Dry Creek: Anatomy of a Flood (One of Many Across California Today)

 

After days of gloomy and wet weather, New Year's Day dawned bright and sunny, and we couldn't resist driving out into the California prairie to have a look at the beautiful landscape. The streams across the prairies east of Modesto were full and flowing in a way we haven't seen for a number of years. And that's the problem of course. 

This creek, normally dry, was just one of many dozens of tributaries to Dry Creek, which is itself an unregulated, undammed tributary to the Tuolumne River. This entire area received upwards of five inches of precipitation in the last day or two, and all the water had to go somewhere.

I included a picture of Dry Creek in my post yesterday, when it was flowing at about 600 cubic feet per second (cfs). I am including another picture below, taken at the same time, but from an angle that shows the pasture to the left. I knew that more water would be coming downstream, maybe as much as 1,500 cfs, an amount that would actually be more, by a wide margin, than the main drainage in the area, the Tuolumne River.

That's not quite what happened...

When we crossed the Dry Creek Bridge north of Waterford today, the creek was running at 6,000 cubic feet per second, more than ten times the flow of the previous day. Take a look below at what happened to the pasture (not to mention all the shrubs and brambles at the base of the oak trees).

By the time we arrived in the prairies in the afternoon today, most of the floodwaters had subsided in the upper watershed, but we could see evidence everywhere that a significant flood event had taken place. Rocks were strewn across the roadways, and every watercourse showed evidence of having been feet deeper the previous day. One bridge we crossed would have been four feet underwater during the height of the storm. 

The flood hydrograph below tells the story. The data is taken from a stream gage downstream in Modesto. The bar graph at the top shows the pattern of the rainfall in the storm up in the watershed, and the subsequent rise of Dry Creek. Notice how the rise of the creek lagged behind the precipitation. This so-called lagtime makes sense because it takes time for the water to gather into the tributaries and then to flow the twenty miles or so downstream. Lagtime represents the critical hours that residents downstream can prepare for the oncoming flood.

Wouldn't it be nice if there were a government entity that could monitor all rivers and all flood events so that when such events unfold, there could be timely warnings? Perhaps even keeping records of storms over the course of a century or more, so that specific warnings could be made about the timing and the expected intensity of the oncoming flood? Unlike earthquakes, floods can be predicted, and there are in fact government institutions that are tasked with this job, mainly the United States Geological Survey (across the entire US), and the Department of Water Resources specifically in California.

Which brings us to the handy-dandy bottom portion of the hydrograph. The blue line on the graph is what happened already. The pink line is the prediction. We have another intense storm coming on Wednesday, and after dropping to around 300 cfs, Dry Creek is going to rise again to at least 6,000 cfs and maybe more. Isn't it nice that we have several days warning? That's just one huge example of the value of science in our society.

Of course, no one is perfect, and all models and predictions can be affected by unknown and unexpected factors. The storm this week offers one tragic example. Although most streams and rivers behaved more or less as predicted, the Cosumnes River defied the predictions and produced record flooding, well beyond the predicted levels. 

What went wrong? Those who do science fully understand that errors happen, and it their goal is to understand the reason for such errors. The factors in the Cosumnes River flooding are being analyzed and may include an unexpected slowing of the storm front causing increased precipitation, two or three broken levees, and the Caldor Fire of 2021 that ravaged much of the watershed upstream. If you want to follow the analysis, check out the Weather West blog by Daniel Swain (@Weather_West on Twitter).

So, there is my science homily for the day. But we were out to explore some nature, and in any case, we need to appreciate the gifts we have been given. The day, a respite from a long series of expected storms, was beautiful. 

Mountain Bluebirds are not common on the valley floor, but we saw a small flock along the road.

An American Kestrel is a sharp-looking small member of the falcon family. This one remained perched near our car for a few moments.

Bald Eagles are not especially abundant in the region, but we found one. So had an 'unkindness' of Common Ravens, and they were making their displeasure known to the eagle.

And finally, an old horse seemed to appreciate the sunshine. The horses were brought to the continent by the Spaniards in the 1500s, but they actually have a long heritage here. They evolved in North America tens of millions of years ago! They migrated across the Bering Land Strait and spread throughout the world, but for some reason went extinct along with many other large mammal species in North America about 12,000 years ago.

California's Rarest Ecosystems: The Prairies and Vernal Pools (Part One of a Two-part Miniseries)

The lower foothills of the Sierra Nevada are one of the most unique and almost universally ignored parts of California. Caught between the famous and heavily-visited forests and alpine regions of Yosemite's high country and the utilitarian agricultural fields of the Great Valley, the lower foothills are made of soils too poor to be developed, and too "plain" to be noticed by people rushing up the highways to the mountain resorts. For much of the year they seem barren and lifeless. But these prairies are one of the most precious of California's diverse ecosystems.

The prairies of California once extended the length of the Great Valley, a 400-mile-long grassland supporting millions of grazing animals, birds, and predators, including Tule Elk, Pronghorn, Deer, Grizzly Bear, Mountain Lions, Coyotes, Foxes, and numerous rodents, reptiles, amphibians, fish, and insects. In earlier times (more than 10,000 years ago) there were camels, horses, bison, giant ground sloths, Columbian Mammoths, Saber-tooth Cats, and Dire Wolves.

There were also modern-day wolves, and it is ironic and coincidental that a wolf returned this week to the Great Valley for the first time in a century. It is a young male from Oregon who made an epic journey through Modoc County into the Sierra Nevada near Yosemite Valley, and then into the Sierra foothills of Fresno County, and across the Great Valley (including two freeway crossings). At last report it's now in Monterey County (it's wearing a radio transmitter). 

The prairies were covered with native grasses that were quickly replaced by invasive species when European colonizers arrived and started herding sheep and cattle. Farms eventually replaced the ranches, and the remaining prairies, no more than 5% of the original land cover, were relegated to the fringes where the soils were poor and no water was available for irrigation. Those remaining prairies form a belt through the foothills of Sierra Nevada.  A narrow remnant can be found along the Coast Ranges and the Carrizo Plains west of Bakersfield.

The prairie soils in the Sierra Nevada foothills are notably poor in nutrients because they have not been part of an active floodplain in tens of thousands of years, and the fertile components have been leached away, leaving behind reddish iron oxides and thick clays. Differential settling and wind deflation has left small hollows and depressions that collect rainfall during the relatively rare winter storms. The clay prevents the water from seeping underground, so pools will persist for weeks or months at a time. These pools are the basis for one of California's rarest ecosystems, the vernal pools.

The vernal pools are islands of diverse life in the otherwise barren grasslands. Numerous species of plants and wildflowers, amphibians, and invertebrates have evolved to survive the weeks of water and the months of dryness. As the waters evaporate away, one species of plant after another germinates, grows, and goes to seed. The pools are often rimmed in colorful zones of different flowers.

I'm writing about the vernal pools today because the limitations of travel in the times of a pandemic have kept me pretty close to home. I live on the edge of the prairie between the Tuolumne and Stanislaus rivers and I was out there today. A few pools are left, and flowers are blooming across the region. The views are expansive, extending to the snow-capped peaks of the High Sierra.

The pools and plants provide food and cover for multitudes of bugs and other wiggly organisms, so the pools are trafficked by a large variety of birds, and dozens of species can be identified by discerning and observant individuals, and occasionally by me as well. 

I saw a half-dozen American Avocets in one pond a few weeks ago.

The prairies are one of the best places to see the rare and threatened Burrowing Owls. The area around Crabtree, Willms, and Cooperstown Roads are among the most dependable places to see them in our region.

Savannah Sparrows are a common species, and they'll occasionally cooperate with photographers.

The Western Kingbirds seemed to arrive from their winter homes in Mexico all at once last week. I saw none a week ago, and dozens today, including this one that hung about long enough for a picture.

I have no picture, but I came upon a Gopher Snake in the road that was in serious danger of being run over. I stopped, looked at it carefully to make sure it wasn't a rattlesnake, and then I picked it up to put it in a safer spot. I was suddenly reminded of how Gopher Snakes sometime pretend to be rattlers! They can flatten their normally narrow heads into the triangle shape of a rattler, and will strike in a similar manner. I didn't hold it for long...how do you explain to the EMTs that you picked up a rattlesnake that you thought was something else?

If you are lucky enough to live near a California prairie, this is a good time of year to explore them. If you tend to rush through this unique part of the Sierra Nevada and the Great Valley on your way to Yosemite or some other mountain destination, slow down and give these precious and unique lands a quick look. You won't be disappointed.