outdoors stream fun awomanlessordinary.com

Outside fun: race rubber ducks in streams and make meanders (Hydrology)

Streams are great places for kids to play and learn at the same time.  I’ve tried several ways of explaining in words to my Geography students aged 16+ what can be so much more easily be shown with play.  If only there were more opportunities for us to get out of the classroom!

Anyway, next time you’re near a stream with kids, I hope you’ll enjoy putting some theory into practice.  Below I explain some hydrology basics in a way that parents and older children can understand, then make suggestions that kids of any age can then enjoy.


Velocity: Size matters

First the theory:  Water that is in contact with the stream bed and to a lesser extent the air above it flows more slowly than water that is in the centre of the channel due to friction (drag).  This is why water that is flowing through a deep, wide channel does so at a higher speed (velocity) than water in a shallow, narrow channel.

Here’s the maths, in case you’re interested / don’t believe me (white water upstream certainly looks faster than calm water downstream).  My drawings are of river channel cross sections.  I’ve made them simple to aid understanding:

Hydraulic Radius is a calculation of stream efficiency – i.e. how much of the water is affected by drag in proportion to how much water is in the centre of the channel.  It’s calculated using the cross sectional area (for these examples that’s just depth x width, although if you want to be more scientific, visit the Royal Geographical Society) and the wetted perimeter shown on the diagrams in red.  The calculation is :   Hydraulic Radius = Cross sectional area / wetted perimeter.  The larger the hydraulic radius, the faster the stream will flow.

stream river velocity awomanlesordinary.com


Getting the kids involved: Take brightly coloured objects that float like oranges or rubber ducks to the stream.  Throw them in an see how fast they go!  (Hydrologists use metres per second).  Can you alter the stream to make it any faster.  HINT: you’ll need something to dig with.

Man-made intervention: Alter the water level

Billy, my husband and I love trying to dam streams.  It’s really hard to do it well with natural materials – the water usually finds a way through.  Use sticks, soil and stones to block off the stream’s flow.  Your success may well depend on the type of soil that is available.  We usually make a good attempt before lunch, mark the “reservoir” water level, eat and then see after lunch what affect it has had.  Remember though to burst your dam before you leave in case your alterations affect local wildlife!


Velocity: Volume matters

First the theory:  Recent weather can affect stream velocity.  A stream that is full will flow faster than one that has less water in it.  Here’s the maths again to prove it:

stream volume hydraulic radius awomanlessordinary.com


Getting the kids involved:  Visit the same stream at different times of year, after heavy rains and after droughts.  Each time, race your ducks / oranges to see the difference.  Why not also build a dam upstream, have a race, burst your dam and have another race!


Velocity: Shape and load also matter

These two examples have the same cross sectional area.  Why do they not flow at the same speed?

stream shape channel efficiency awomanlessordinary.com


It’s because the wetted perimeter is greater on the stream on the right.  Think about the friction that must be overcome in both examples.

It’s the same reason why the stream on the right in the diagram below flows more slowly than the one on the left.  Despite the channels having the same dimensions, large boulders on the stream bed increase the length of the wetted perimeter.  A larger proportion of the water in the stream is in contact with the bed, increasing drag.

river load hydraulic radius awomanlessordinary.com

Getting the kids involved:  Try moving the sediment (that’s soil, gravel, pebbles etc.) around in the channel.  Can they make the water flow faster or slower by its position?  How much impact does a large boulder have in comparison to some small pebbles?


Velocity: Position matters

Still got those oranges and rubber ducks?  Does it make a difference where on the stream’s surface they start from?

It does!  Even in my idealised diagrams it would make a difference but more so in real stream channels where the sides are more sloped, or across a meander.  In the diagram below, the yellow cross marks where the water would be flowing most quickly in a straight channel or on a meander.  Aim your duck for just above it if you want to win!

river velocity awomanlessordinary.com

Make meanders

Meander is the term Geographers use to mean a bend in the river.

First the theory:  Watch this short video to understand why a meander forms.

Getting the kids involved:  What can they do to a straight section of stream channel to get it to meander?  Sure, they could just dig a new channel but is it possible to get the river to erode a new channel?  Use all you’ve learned above to attempt this.

Have fun!


streams rivers learning hydrology  A Woman Less Ordinary lives, parents, purchases and thinks differently.  With 10 years of teaching experience, she has many effective techniques for managing kids’ behaviour (and a lot to say about finance if you’re interested) BUT YOU DON’T HAVE TO DO ANY OF IT!

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