When Water Stops Flowing: Stream Types Explained
Learn to Identify Stream Types

Do you have a stream in your area that stops flowing occasionally? If so, it's most likely supposed to. That's because of its stream type.

There are three types of flows to describe streams: year-round (perennial), seasonal (intermittent) and rain-based (ephemeral). Read on to learn about these stream types and some vocabulary words.


Year-Round | Perennial
This type is easy to recognize because, just like its name, water always flows no matter what time of year, unless it has been particularly dry. Think of rivers or larger streams. These streams flow year round because they receive water from smaller streams that drain into them and because they are connected to groundwater.

You may already be familiar with the word ‘perennial’ through choosing plants for your yard. Perennial means ‘lasting or existing for a long or apparently infinite time; enduring or continually recurring.’

A year-round perennial stream in the Turtle Creek Watershed.

Seasonal | Intermittent
‘April showers bring May flowers.’ Here in PA, we usually experience more rain in the spring and fall, and because of that increase, seasonal streams start flowing. This flow is related to groundwater, which is the water beneath the Earth’s surface stored in soil and rock pores. Although these streams may look dry during periods of the year, water still flows below the surface because of groundwater.

The word intermittent means ‘occurring at irregular intervals; not continuous or steady.’ Think: ‘It rains intermittently during the year.’

Rain-Based | Ephemeral
These streams only flow after a rain event, which means that rain or melted snow are the only source. This is different from the above stream types because those streams are linked to other sources like groundwater.

Ephemeral means ‘lasting for a very short time.’ As an example, ‘This flower has an ephemeral bloom.’

One thing is sure about each stream: they are all important! Each has an impact on the quality of our drinking water and on the creatures that live or depend on it.

A rain-based ephemeral stream in the Saw Mill Run Watershed

If you are interested in learning more about all of our creeks, or cricks, check out the links below: