How Rivers Form in the Desert

 Today we woke up to rain. The smell of it was delicious on the patio in the dark before sunrise. 

It was the good soaking rain of solid grey skies, the kind we have not had it a long while. After sunrise I put on my rain gear, took my hiker's umbrella from the garage, and went walking out to my patch of desert. I wanted to see the washes, and the water that might be flowing in them.

The main channel was already flowing solidly carrying water from under the bridge and up along the side of the road. The little side wash which I call the Sandy Bottom was not yet under water. There were small pools at the downstream end by the ironwood.

The transition in the desert between a dry wash and flowing creek is fascinating. How does it happen? 

It happens this way: small puddles begin to form in the bottom of the wash. These puddles grow in number and size over time. As they grow they link up and form larger pools. Sometimes all that happens is you see these pools form.

But if the rain continues then at some point the pool is large enough that the gravitational difference between the top and bottom of the pool is large enough that the water wants to start flowing.

If it does that it can creep along the bottom of the wash like a column of ants. It looks like water crossing a table after glass has been spilled. Typically it encounters some obstacle. It can be something tiny like a loose scattering of dry twigs that have fallen from a nearby bush, and which are scattered among a few small rocks. A loose thatch of debris and a tiny rise in the bottom of the creek can serve as a barrier. As the thin creeping water column hits this, it pushes the material together and compresses it, making the barrier stronger. It doesn't need to make a solid dam, just enough to slow the flow down and disperse it into a trickle on the far side of the barrier.

I saw that this morning by the Palo Verde at the top end of the Sandy Bottom. I wondered if it would break through, and achieve steady state flow through the debris. The water would then cross into the open sandy bottom. It could then link up with the still stationery pools by the ironwood on the other side of the Sandy Bottom, The mass of water column at this point would have momentum to carry it further downstream, merging with pools as it does, until it achieved laminar flow as a creek.

This morning the rain slackened before that step happened, and the column of water that was stuck by the palo verde started receding into the thirsty soil for the moment. I saw that the night layer of clouds was breaking up as the sun warmed the air.

At some point of course, if the rain were to continue while the ground is still wet, the pools could rise and link up, and the flow could start again. The pools would rise quickly in a second round of rain. In Arizona you never know. I take my opportunities to observe while I can. It somewhat makes up for the lack of running water anywhere near me, that I can how the rivers form from the bare ground.