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.

Year-end Look at the California Water Situation

 

It's the last day of 2022, and the third year of a stunning drought in California. It's been raining pretty much across the state during December, and there are hopes of alleviating the drought a bit. Let's hope so. The map above from the California Data Exchange Center gives a pretty clear idea of the situation. Reservoirs across the state are for the most part well below normal, and in the case of the biggest (Shasta, Oroville, etc.), still ominously low. I hope to revisit this diagram in a few weeks and see some changes, but we will see.

The long-term predictions earlier this year were for a continuation of dry conditions, so the current onslaught of atmospheric river storms is somewhat of a surprise, albeit a welcome one. But of course, one has to be wary of what one wishes for. There are flood watches up all around the state as one more storm will blow through to end the year.

There is also the cautionary tale of the previous rain year. We had some record storms in October and December last year, and things were looking great, but then January and February were about as dry as can be. I recorded a mere 0.08 inches in those two months.

Still, this year has some promise. From the weather station in the Geotripper backyard 13 miles east of Modesto, we've had 7.14 inches of precipitation in December, the highest total in the 32 years that I've been keeping statistics (I've recorded more than 5 inches in six different years, but never more than 6). Even with the earlier dry months, we should round out 2022 with about 8.6 inches as we move into the critical months of January and February when most of the precipitation should happen.

Despite the rain, I felt a need to check out our local barometer of runoff conditions. The Tuolumne River cannot serve in this capacity because of the numerous reservoirs upstream that very carefully control the daily flow levels. I checked instead at Dry Creek, of which there are many in California. This particular Dry Creek has its headwaters in the lower foothills of the Sierra Nevada Mother Lode, and flows for about 40 miles before joining the Tuolumne River in Modesto. Ironically, there is almost always some water in Dry Creek, obviously from rain runoff during the winter season, but also from irrigation overflow during the dry summer months. Without any substantial flood control structures, it is a good measure of runoff conditions during storms.

The creek was running about 600 cubic feet per second when I got this picture, which is about twice the current flow of the reservoir-controlled Tuolumne River. As can be seen from the discharge graph from the USGS Water Resources site, it has already been well over 1,500 cfs a couple of times in the last week. I haven't seen this much water in the creek in a couple of years.

Of course, the local picture is not the most important statistic. Everything in California's water infrastructure depends on the snow conditions, especially in the Sierra Nevada. Although these atmospheric river storms we are experiencing derive from tropical sources and are warmer than we might want, the current snow conditions are promising. Check out the report:

Let's hope this keeps up. A lot of forests, rivers, animals and people are depending on it.

What are conditions like in your region?

Liveblogging the Deluge: Following Up on the Dry Creek Flood

Dry Creek in Modesto flooded to the greatest extent in two decades yesterday, with peak flows exceeding 6,000 cubic feet per second. I was out documenting the flood surge, and I managed to get to the Dry Creek Trailhead ahead of the high water around 4:30 or so. The water was rising but as you can see in the picture above, it had not breached the small levee that kept the river in the channel. I know from other reports that the flood hit a short time after I left.

I was out early this morning, and though the peak of the flood had passed hours earlier, the flow was still high, about 4,000 cubic feet per second, and there was a lot of water covering the flood plain. It was an impressive sight.

Now that the flows in Dry Creek are receding, the dam operators at Don Pedro Reservoir will probably start ramping up their releases from the lake. The goal is to keep flows in Modesto to 9,000 cfs or less to prevent flood damage. The reservoir has been accumulating water with inflows around a high of almost 45,000 cfs to the current 26,000 cfs. The lake stands at 812 feet, or 1,805,600 acre-feet, and has risen 26 feet since the beginning of the storm sequence. Since the lake will fill at 830 feet (2,030,000 acre-feet), it is now at 89% of capacity, and more than 130% of average for this time of year. They will need to free up more space for additional storms and spring runoff

Liveblogging the Deluge: "Dry" Creek Takes Center Stage Today

Dry Creek in December of 2016 before the current storms

 What happens when a small insubstantial creek that is dry for much of the year...

...gets hit with a flash flood amounting to 6,100 cubic feet per second? Let's find out!

Dry Creek at about 1,000 cubic feet per second on January 9, 2017

Dry Creek (one of many such "Dry Creeks" in California and elsewhere) is a "minor" tributary to the Tuolumne River that has its headwaters in the lower parts of the Mother Lode northeast of Modesto Reservoir. It actually isn't dry as much as the name might suggest because irrigation overflow keeps some water in the channel during the dry season. But it has no dams or reservoirs to control flows, so it has a history of sudden flashfloods during intense storms. I've been here in Modesto for nearly thirty years, and I've seen several events that resulted in flows of 3,000 cubic feet or so, but only once have I seen flows that approached that which took place today. It was in 1997 during some of the worst flooding ever experienced in Northern California. The opening scene in the video that I posted the other day about the 1997 floods shows Dry Creek in Waterford at that time. Compare that scene with the picture below.

Oakdale-Waterford Highway at high water on January 11, around 6,000 cubic feet per second.

There is only one stream gage on Dry Creek, and it is located downstream in Modesto at the Claus Road Bridge. The peak flow was predicted for around 5 or 6 PM, which meant the surge in Waterford upstream would be several hours earlier. I took these first pictures at the Oakdale-Waterford Highway Bridge at about 2 PM, and the river was indeed very high. Compare to the flow on December 9 in the picture above. The entire floodplain was covered by several feet of water.

A few hours later I headed downstream to see if I could get ahead of the flood crest, and I found high water at the Church Street Bridge north of Empire. The view was astounding. In the picture above one can see how entire orchards are under water. And it wasn't standing water, either. The entire channel was flowing rapidly downstream.

I then went just a few miles west to the Claus Road Bridge, and found that the highest flows had not yet arrived! The Dry Creek Trail had been closed because some portions were already under water, and the creek was expected to rise another couple of feet. I walked down to the river edge and found a sheriff's deputy and a crew from the office of emergency services who were using a drone to take video of the coming flood. Modesto friends: let me know if you see the videos posted anywhere!

It started raining again, and it was getting darker, so I headed east again to check out some of the other bridges.

Wellsford Road bridge was the most interesting. The main channel was moving fast, and the muddy water reminded me of running the Colorado River through the Grand Canyon a few years back. Not because of any canyon walls, of course, but because of the surging muddy water with eddies and whirlpools. The flows weren't all that much different; Dry Creek was at 6,000 cfs, and most of the time the Colorado was running at about 15,000 cfs. The big difference is that the Colorado flows at that rate more or less the entire year, and Dry Creek does it once in a generation! I found a bit of information somewhere (source has been forgotten) that suggested that about once every 500 years a flood on Dry Creek could exceed 18,500 cubic feet per second. It would be something to see something like that happen, but such a flood would do incredible damage, and I wouldn't wish such a thing on anyone.

The morning will tell if there has been any serious damage downstream. The dangers of flooding are well-known today, but a number of very nice homes were built decades ago along the lower reaches of Dry Creek near the confluence with the Tuolumne. The dam operators at Don Pedro Reservior work very hard to modulate flows of the Tuolumne River to prevent the combined flows of the two rivers from exceeding 9,000 cubic feet per second. If flows go any higher, water backs up in Dry Creek, causing flooding in the neighborhoods there.

I head to work before sunrise, but I'll probably try to catch a few more photographs in the morning. It will take a day or so for flows to recede.

Liveblogging the Deluge: Perspectives on the Biggest Storm in a Decade, Part Five

We finally got a taste of some of the kinds of squalls that are leading to flooding across much of Northern California. Luckily it was brief, and we have not had problems in our immediate vicinity. The view in the video above is out our front door a few moments after I got home from work, driving in conditions that were much the same as what you can see here.

Radar signal of the storm in the video above, courtesy of  Ryan Hollister @phaneritic

My dear friends who live in hurricane-prone regions are welcome to snicker a bit at my sense of awe at the violence coming from the sky. I live in a dry region, and downpours like this are pretty rare. We got 1.67 inches today, and only six days have had a higher total in the 26 years I've been measuring precipitation in my back yard. That brings our four day storm total to 3.35 inches. Again, that can hardly compare to some of the high numbers coming from the coastal mountains or the Sierra Nevada, but it represents about a quarter of an entire year's rain total in an average year in my village (just over 13 inches in the years I've been measuring).

National Park Service Photo

Meanwhile, the big flood danger in Yosemite has passed as the storm surge topped out at 12.7 feet and subsided a few hours later. Yosemite is in the process of reopening the valley floor to tourism. Highway 140 on the Merced River was closed by mud and rockslides (good pictures at the link). Snow has been falling in the valley and the adjacent high country, which is a good development. We need to build the snowpack to have any hope of putting a dent in the drought.

Dry Creek at around 1,000 cfs yesterday. We'll see how it looks tomorrow at peak runoff. 7,000 cfs is expected.

The Tuolumne River continues to cause some headaches as dam operators try to tread a delicate path between high flows from Don Pedro Reservoir and high flows along Dry Creek, an unconstrained waterway that has been flowing at more than a thousand cubic feet per second for several days. The creek has my undivided attention right now, as flows are expected to crest at more than 7,000 cubic feet per second tomorrow. I hope to get out and snap some pictures.

I have had little to say about the Stanislaus River. New Melones Reservoir is huge, and the Stanislaus is a relatively small river compared to the Tuolumne. As a consequence, the water level in the reservoir has been low throughout the drought. It began this week at 27% of capacity, about 47% of normal for this time of year. The lake has risen about 20 feet this week, and now stands at 31% of capacity (54% of normal).

It's been an interesting week...

Liveblogging the Deluge: Perspectives on the Biggest Storm in a Decade, Part Four

Check out some newly "discovered" footage from the floods of 1997!
We've been following developments on the Stanislaus, Merced, and Tuolumne Rivers as the biggest atmospheric storm in a decade or more has been sweeping through California. Many references have been made to the record-setting floods of 1997, and I was reminded that I was actually around back then. I even had a thing called a "VHS video camera" that recorded images on, of all things, magnetic tape. How so very quaint! I finally remembered the tapes, and even more importantly remembered that I had the tapes recorded to a DVD a couple of years ago to assist someone's research on the 1997 floods. I dug it out and isolated my videos of the Tuolumne River in Waterford, CA, and Dry Creek in Waterford and Modesto (it was not very dry at the time).

Some important numbers can be compared to the events transpiring right now. Don Pedro Reservoir is a huge dam upstream of Waterford and Modesto. It has a capacity of 2.04 million acre-feet. In 1997 it was relatively full, but not out of sync with the seasonal guidelines (about 1,690,000 acre-feet, around 83% of capacity; that's the level of the reservoir today, January 9). The storm of 1997 was not normal, however, and awareness was growing that the dam might fill and start flowing in an unconstrained manner. Operators ramped up flows on the river downstream to near-flood level at 9,000 cubic feet per second, and realized eventually that they were going to have to make a difficult choice. They were going to have to flood out a number of homes and ranches in order to make room in the reservoir for the even bigger flood to come. Several feet of warm rain fell on a near-record snowpack, and soon river flows well over 100,000 cubic feet per second were entering the reservoir. The lake level rose at an astounding rate of a foot per hour, and in less than two days, the level rose to 830 feet, overtopped the dam weir and floodgate.

For several hours, the Tuolumne River below the dam was flowing at 60,000 cubic feet per second, the highest such flow ever recorded. This was a disaster for several hundred homes downstream, but had the dam not been there, and had the operators not made the choice to produce an artificial flood, there would have been a flood in Modesto that was twice as large.

To compare to the flood going on right now in 2017, inflow to Don Pedro has been ranging as high as 38,000 cfs, and the controlled outflows have been as high as 10,000 cfs. The lake has reached an elevation of almost 802 feet, but the main part of the storm has passed, so I don't expect major problems on the Tuolumne River (but of course predictions are just that, predictions). Keep your attention on the news until this storm is over with).

And so that is the background to the video above. I was filming what I could from the various bridges before some of them were closed. I had to take a circuitous route to get to town that day to get video of Dry Creek in the La Loma Park area, which was completely inundated. I couldn't even stop, and had to take the video from a moving car (Mrs. Geotripper assisted with the driving in case you were wondering).

So here it is in all the glory of hand-held amateur film-making. Be thankful that it isn't happening again, at least not here (but give a lot of thought to those who are struggling with high water farther north in the Great Valley and along the Truckee River in Nevada).

UPDATE (1/10/17, 12:20 PM): It looks like the inflow at Don Pedro reached 44,905 cubic feet per second yesterday. Lake level reached 802.6 feet, about 1,698,000 acre-feet.

UPDATE (1/10/17, 4:40 PM): Don Pedro Lake level reached 804.9 feet, about 1,723,000 acre-feet. That's about 84.5% full.

Liveblogging the Deluge: Perspectives on the Biggest Flood in a Decade, Part Three

Pictures are emerging from Yosemite Valley! See the update below.

NPS Webcam Photo from Happy Isles at upper end of Yosemite Valley

The flood on the Merced River in Yosemite Valley has crested at 12.7 feet (10,488 cubic feet per second). The highest flow was recorded at 4AM, and the waters are beginning to recede. There are more storms to come in the next few days, but they will be colder, and will start producing snow instead of rain. Although there has been some damage around the valley floor, this has to be seen as one of the best possible outcomes for Yosemite, given the power of this particular atmospheric river storm, and the level of flooding seen elsewhere in Northern California. It is still dangerous out there.

Meanwhile, on the Tuolumne River, releases have begun again from Don Pedro Reservoir, ramping up from almost nothing to 5,320 cubic feet per second. The operators are trying to modulate flows from Don Pedro with the unconstrained flows from Dry Creek to prevent flooding in the low-lying areas of Modesto downstream from the confluence of the two waterways. Flooding commences at about 9,000 cfs in that area. Inflows at Don Pedro are around 15,000 cfs right now (*see the new update below); if the dam didn't exist, the river in Modesto would be approaching 20,000 cfs.

There is a lot going on with the flooding around Northern California, and Southern California is finally getting some precipitation as well. I've been concentrating on the activity on the Merced, Tuolumne, and Stanislaus River drainage basins. Updates will follow as conditions warrant.

UPDATE (1/9/17 11:20AM): The LA Times has a pretty good roundup of flood news from around the state:  http://www.latimes.com/local/lanow/la-me-ln-sierra-storm-20170108-story.html.

UPDATE (1/9/17 11:30AM): The inflow to Don Pedro Reservoir actually reached nearly 38,000 cubic feet per second  yesterday! Water level in the dam jumped 7 feet to 796 feet, for a total of 10 feet since the storm began. The lake gained just over 100,000 acre-feet, to 1,620,000 acre-feet. The lake would overflow at 830 feet, or just over 2 million acre-feet.

UPDATE (1/9/17 12:50PM): Yosemite National Park has posted some photos of flood related effects in the Yosemite Valley area. Here are a few of them...

NPS photo

NPS Photo

NPS photo

UPDATE (1/9/17 7:30PM): At about 3PM, the Merced River receded below flood stage in Yosemite Valley. It's a relief that the flood wasn't worse, especially given greater runoff to the north where there have been some really serious problems during the day. Another storm is moving in, but it's colder and will be mostly snow, which is a very good thing.

UPDATE (1/9/17 9:00PM): As of this evening, Don Pedro Reservoir has surpassed the 800 foot elevation, gaining 5 feet to 801.84 feet, for a gain of another 50,000 acre-feet to 1,690,000 acre-feet. That's about 83% of capacity.

Liveblogging the Deluge: Perspectives on the Biggest Flood in a Decade, Part Two

Check back for updates at the end of this blog...

First off, let me emphasize that I am writing almost exclusively about the central Sierra Nevada and adjacent Great Valley, particularly the drainage of the Merced, Tuolumne, and Stanislaus Rivers. I say this because some very serious things are happening farther to the north, and flooding is occurring in a number of places. If you live in those areas, you need very much to be listening to emergency response officials, and not reading random blogs!

Screen capture of Yosemite Falls from video on NPS Facebook Page

That being said, it looks like we have good news emerging from the storm. Although the rain has been heavy at times, it did not reach the apocalyptic levels that seemed possible earlier in the week. The Merced River in Yosemite Valley is certainly rising towards, and will exceed, flood stage (below), but not nearly to the extent that had been predicted in the previous week.

Early predictions had suggested discharges in the range of 20,000 cubic feet per second, but the latest projection is pointing more towards 9,000 cfs (below). This will raise the river level to nearly 12 feet, and that will inundate some roadways, but nothing like the floods of 1997 when wide parts of the valley floor were under 8-10 feet of floodwaters. That's good news.

There is also good news indicated by the actions of the watermasters at Don Pedro Reservoir on the Tuolumne River. Two days ago, a lot of water was being released from the reservoir, trying to make room for possibly catastrophic river flows. Flows were up to 10,000 cubic feet per second for a while (see some pictures here). This morning, I checked the flow (below), and it has dropped back to the usual level of several hundred cubic feet per second. This means that the dam operators are confident that they can capture and save the runoff and are not worried about the dam being overtopped the way it was in 1997 (the damage from that event is still visible 20 years later). (*see new update below)

Discharge of the Tuolumne River at LaGrange Jan 8, 2017

The river surge will have some benefits: River Hyacinth, an invasive aquatic plant has been choking the channel of the Tuolumne River during the last few years. The surge of the artificial flood has no doubt cleansed the channel of the river in many places. A lot of silt that has been smothering the best spots for salmon eggs has probably been swept way, and new sand bars have perhaps been formed. Although some minor flood damage may have taken place, there is a lot of good too.

River hyacinth in the lower Tuolumne River in 2015. Those mats of green aren't supposed to be there.

The other area of concern is Dry Creek. It is an unconstrained drainage, and for a time it seemed possible to have a record flow of 8,000 cubic feet per second. Those projections have backed off a bit too, but the expected 5,600 cfs will still be the highest runoff that I've seen, and some minor flooding damage will be possible in Modesto. Notice in the diagram below that there are three peaks predicted.

Source: http://www.cnrfc.noaa.gov/graphicalRVF.php?id=DCMC1

I've been thinking about something else today. Will this spell the end of the drought that has afflicted California since 2011? The short answer is no, it will not. The storm is building reservoir levels to more healthy levels, but filled reservoirs are but a single metric in determining droughts. This warm storm has melted the snowpack, and the snowpack is what is critical to sufficient supplies during the dry summer season. The good news is that the storms later in the week will be much colder, and the snowpack may be building up. The storm does nothing about global warming, which will continue to put a "finger on the scale" each year in California towards continued droughts.

Even more concerning is the fact that the storm will only help a bit to recharge the groundwater of the Great Valley. We have developed a huge deficit in groundwater storage during the drought, as agricultural interests have pumped increasing amounts of groundwater to replace the missing irrigation supply. Even if we have the foresight to put water back into the ground, some of the storage space has been lost due to compaction. This becomes obvious when one observes the amount of subsidence of the ground surface throughout much of the San Joaquin Valley (the southern portion of the Great Valley).

Source: http://www.sacbee.com/news/state/california/water-and-drought/article31527953.html

UPDATE (1/8/17 4:50PM): The story on the drop in the discharge on the Tuolumne River is a bit more nuanced than I suggested above. I've been notified that they are trying to modulate the flow with the high discharge in Dry Creek to prevent flooding downstream of the confluence of the two watercourses in Modesto. In other words, flooding occurs on the Tuolumne at about 9,000 cfs. Dry Creek may contribute as much as 6,000 cfs tomorrow, so they've cut back on the flow of the Tuolumne to compensate.

UPDATE  (1/8/17 8:50PM): The Merced River in Yosemite Valley has just reached flood stage at 10 feet (6,600 cfs). The river is expected to rise another 1-2 feet before subsiding.

From https://waterdata.usgs.gov/ca/nwis/uv?site_no=11266500

UPDATE (1/8/17 11:20 PM): Among all the other stories of the day, word comes from Calaveras Big Trees State Park that the beloved "Tunnel Tree" has fallen in the windstorm. This is sad in one way, but it's clear that these kind of "touristy" developments like carving big holes in the base of the tree is damaging to them. We are changing nature for our own amusement instead of appreciating them for what they are. It's an unfortunate parallel that the Orca in the "Blackfish" story died this week as well.

Liveblogging the Deluge: Perspectives on the Biggest Flood in a Decade, Part One

See updates at end of post... BTW, if you are on the scene in places like Yosemite Valley, please send photos or updates! I will gladly post them here.

Printable PDF of current conditions can be found at this link 

It's clear by now that I am fascinated by the current atmospheric river storm that is blanketing the state of California with precipitation from one end of the state to the other. I'm right in the thick of it, in the Great Valley, sandwiched by mountain ranges that will be getting rain amounts measured not in inches, but in feet. On this blog, I've already talked about the last record-breaking storm, the 1997 event that caused unprecedented damage across California, but especially in Yosemite and along the Tuolumne River. I also blogged about the preparations being made in anticipation of the storm: the operators of Don Pedro Reservoir have ramped up the flow of the Tuolumne River to near flood stage to make room for storm runoff. Then, last evening, we had a spectacularly clear view of the Sierra Nevada from the valley floor, covered with snow, and seeming to wait for the coming storm event. From here on, I'll be liveblogging, sort of, the storm (it will be separate posts at times).

Radar of precipitation in Central California as of 8:00 PM Saturday. Source: Intellicast

And now the storm has arrived. Atmospheric river storms are linear streams of moisture-rich air coming northward out of the tropics (they have been called Pineapple Express storms at times). In "normal" front-related storms, the period of precipitation is fairly limited as the front sweeps through the state. Atmospheric river storms are different; they spray the state with storms that can last for days, and in some extreme examples, weeks. They are also warm storms. The snow level can reach very high elevations so that rain falls on the snowpack, melting vast amounts of ice. It all adds up to a potential for the worst kinds of floods in California.

I don't know what all is going to happen in the next few days, but I want to establish some baselines so it can be understood just what the storm means for California and our water situation. We have been in the grip of a crippling drought since 2011, and reservoirs across the state have been at historical, even catastrophic, low levels. The first diagram in this post provides a benchmark to track the changes in the next few days. It's already been a fairly good precipitation year, as some reservoirs already sit at 100% or more of their desired levels. A few are still very low, including especially New Melones Reservoir on the Stanislaus River just north of me. It is only at 27% of capacity. Lake McClure on the Merced River below Yosemite Valley is about where it is supposed to be at 47% of capacity. My special river, the Tuolumne which also flows out of Yosemite National Park, is a bit too close to full at 76% of normal. If the storm dumps too much rain in the park, it has a small chance of overflowing the floodgates in an uncontrolled manner as it did in 1997. That's the reason the dam operators ramped up flows in the last two days.

Source: California Nevada River Forecast Center

It's going to be especially interesting to see how the storm predictions pan out. The flood hydrograph above is the expected outcome in Yosemite Valley. The red dotted line is flood stage (at 10 feet, or around 7,000 cubic feet per second). Earlier predictions for the flood peak have wavered between 15,000 cfs and 24,000 cfs. The high end prediction would be of 1997 magnitude, but even the low prediction would be enough to flood valley roads and possibly close the park. The high flow history at Pohono Bridge in the valley is shown below:

Historic Crests

(1) 23.43 ft on 01/03/1997
(2) 23.43 ft on 01/02/1997
(3) 21.52 ft on 12/23/1955
(4) 20.98 ft on 11/19/1950
(5) 20.10 ft on 12/11/1937
(6) 16.96 ft on 12/23/1964
(7) 13.11 ft on 04/11/1982
(8) 13.01 ft on 01/13/1980

The current outlook is for a peak of 21,590 cfs (19.7 feet).  If it happens, it would be the 6th highest flow ever recorded. As you might expect, I'll be watching this one closely.

Flood hydrograph for Dry Creek. Source: http://www.cnrfc.noaa.gov/graphicalRVF.php?id=DCMC1

The other local river that has my undivided attention is Dry Creek. It's not a well-known waterway outside our region, but it is capable of causing some havoc at times. Although it doesn't have a vast drainage basin (it arises in the Sierra Nevada foothills in the Mother Lode), it has no dams or flood control structures. It is therefore very responsive to variations in storm intensity. I've seen it flowing at 3,000 or 4,000 cfs, but the prediction as of this evening is a bit ominous: it may peak at 8,400 cubic feet per second. Let's be clear what that means: yesterday, the Tuolumne River was flowing at 8,000 cfs, and that's considered very close to flood stage. For the Tuolumne River. A minor tributary to the Tuolumne River may be as big as the Tuolumne, but in a channel that is many times smaller. I will be out and about on Monday to get pictures, but to set a baseline of sorts, here's how the creek normally looks at the Oakdale-Waterford Highway bridge:

Dry Creek on Dec. 17, 2016 just prior to a previous high water event late last year.

So, here is where we stand with the atmospheric river storm in my backyard. We've received about a half inch of rain today, adding towards an expected 4 inches by the end of the week. If things transpire that way, it will represent about a third of the precipitation expected in my town over the course of an entire normal year. We of course are in the driest part of Central California (aside from the desert east of the Sierra Nevada), so literally everywhere else will be getting a lot more. As I've been saying in my previous blogs, be safe out there! Don't travel if you can avoid it, don't try crossing flooded bridges, and don't try walking either. If you are headed into the mountains, imagine being stranded by yourself for a few days and pack accordingly, especially with warm clothing and extra food rations. My distant relatives, the Donners, failed to do that in the Sierra Nevada in 1847, and look how they ended up. If authorities issue a flood warning, take it seriously. They're getting their advice from scientific professionals, and that still means something.

UPDATE: Officials closed Yosemite Valley roads and visitor services as of yesterday, and will reopen after the flooding subsides.
UPDATE: (although more like an addendum) For comparison's sake, here is where the reservoirs of the state were at a year ago:

A huge difference!
UPDATE (1/8, 1:52 AM): Yes, up late, listening to intensified rainfall. Gauge has picked up another .40 inches in two hours.

UPDATE  (1/8, 10:18 AM): Good morning! There was another 0.20" in the backyard gauge this morning, and now we are in a lull between major storms (most of the action is north of here at the moment). Another 2/3 inch is expected later today on valley floor. There is better news in Yosemite; the expected flood is now projected to be less than catastrophic, with a projected high flow of 9,600 cubic feet per second. Yesterday, the projection was twice that. It's still above flood level by 2 feet, but would not cover valley roads. Several more inches of warm rain is expected (~4"), but tomorrow the precipitation will turn to snow, and that is a good development.