flood behaviour, and long‑term ecosystem function.