Reading river flow and rainfall data before a trip

A generation ago, the only way to know what a river was doing was to walk to it. Now most regional councils run networks of river and rain gauges and publish the readings online, often within minutes. Used well, that data is one of the best planning tools a tramper or paddler has. Used badly, it gives false confidence. This note covers where to find it and how to read it.

Where the numbers come from

Regional councils measure rivers because they manage floods and water takes. Horizons Regional Council, which covers Manawatū-Whanganui, says its environmental data network has more than 800 monitoring sites collecting river, rainfall and other data, and that the data is free for anyone to use. Other councils run similar networks for their own regions.

If you don’t know which council covers a river, start with LAWA’s water quantity pages, which bring together live river flow, rainfall and groundwater readings by region. LAWA notes that some regions can’t yet supply a full set of data, so if a site is missing, check the council’s own website.

For rain, the place to start is MetService’s severe weather warnings, which include heavy rain watches and warnings, alongside the ordinary forecast for the ranges you will be in.

Level and flow: two different numbers

Most gauge pages offer two readings:

• Stage or level, in metres: how high the water is at the gauge, measured against a fixed point. It is not the depth of the river, and a reading at one gauge tells you nothing about another.
• Flow, in cubic metres per second (often called cumecs): how much water is going past. Flow is worked out from the level using a rating curve that the council maintains.

For trip planning, the useful question is rarely “what is the number?” and more often “is it normal, and which way is it heading?” Compare today’s reading with the usual range shown on the gauge’s graph, and look at the trend over the last day or two.

Reading a hydrograph

The graph of flow over time is a hydrograph. After heavy rain it usually has a recognisable shape.

• The lag. Rain falls in the catchment, then takes time to reach the gauge. On a short, steep West Coast river the lag can be brief; on a big river draining a large catchment it is longer. Either way, a fine morning at the road end does not mean the flood has passed. It may not have arrived.
• The rising limb. A steep rise means the river is responding fast. If the graph is climbing when you look, assume it will keep climbing for a while.
• The peak. You only know you have seen the peak once the line has turned down.
• The falling limb. Rivers usually fall more slowly than they rise. Water keeps draining off the hills and out of the ground for hours or days after the rain stops.

Snowmelt adds its own pattern: in spring, rivers fed from high ground can rise in the afternoon on warm days, without any rain at all.

Traps to avoid

The gauge is not your crossing. Gauges are often well downstream of tracks, on the main river near a road or bridge. A side stream you have to cross may be in flood when the main gauge looks calm, or the other way round. Use the gauge as a clue about the catchment, not as a reading for your crossing.

Old data looks like current data. Check the time on the last reading. Gauges stop reporting in floods, power cuts and equipment faults, and the graph may simply stop at the last good value.

No gauge, no information. Many small backcountry rivers have no gauge at all. Then the weather in the catchment is your main clue.

Numbers don’t give permission. Other people’s trip notes sometimes say a crossing is fine below a certain gauge level. Treat those as rough guides from someone else’s day, in someone else’s conditions. What you see at the river decides, and our river crossing basics cover the warning signs to look for.

A simple routine

1. A few days out: look at the forecast for the whole catchment, including the tops, and any heavy rain watches or warnings.
2. The day before: find the nearest gauges upstream and downstream of your route and note the usual range on their graphs.
3. On the morning: check the latest readings and the time stamp. Is the river rising, falling or steady?
4. On the track: keep watching the sky and the water. Rain upstream will arrive later.
5. At every crossing: decide on what you can see, with a plan to wait or turn back.

For paddlers

Kayakers and packrafters use the same gauges, usually more closely, because a river that is too low to paddle one day can be dangerously high the next. Club river guides often list the flows at which a run is usually done. As with trampers, those are starting points; the river on the day decides. More in our note on getting started with packrafting and river kayaking.