River flow data

Daily river flow information can be sent to your mobile. The cost is 30 cents per message, charged by your carrier, not Environment Canterbury. This river flow information is not to be used for resource consent compliance purposes.

How to receive text messages in 3 simple steps

1. Look up the shortcode for the river you want (see list below)

2. Text "RV" (for river), a space and then the shortcode to phone number ECAN (3226). For example, send RV CLAR to ECAN for information on the Clarence River. Remember to include the space between "RV" and the shortcode.

3. You will receive the information in a text message.

All the data we collect from our sites in the field are collected in accordance with National Environmental Monitoring Standards (NEMS). 

Flow derivation from water level

To produce the flow plots as seen on our website, our systems derive flow from a relationship with water level (and in a few cases, velocity). Flow is almost never continuously measured directly.

The flow sites on our website measure water level (also called stage) every five minutes and send the data to our database usually once per hour, 24hrs/day.

• Visit Water levels|NEMS website for more information.

Maintenance of "water level to flow" relationship

To establish and maintain the “water level to flow” relationship, our staff undertake flow measurements (gaugings), routinely one per month, and more frequently during unusually high or low flows. A change in the relationship is brought about by a change in the effectual riverbed shape, level and or slope.

Aquatic weeds can also affect the water level and subsequent flow. Some of our sites are affected by tides, which are removed from the data used for analysis, but not from the data displayed on the website, for example, Waimakariri River at OHB. 

Decision-making for site visits

Each site is different. Our staff understand what flows and conditions are likely to produce a change in the relationship at the sites they visit and balance this against a number of factors when considering an unscheduled site visit.

Gauging accuracy

Gaugings are our primary means of establishing and maintaining the flow record. Using the best methods and equipment at a good gauging site produces a measurement that can be 95 percent certain and within five percent of the actual flow. At a less favourable site, for example, during a flood, this will usually extend to eight percent.

• Visit NEMS website | Open channel flow measurement for more information.

Rating curve for flow conversion

To convert water level data into flow, we use a rating curve. We plot our gaugings on a grid with water level on the Y axis and flow on the X axis. Gaugings are given eight percent error bars to allow for uncertainty in the gauging, water level measurement and rating curve drawing.

A curve is drawn through the gaugings, and this curve will then allocate a flow for any given value of water level.

• Visit NEMS website | Rating curves for more information.

Other flow anomalies

Some of our medium-to-large rivers have two or more flow sites – one near the coast and one or more further up the catchment. This allows for some easy flow comparisons, but even this relationship varies according to several factors. Using the Waimakariri River as an example, northwesterly rain will usually see similar flows at the upper and lower sites, whereas rain from the southerly quarter can see considerably more flow at the lower site. This in-river relationship can also be affected by water abstraction regimes, exchanges to or from groundwater, evapotranspiration and local topography.

Another reason comparisons can be difficult to make is the distance between the sites. This causes a time lag in the data and makes comparisons tricky until the river is in a stable flow condition.

Rating changes

Because gaugings are our check of the water level to flow relationship, we do not know for sure if the relationship is still accurate between gaugings. If a gauging shows that there has been a change, we need to look back through the data to see when that change may have occurred and then apply the new rating curve from that date. This means the near real-time flow data is operational flow data that is based on our best knowledge at the time, but can change retrospectively when a new rating is applied.