What is streamflow and why does it matter?

You’ve probably heard the word streamflow when we talk about rainfall and dams. It sounds technical, but it’s actually a simple idea and a really important one when it comes to understanding how we get our drinking water. 

Let’s break it down.

First, what exactly is streamflow?

Streamflow is the water that runs off the land and into our dams as a result of rainfall. But it doesn’t happen straight away. Before water can flow into dams, the ground usually needs to be fully soaked like a sponge that’s finally reached its limit. Then does the extra water start moving across the surface and into our water supply. 

This is why streamflow depends on more than just rainfall. Even if it rains a lot, if the soil is dry or plants are thirsty, most of that water gets absorbed before it ever reaches a dam.

So where does streamflow come from?

It all starts in catchments, natural areas where rain collects and flows downhill, guided by gravity. These catchments feed into creeks, rivers, lakes or dams. Some water also seeps underground, helping plants grow and topping up our aquifers (which store groundwater). 

When catchments are wet enough and the soil can’t hold any more water, streamflow kicks in. It’s the leftover water that finally makes its way into our dams.

It used to be different

Before the mid-1970s, streamflow was much more reliable. Perth and the South West had more consistent rainfall throughout the year, especially in autumn and spring. That meant catchments stayed moist for longer and streamflow started earlier and lasted well into the season.

On average, we used to get around 420 billion litres of streamflow into our dams each year. These days, we’re lucky to get even a fraction of that. A 15–20% drop in annual rainfall has led to an 80% reduction in streamflow. That’s a huge change and it’s mostly because our catchments are drier and rainfall is less consistent. 

In 2025, even with good winter rainfall in Perth, streamflow has been slow to pick up. Most of it arrived late in winter and some catchments in the south had below-average rainfall in June and July. So while it might look wet, not much water is reaching our dams. It’s unlikely streamflow will return to average levels before the rain stops and warmer weather sets in. This shows how dry conditions and less rainfall in autumn and spring are making it harder for catchments to do their job. 

How much streamflow is in our dams?

You can check how much water from streamflow is in our dams on our rainfall and dams pages. Keep in mind that streamflow can change a lot from year to year and that the data is updated at different times each year. The total numbers show streamflow at a specific point in time, which may differ from the most current levels.

View Streamflow data

Why does streamflow matter?

Streamflow used to be Perth’s main source of drinking water. In 2025, it only makes up about 26% of our supply. Most of our water comes from groundwater and desalination. But that doesn’t mean streamflow isn’t important. 

Today, streamflow only makes up about a quarter of Perth’s water supply. Most of our water now comes from groundwater and desalination. But that doesn’t mean dams aren’t important, they still store water from all sources, ready for when we need it most.