Learn about series
A series of information pages on the basics of water.
What is the Strahler stream ordering system?
Strahler stream order classifies the hierarchy of streams and rivers in a river system from smallest to biggest.
- First-order stream: A first order stream starts at the top of a catchment and has no other streams flowing into it. First-order streams are the smallest in a river system.
- Second-order stream: Second-order streams form when 2 first-order streams join. They are bigger in size due to the water from both first-order streams combining.
- Third-order stream: Third-order streams form when 2 or more second-order streams join. And so on.
The stream order keeps increasing as streams and rivers of the same order join. The highest stream order is found on the river's main stem in the downstream section of a river system.
When 2 streams with different orders join, the resulting stream has the same order as the highest of the 2 joining streams. For example, when a first and second-order stream join, the resulting stream is second order.
A stream may separate and come back together. This is called a braided stream or anabranch. A braided stream retains the same order throughout the braid, as if it were a single stream.
Sometimes a lake may be located along a stream. A lake does not change the order of a stream.
What can Strahler stream order tell us about a river or stream?
By knowing the order of a stream, we can better understand our rivers and how to best protect and manage them.
Because no other watercourse flows into them, first-order streams originate from rainfall, snowmelt, or springs. They are usually found in more hilly areas and move quickly downhill. While first-order streams might move quickly, the overall volume of water is low. Many do not flow continuously throughout the year and may only have water after rain or during snowmelt.
Higher-order streams have wider channels, deeper water, and move slower. They carry larger volumes of water because they receive input from many tributaries. This makes them important for maintaining water supply for various uses, including drinking water, agriculture and industry. Higher-order streams also provide habitat for more biodiversity. Because of their large size and slower moving flow, they can support a diverse range of plant and animal species. By identifying a river's order, we can better protect its unique ecosystem.
Higher-order rivers are also more prone to flooding. Understanding stream order can help predict where floods might happen and plan flood prevention measures.
Stream order can also be used to monitor water quality issues. Pollution in smaller, first-order streams can eventually flow into larger rivers. By knowing a stream's order, we can track and control sources of pollution to protect water quality downstream.
How does the Strahler stream order affect water users in NSW?
Under the Water Management (General) Regulation 2018, Strahler stream order is used to determine which watercourses are subject to certain licensing and approval requirements.
In certain circumstances, water users may be exempt from needing an access licence or approval to take water from a waterbody. However, these exemptions may only apply to first, second, or third-order streams.
Strahler stream order is also used to help identify where dams may be located on a landholder's property.
Find more information on how to determine stream order to check for exemptions from water licences and approvals, or to help identify suitable locations for harvestable rights dams.