Cherax - Destructor, or Constructor?

Cherax Destructor - the common yabby of farm dams and creeks across a vast swathe of Australia, has made a home for itself in all sorts of odd places.  It is not a native of the upper Blue Mountains, but has been here for quite a while.  The grandkids and I have caught them on many occasions in Jamison Creek, beside the orchard, or in the Wentworth Falls Lake.  Recently I found a number of small specimens inside the stomach of a Redfin Perch caught in the lake, and I am sure they are one of the reasons that the cormorants hang around the lake, as well.

It surprised me though, when I found it lurking in a tiny little rill that runs down the back of our place.  I had seen an adult in there a while back, over 50 metres up from the main creek, and wondered if it was lost - and then the holes appeared in the banks of that miniature creek.

The first hole - angled down into the left hand bank of the stream, beneath the long blades of grass.

The debris from the excavation ended up strewn across the stream, just downstream.  Heavy rain washed leaves into the pool that had formed, and when I cleared the blockage, yabbies only a few centimetres long scurried back into the hole.  Within a day or two, the stream was blocked by another wall of sand and gravel.

Mixed in with the sand and mud was gravel up to 40mm in diameter - not bad work for a little creature that is often only 100 - 200 mm long itself

I cleared it again, and again, and each time it was back within a night or two (cherax are prey to kookaburras, currawongs, foxes, and who knows what else - darkness is their friend) and each rebuild was bigger and better than the last.  It took me a while to understand that this was no random construction - all the material excavated from the tunnel was neatly piled across the stream, forming a small lake around the tunnel entry.  Not only that, but the water in the little pool above the dam was running through the tunnel and re-entering the stream about 750 mm downstream.  Perfect for a female yabby nurturing eggs or babies.

At the beginning and end of the above video I focus on the point at which the water flows out of the downstream end of the tunnel and back into the stream.  I have never seen a yabby do anything like this before, and wonder if anyone else has seen such behaviour.