Rain in the Sonoran desert feels different depending on the season. Summer monsoon rains are violent events bringing thunder and downpours. Winter rains are more peaceful events, bringing cool air, low clouds, and steady drizzle. And for the rest of the year, there is no rain at all. Seasons have a very different rhythm here.
When a large winter storm passed through Tucson a few weeks ago, it felt like the world was changed. Dark grey skies, the smell of water and creosote on the air, and the sound of raindrops on tin roofs and hard rocks are some of the joys particular to this part of the world.
When there is rain in the mountains, the runoff flows into our canyons and washes. Because many of these surfaces are relatively impermeable, often with bedrock bottoms, the result is a series of flash floods – one of my favorite parts of desert life.
I went with a friend on an exploration of Bear Canyon in the front range of the Catalina Mountains. Normally this is a quiet canyon with seasonal flows, usually smooth and clear, no deeper than a few inches. Here you can see the canyon in spring – part of a Sierra Club ICO trip.
Here is what the canyon looked like after that winter rain. We made several crossings of the canyon in its safer and wider sections, but the water was swift and more than hip deep. Bouldery sections created long whitewater wave trains, and larger bedrock sections become fast-flowing flumes and waterfalls.
Further down the valley, where Bear Canyon meets Sabino Canyon, the scene was similar. Waters were high above riparian mesquite and sycamore trees, flooding a wide expanse of normally sandy and quiet terrain.
The United States Geological Survey monitors many streams, including this one. Today, as I write this, the flow rate is only 2.4 cfs (cubic feet of water per second). The day I took the previous photos, the flow was 1290 cfs – almost a factor of 600 higher. This flood wasn’t even a very large one – it was produced by only approximately two inches of rain – fairly average for a winter storm.
Rarer much larger storms can raise flow rates by factors of 100,000 – these floods are the ones that transport immense quantities of sediment, scour and downcut further the canyons, and disperse animals and plants to entirely new habitats. One of these days I hope to see one of these landscape-transforming flows.
The day after this storm, sun returned to the desert; two days later, flow rates were nearly back to baseline; by midweek, the rivers were dry again. Just another day in the desert.