Thursday, February 23, 2017

Liveblogging the Deluge: Checking out the Spillway at Don Pedro Reservoir

It's probably not too hard to figure out what happened. I get home from a five day trip only to find that the spillway at Don Pedro Dam has been opened for the first time since the floods of 1997. It's a big event in these parts, an acknowledgement that the reservoir was full and in danger of spilling over in an uncontrollable manner. So I had to go and have a look. Mrs. Geotripper and I had a few hours this afternoon and headed on up the river. We had no idea what to expect, or whether we would be allowed anywhere near the spillway.
We were pleasantly surprised to find that there is some limited access for the public. If you want to have a look, you need to drive Bond's Flat Road to the Blue Oak Campground at Don Pedro (trying to approach from the south side - the Fleming Meadows side -  doesn't work as Bonds Flat Road is closed two miles away from the dam). The campground and day use area is open, and provides a limited view of the spillway and the spray from the water releases.
The spillway opens up into a former meadow that was carved down to bedrock during the floods of 1997. A portion of the flow can just be glimpsed from the viewing area. The spillway is similar to the one that failed at Oroville Dam, but at Don Pedro the slope is more gentle, so uncontrolled erosion of the rock is unlikely (especially since the flow rates are not nearly as high as they were in 1997).
The long concrete structure in the top two photographs is the weir that serves as kind of a last ditch spillway. The crest lies at an elevation of 830 feet, and the water this afternoon had reached 829.5 feet. As a consequence, the outflow through the spillway has been increased to 14,600 cubic feet per second (well above flood level) to start bringing the water level down.
There is a high point in the campground that provides a better look at the spillway from the lake side. On this sparklingly clear day, the view of the valley was also spectacular.
For the first time in twenty years the lake is absolutely full. It is quite a sight!

Just a note, in case you want to see the sights for yourself...stay behind the fences and follow the directions of the authorities. I didn't find out for myself ("But sir, I'm a GEOLOGIST!"), but the ranger told me stories of the people that they had caught and cited. They were rather vigilant.

Tuesday, February 21, 2017

Liveblogging the Deluge: Big Changes on the Tuolumne River

I've been away for the last five days, experiencing California's storms from an entirely different perspective, that of being exposed and out in the open country of Death Valley National Park. There will be plenty of information and pictures about our adventures soon enough, but there were plenty of events back home that have been rather significant as well. A great many people have been affected as the waters of the San Joaquin River and its tributaries have risen past flood level.

First and foremost, at Don Pedro Reservoir on the Tuolumne River the emergency spillway has been opened for the first time since the floods of 1997. This became necessary when the lake filled nearly to capacity, and a new storm battered the north state over the weekend. The channel downstream floods at around 9,000-10,000 cubic feet per second, but storm runoff threatened to exceed that level. The dam operators had to make a decision to cause some flooding downstream to avoid even greater flooding if runoff was too high.
Flows were ramped up to around 16,000 cubic per second over the weekend, and have since settled back to 13,500 cfs at the latest reading (below). Rain and snow are still falling in the region, so the concern about flooding remains for the time being. The lake fills at 830 feet, and the current level is 828.82 feet. There isn't much room for error.
Source: USGS (

People are understandably concerned about Don Pedro Reservoir as they compare notes with the events at Oroville Dam in the last few weeks. Both dams have an emergency spillway that flows over unlined and unreinforced rock rather than a concrete channel.

There are some important differences, however.

First, the emergency channel below Don Pedro is much less steep than Oroville's, and is therefore less subject to the headward erosion that threatened to undermine the spillway at Oroville. Second, the spillway was tested previously, during the floods of 1997. The runoff amounts in that event were almost astronomically higher than any flows expected in this year's event. A forty-foot channel was carved through the meadow, and the flowing waters are unlikely to do anything worse this time around.
Still, these are nervous times. I had my first chance to see the higher flows this morning as I headed to work. I stopped at the head of the Tuolumne River Parkway Trail to have a look. The picture below was taken on February 5th when flows were around 10,000 cfs. Note that the entire trail was above water level.
The picture below shows the same scene this morning with flows closer to 14,000 cfs. As you can see, the entire trail is under water now, which means I'll have to find another place for my birdwatching for a few weeks or so.
One senses that authorities are just barely keeping ahead of events, which is not surprising given the nearly nonstop barrage of atmospheric river storms battering California. The situation has not quite spiraled out of control, and the storms are letting up for a few days, which will allow for more floodwaters to flow into San Francisco Bay. I hope we'll have a breather.

Wednesday, February 15, 2017

Liveblogging the Deluge: San Joaquin River National Wildlife Refuge - What a difference a few months made (and Rabbits)

The San Joaquin River at the National Wildlife Refuge in November 2017
Just a few months ago, the San Joaquin River, and its major tributary the Tuolumne River, were in big trouble. The worst drought in California's recorded history had left the rivers at a mere trickle, and many parts of the San Joaquin were dry, despite recent efforts to bring back some sort of minimal flows to stretches of river that had been dry for decades (after dam and irrigation levee construction upstream). The invasive weed River Hyacinth had choked channels, crowding out native vegetation and making life difficult for fish and other organisms.

I hiked the Pelican Nature Trail in the San Joaquin River National Wildlife Refuge back in November, and despite a few early storms, dryness was the overwhelming feeling. Volunteers have put in a vast amount of work replanting former riparian woodlands, but it felt like the river overflow areas wouldn't see water any time soon. The river was flowing at a sluggish couple of hundred cubic feet per second.
The San Joaquin River in the same exact location on February 14, 2017
I returned to the Pelican Trail yesterday to a changed world. A series of atmospheric river storms over the last six weeks have dumped record amounts of precipitation across northern and central California. Watermasters at the reservoirs upstream joyfully let their dams fill with the prodigious amounts of water, but then the storms kept coming and they realized that some of the dams were in danger of overflowing. They ramped up flows out of the reservoirs and the rivers downstream swelled. The Tuolumne River, for instance, has been flowing at near flood levels (~9,000 cubic feet per second) for the last five weeks, and this week the flows were increased to over 11,000 cubic feet per second. Don Pedro Dam is only 2-3 feet below full capacity, and has only about 30,000 acre-feet of capacity (out of 2.03 million acre feet of storage space). The Merced River has been experiencing high flows as well, and water is high in the usually dry San Joaquin riverbed. Yesterday the river at Vernalis, just downstream of my trail, reached official flood state at 34,000 cubic feet per second.

It was rather stunning to stand on one bank of the San Joaquin, the neglected trickle of water from the southern Great Valley, and not be able to discern exactly where, off in the distance, the other bank was located. And there is this: the water level is probably going to rise soon. Yet another intense storm is headed towards California, and there is going to be another week of rainfall and snow in the mountains upstream. Don Pedro Lake, for one, is just about full and it is possible they will have to let additional water through the system to keep the lake from overflowing.

More than a month ago, on January 4, I wrote the first of a series of blogs that I soon called Liveblogging the Deluge. I quite honestly thought it was going to consist of four, maybe five posts as the first atmospheric river storm passed through California. But another storm soon followed, and then another. I look back now and see that I've published 24 posts on the storms, and they keep coming. This has been an extraordinary storm event and, for some people, a scary experience. Nature cares little for human beings and our puny attempts to control it can sometimes result in failure and tragedy. Despite the evacuation of 180,000 people because of an unstable situation at Oroville Dam, we've kind of held things together in the north part of the state, but the system is presently strained to the limits. Levees are showing signs of failure in places and reservoirs are getting dangerously full. It is imperative that residents in California keep their eyes open, and when the warnings come and evacuations are ordered that they comply as quickly and orderly as possible. Making a principled stand to defend your house against a rising flood is dangerous and may be deadly.

By the way, the other strange sight on the Pelican Nature Trail was the prevalence of Desert Cottontail Rabbits. There were perhaps hundreds of them along the trail. I assumed they had been flooded out of their habitat on the flats below and had gathered on the high ground of the levee where they were nibbling at the smallest shoots of grass, or reaching for leaves on the lower branches of the shrubs.

They barely seemed to register my presence. I was stepping only two or three feet away from them before they would move into the brush along the trail. They only scattered rapidly once, when a Northern Harrier flew overhead. Human beings are not the only beings who are affected by the floods.

I'm not going to be able to liveblog the coming storm, as I will be out in the middle of it. We're headed out to the Death Valley region where we will be navigating between storms and flash floods. I expect to have some interesting observations once the storm is past and we are home again. And yes, we will be careful out there. It's not often that one gets to observe rain in the driest place in North America. See you in a few days!

Tuesday, February 14, 2017

Liveblogging the Deluge: A Tuolumne River Update, and Scenes on the San Joaquin

The travails at Oroville Dam are appropriately dominating the news this week as nearly two hundred thousand people remain isolated from their homes as the operators work to temporarily shore up the failing spillways at the lake. It should not be forgotten that most rivers in Northern California are running very high, and with yet another storm arriving this week, the threat of flooding elsewhere is still very real.
San Joaquin River near Stevinson
I don't have much information on the San Joaquin River, but I crossed the river yesterday near Stevinson on Highway 165, and it was well out of its banks. There are reports of flooding downstream west of Modesto. Although the river was clearly flooding, it was still well below the levees near the bridge I crossed.
San Joaquin River near Stevinson

I made it down to the Tuolumne River Parkway Trail this afternoon, and found that the river has reached the highest flows since the recent storm assault began in January. It is swollen with water released from Don Pedro Dam upstream.
The capacity of Don Pedro is 2.03 million acre-feet (water elevation 830 feet), and the latest measurements show the lake at just inches below 828 feet. Inflow has declined in the last day or so to 12,544 cubic feet per second, while outflow has been ramped up a bit past flood level, to 10,700 cubic feet per second, so the lake is still rising, albeit much more slowly than a few days ago. An announcement from Turlock Irrigation District indicates that they have no current plans to open the emergency floodgate.  If they do, it will be for only the second time ever. The first time was in 1997, and that event carved a new channel through a pasture area that reached a depth of 40 feet.
It's important in these days of evacuations in the Oroville Dam area to point out that there are big differences between the spillgates at the two dams. The floodgate at Don Pedro is more heavily reinforced and the former meadow slopes more gently, so undercutting of the gate is unlikely. The overflow channel, although not lined with concrete, passes no major infrastructure other than Bonds Flat Road, which washed out in 1997 (but was later repaired). If the storms fill the reservoir, the floodgates could indeed be opened, and some flooding would occur downstream, but not to the extent of the potential flood that threatens Oroville and other towns in the north valley.
Don Pedro Dam, courtesy of Google Earth

In the meantime, I am watching a powerful river changing things along my exercise trail. Portions of the trail were underwater, and the normally placid river is surging noisily downstream. High flows are expected to last for many weeks as the reservoir is prepared to accept the snowmelt that will begin soon. It has been a long time since things have been anything like this.
I'll repeat what I said in the last post. Flooding is a serious matter, one of the most dangerous of geological hazards. One thing that sets river floods apart is their predictability, in that rivers are closely monitored, and flood levels are almost always known with good accuracy hours and days ahead. This means that if you are told to evacuate, your life is in danger, and you should follow any directions and requests made by the authorities. Unlike the give and take of politics, there is no place for "unbelief" that the river will hurt you or your property. The river doesn't care what you believe. Just get out of the way.

Monday, February 13, 2017

Liveblogging the Deluge: Reflections on the Events at Oroville Dam. What Will it Take to Change Things?

This picture is on the San Joaquin River, not the Feather. It's a metaphor for floods all around.

Let me be absolutely clear: this is a serious situation. Nearly two hundred thousand people have been evacuated as officials try to deal with two crumbling spillways at Oroville Dam as high river flows have filled the reservoir past capacity. The situation has briefly stabilized as the lake has been lowered to a level below the auxiliary ("emergency") spillway so temporary repairs can be started. But again, this is serious. If you live in the evacuation zone, follow the orders and get out.

I have a story that will seem pointless at first, but bear with me. I have a fence in the backyard. It was a nice fence at one time. But the years took their toll. Heavy storms would knock some of the slats loose, but I would nail them back in. Dry rot attacked some of the crossbeams, but chicken wire and bungee cords sort of sufficed to hold things together.

The years went by and it got worse. But there never seemed to be enough money, and there were always excuses to do other things instead. But the storms of January hit, and hit the fence hard. Whole sections just fell to pieces. The fence was totally destroyed and finally we had to act, whether we could afford it or not.

What does this small story from my life have to do with Oroville Dam?
From the Los Angeles Times

First, a recap...

Oroville Dam is the highest dam in the United States, at 770 feet, and the second largest reservoir in California with a capacity of 3.5 million acre-feet. It is a major component of the State Water Project, providing water for millions of people in central and southern California. It was completed in 1968.

California has finally ended the worst drought in the state's history with what may turn out to be the wettest year on record (there are still several months left in the season). A series of atmospheric river storms have dumped incredible amounts of rain and snow across the state, and rivers are swollen. Most of the state's reservoirs are nearly full, and operators are carefully draining what they can to avoid flooding now and in coming weeks as more storms roll through. But the big problems cropped up at Oroville Dam.
The main spillway early in the crisis. The emergency spillway is to the left.
When flows get too high, the lakes have to be drained through spillways which are supposed to be designed to accommodate the largest credible flood events. Oroville has such a spillway, but when flows ramped up earlier this week, a huge section of the concrete collapsed and water started tearing away at the adjacent hillside, threatening some of the major electrical transmission lines. It turns out that even though it was never meant to be used, a second spillway had been constructed. No effort had been made to prepare the slopes below for floodwaters, and the cement lip was never really properly reinforced.

Hard choices had to be made. The lake was filling fast, and the longer the main spillway was flowing, more and more concrete was being ripped away. The decision was made to let the lake drain over the auxiliary spillway and only yesterday morning the water started seeping over the edge. By the end of the day, the water was one or two feet deep across the entire structure, and if what I heard is accurate, the flow was around 10,000-15,000 cubic feet per second. For perspective, that's the current near-flood level flow of the Tuolumne River in my own backyard down the valley (below).
Tuolumne River flowing at 9,000 cubic feet per second

It was only a day before a big problem cropped up. A large hole had eroded from the spillway that threatened to cause its collapse. This was the event that precipitated the evacuation event, with warnings that the collapse could happen within an hour. I can't imagine what it must have been like to be living there and hearing such a warning.

What was about to happen? It wasn't a collapse of the entire dam. That's not really in the cards here. Had the spillway collapsed, the water would have starting flowing over the bedrock and loose soil beneath the concrete lip. The breach would have concentrated the flow in one spot, and even though the bedrock is solid metavolcanic rock, the smallest fissures and joints would have been exploited by the rushing water. The result would be a cut of several tens of feet, and that would have drained a significant portion of the reservoir (the top 30 feet out of the 770 feet). That would have been enough water to inundate a wide swath of the floodplain and adjacent areas downstream, and that is what prompted the evacuation order.

The operators of the dam ramped up the flow on the original spillway to around 100,000 cubic feet per second, and that was finally enough to start lowering the level of the lake. By 8 PM this evening the water dropped below the level of the auxiliary spillway, and repairs started immediately (six helicopters are dropping huge bags of boulders into the hole). The immediate crisis passed, but they are still depending on flow down a crippled spillway for the foreseeable future with a stated hope to drop the lake fifty feet beneath the emergency spillway. And we have more storms on the horizon.

What should we take from this perilous situation? My first thoughts as this event unfolded is that this dam and a great many others across the country are decades old. Concrete gets old and crumbles, steel reinforcement beams rust away deep within the concrete, and equipment suffers intense wear and tear. Congress and state legislatures are almost always generous with initial construction funds, but when the time comes for maintenance and renovation, suddenly the deficit becomes important and funds can't be found.

We have chosen to allow our population to grow beyond the capacity of our landscape to support us, and many times over we have chosen to place giant urban centers in places that make no sense (yes, I'm looking at you Las Vegas and Phoenix). We thus depend on gigantic projects bent on controlling nature to provide our water for irrigation and domestic use. We depend on them, and their failure would be catastrophic, both in lives lost, and in damaged infrastructure. And yet we refuse to provide enough funding for their upkeep.

I don't know what caused the failure of the original spillway at Oroville Dam, although I have some suspicions. I think I know the dynamics of what happened with the emergency auxiliary spillway. Whatever the reasons were, they were predictable to those knowledgeable in engineering design. They were in fact predicted in 2005 by environmental groups testifying during the relicensing of the dam according to a note by the California Water Research:

A single operational use or multiple operational uses (with failure to repair any preceding or cumulative damage) of the ungated spillway could result in a loss of crest control of Oroville Dam. A loss of crest control could not only cause additional damage to project lands and facilities but also cause damages and threaten lives in the protected floodplain downstream. An unarmored spillway is not in conformance with current FERC engineering regulations. (emphasis added.)

And that's where we are tonight. A dam in danger of spillway failure, as many as 180,000 people displaced suddenly from their homes, and a government unwilling to take care of infrastructure because it is just too expensive and inconvenient. This is a story that is going to be repeated over and over in coming years. The failure of the levees and the deaths of 1,800 people in New Orleans during Hurricane Katrina was a harbinger. I frankly don't think the events of this evening are going to be enough to convince legislatures to do the responsible thing. What will the tipping point be? The death of hundreds? The death of thousands?

Our country went collectively nuts after terrorists killed 3,000 people in the Twin Towers on 9/11 and we spent more than a trillion dollars on a useless war on terror. Fixing and investing in our failing dams and bridges makes so much more sense, and the benefits go far beyond just safety. It also means putting hundreds of thousands, maybe millions of people to work, and that will strengthen the economy immeasurably.

It's just like the way I put some people to work this week fixing my broken fence. They benefitted and so did I. And my yard is a much nicer place now.

UPDATE (2/13/17): A picture of the damage that was expected to impact the auxiliary spillway, from Peter Gleick on Twitter (@PeterGleick). I certainly looks like the jointed metamorphic rock I expected. This is similar to what happened at Don Pedro in 1997 when a 40 foot channel was carved in a few days.
Circled are people for scale.

Saturday, February 11, 2017

Liveblogging the Deluge: Meanwhile, Back at the Local Dam...Don Pedro Reservoir Today

With all the attention being directed at Oroville Dam today with the broken spillway, it's a bit too easy to forget that similar conditions are being experienced all across Northern California. Several reservoirs are approaching full capacity, including Lake Shasta (96%), and Don Pedro Reservoir (97%). Since it is a few miles upstream of my village, Don Pedro has been the center of my attention since the present series of atmospheric river storms arrived in Northern California.
The capacity of Don Pedro is 2.03 million acre-feet (water elevation 830 feet), and the latest measurements I've seen show that the lake rose 6 feet in the last day to just over 825 feet. Inflow continues in the range of 30,000 cubic feet per second, while outflow continues at just below flood level, about 9,200 cubic feet per second. It may very will be that the inflow will decline (in the past day it dropped from 32,000 cfs to 28,000 cfs) just enough that the reservoir will stop rising, but it will be a close thing. I've heard no word about plans for opening the emergency floodgates, but if they do, it will be for only the second time ever. The last time was in 1997. That event carved a new channel through a pasture area that reached a depth of 40 feet. Since that channel is already present, I don't expect there will be too many problems associated with the overflow, but uncontrolled runoff could result in some minor flooding downstream in Modesto. I would appreciate hearing from anyone in the know who can shed light on the plans for the next few days.
In any case, the reservoir is higher than it has been in many years, and room must be made for the coming runoff from the near record snowpack. That means that the Tuolumne is going to be flowing at thousands of cubic feet per second for the next few months. That is an unprecedented situation that may very well change the configuration of the river in some areas.

It's a good thing that we now have a window of good weather, maybe five days before the next storm arrives. We need some time to make more space in the reservoirs.

POSTSCRIPT (2/12/17): As of this morning, the reservoir has risen to 827.27 feet, and is less than 3 feet from being full. The lake has 1,995,000 acre-feet out of a capacity of 2,030,000 acre-feet (98% of capacity). Source:

Friday, February 10, 2017

Liveblogging the Deluge: The Concerns (Panic?) at Oroville Dam, a Story We've Seen Before

The graphic above (from the Los Angeles Times, Google Earth, and the California Dept. of Water Resources) succinctly explains the serious problem unfolding right now at Oroville Dam on the Feather River. The lake is the second largest reservoir in California backed behind the highest dam in the United States. It is very close to capacity, but the main relief valve, the spillway, has been severely damaged and is essentially crippled. Without the spillway, the operators have only two choices to get rid of the rapidly rising floodwaters. They could let the floodwaters flow over the top of the dam itself, or they open a second emergency spillway.
California Dept. of Water Resources via The Landslide Blog
The first choice is unthinkable. The structure is an earthen-fill dam, made up of a core of impermeable clay covered by other sediments and rock material. If floodwaters top the dam, they would easily cut through the loose material, and the dam could fail. Such a failure would be a catastrophe without precedent. The instantaneous release of more than 3 million acre-feet into the Sacramento River system would be a flood of Biblical proportions. Sacramento and other towns would be inundated, threatening the lives of hundreds of thousands of people. The water delivery system for California would be crippled for years or decades. Postscript: Darn it, I wasn't thinking right. The emergency spillways are below the level of the dam by 20 feet, so this is unlikely to happen, at least due to flooding. It just made such vivid narrative.  Thanks jfmiller for the clarification (see comments below). So that is not going to happen.
Source: CA Dept. of Water Resources via The Landslide Blog
The second choice is magnitudes less catastrophic, but would still be a problem. The emergency spillway would flow across slopes and hillsides that have never been exposed to fast-moving floodwaters. Trees, rocks, and mud will pour into the river downstream, complicating efforts to prevent flooding and levee breaches downstream.

We'll see where this situation resolves over the next few days. But I couldn't help but be reminded of two historical events involving dams and reservoirs that put a spotlight on the choices we've made to build such titanic structures to serve the needs of our society: Don Pedro Reservoir in 1997, and Glen Canyon Dam in 1983.
Glen Canyon Dam in Arizona
We tend to think of these huge structures as monuments to our creativity, power, and control over nature (it's an entirely different issue that I almost wrote "man's control over mother nature", which sounds very patriarchal or chauvinistic). The fact remains though, that we sometimes have flaws in our thinking and planning. Let's take Glen Canyon as an epic example.

Glen Canyon Dam was built between 1957-1964. It is 710 feet high (216 m) and 1,560 feet (475 m) wide, with a volume of 5,370,000 cubic yards (4,110,000 cubic meters) of concrete. It is anchored in Navajo Sandstone. When full the lake is 186 miles (299 km) long, with 1,960 miles (3,150 km) of shoreline, and a total capacity of 26.2 million acre feet (equivalent of two years of the average flow of the Colorado River). The main thing to notice about the picture above is that the dam has no spillway. If water ever came over the top of the dam, the floodwaters could destroy the power generators below, possibly "pulling the bath plug" on the dam. The sandstone that anchors the dam is relatively easy to erode, so such an event could threaten the very stability of the dam itself.

That not to say the operators have no way to deal with floods. When they built the dam, the Colorado River was diverted through tunnels in the cliffs around the dam site. The tunnels were adapted into an underground spillway within the cliffs. When floodwaters threatened to fill the reservoir, they would simply open the spillways and millions of gallons of water would shoot out of the cliffs at the base of the dam.

Which brings us to the events of 1983.
Wait a second...why is there a dam made of plywood here?

After construction was completed in 1964, the lake slowly filled (since water use downstream did not cease, only surplus water was used to fill the lake) and did not reach capacity until 1980. In 1983, the dam came perilously close to failing due to a major flood and design errors. An unexpectedly warm and wet storm caused a tremendous flood upstream of the nearly full Lake Powell. To prepare for the huge influx of water, the dam engineers opened up the underground spillways for the first time. They proved woefully inadequate to the task as cavitation caused the walls of the diversion tunnels to rip out. In places the powerful flow of water cut 32 feet (10 meters) into the soft Navajo Sandstone and threatened the structural integrity of the dam itself. People working within the dam reported rumblings and vibrations that simply shouldn't have been happening.

The diversion tunnels had to be shut down, and the lake threatened to flow over the crest of the dam in an uncontrolled fashion. As noted above, this could have led to catastrophe, as such uncontrolled flow could have eroded and weakened the sandstone abutments of the dam. Failure of Glen Canyon dam would have led to the domino-like destruction of other large dams downstream, and the decimation of the water-supply infrastructure of some thirty million people. The disaster was averted by the construction of an 8 foot high dam of wood flashboards that held back the water long enough for the flood to subside (see the picture above). The structural integrity and survival of the dam came down to less than a foot...the distance between the water level and the top of the flashboard dam in 1983.
The damaged spillway at Glen Canyon Dam in 1983
That is what came to mind as I saw pictures of the Oroville spillway yesterday. The other echo of the past was the need to open up the spillway onto new ground. Something very similar happened at Don Pedro during the floods of 1997 (and if the narrative below seems familiar, yes, I used it a couple of weeks ago as an introduction to the coming atmospheric river storms. I'm revisiting the story because of the similarity of events at Oroville Dam).

Don Pedro Reservoir on the Tuolumne River stores water for the Modesto and Turlock Irrigation Districts, and also serves as storage for Hetch Hetchy Reservoir water bound for San Francisco. The earth-fill dam stands 580 feet high and inundates 26 miles of the Tuolumne River, which flows out of Yosemite National Park. It stores just over 2 million acre-feet of water. The dam was built for irrigation storage, hydroelectric power generation, and recreation. And flood control.

The water year of 1996-1997 was unusual, but unfortunately familiar given the events of the last month in California. A series of large storms in December had built up a record or near-record snowpack in the Yosemite high country. Then a New Years Day Pineapple Express storm took aim at central California. The warm, moist atmospheric river flowed over the Central Valley dropping only a few inches of rain, but when it hit the mountains, it poured as much as three feet of rain at elevations as high as 10,000 feet and onto the record snowpack.
Don Pedro Reservoir was at the proper level for normal flash flood conditions, with about 300,000 acre feet of storage available. But the water coming downstream was not normal. At the peak, the rivers flowed into the lake at an unbelievable rate of 130,000 cubic feet per second (the Feather River, by the way, reached about 170,000 cubic feet per second this week). To put this number into perspective, the Tuolumne River is considered to be at flood stage at 9,000 cubic feet per second. The dam operators had a big problem and they knew it. They had to purposely flood the cities downstream to prevent a total catastrophe. They ramped up the power generating turbines, and for the first time in the dam's history, they opened the floodgate.
The floodgate didn't open into the Tuolumne River. It faced a meadow that had never before had a river flowing through. As can be seen from the pictures here, the meadow was hit by a flood of gigantic proportions. Ripping away soil and solid rock, the river quarried a channel forty feet deep in the space of three days. And it barely worked. At the highest point, the reservoir was flowing uncontrolled over a concrete weir that was the never supposed to be topped. The water was only a foot deep, but spread out over several hundred feet, it ripped away soil, rock, and the highway that passed below the floodgates.
The city of Modesto and others downstream experienced the greatest flood in their history, with top flows of around 60.000-70,000 cubic feet per second. But if Don Pedro Dam had not been there, the towns would have been hit with a flood twice as big.

Only one flood in recorded history could possible compare with 1997. That was the flood of 1861-62, which was so large that parts of the Great Valley turned into a lake for weeks. Sacramento was abandoned as the state capitol for months while the waters subsided. No gauges were present on any of the rivers so we don't know how the numbers compare, but considering that 1997 was considered a 250 year flood (a 1/250 chance of occurring in any one year), it must have been truly extraordinary. And in the last month, only twenty years later, we seem to have reached a similar level of flooding on many of our rivers in California.

And that's the way it is with Oroville Dam today. With a dam and a river that is larger than the Tuolumne. I can't imagine the stress levels of those who are dealing with the situation as they try to navigate their way between undesirable choices.

Since we have chosen to live together as a society in numbers totaling millions of people, we have to make many choices about how much we alter and change the landscape that supports us. Our basic needs of water, food and shelter have to be provided on a massive scale, but the infrastructure at such kinds of scale also have the potential to fail at equally high magnitude. Politicians are fond of keeping taxes low and complain endlessly about "burdensome" regulations and guidelines. They cut the budgets of agencies who provide the expertise that can allow us to avoid catastrophes. When they do this, they are setting up conditions for failure on an astronomic scale. So who will we blame in the end?

I wish the operators of Oroville Dam all the best. I assume they are talented at what they do and that they will be making the best possible decisions in coming days. I also hope that those who represent us in the halls of government will gain some kind of insight about the best ways to care for and improve the infrastructure that supports and protects us all. I hope, but I'm not all that optimistic, seeing the disdain that those in power right now hold for science and knowledge.

POSTSCRIPT: The incoming flow has declined to 120,000 cfs, with 65,000 cfs outflow. If it continues to drop, the operators might not need to use the emergency spillway.

POSTSCRIPT (2/11/17 at 8 AM): According to this Mark Finan at KCRA, this is the emergency spillway this morning at 7 AM. About to go over the edge...

The incoming flow at 8 AM is 89,276 cubic feet per second, the outflow is 55,092 cfs. The lake is still rising, with less than 2 inches of clearance before water flows through the spillway.

POSTSCRIPT (2/12/17): The inflow to Oroville Reservoir is finally dropping below the outflow, so the lake level can start dropping (hopefully enough to deal with the next storm in five days).
POSTSCRIPT (2/12/17, 6:45 PM): Gee whiz, things were quiet, so I went birding for a few hours and all hell broke loose while I was gone. First, if you are in the Oroville area, take the official announcements seriously and follow evacuation orders without delay. It's got to be serious if they take a step like that. In any case, the auxiliary spillway is in imminent danger of failing. If it does so, lake water would be surging over bedrock in an uncontrolled manner. It depends what kind of bedrock there is (I'm working on that), since the rapid flows could erode the rock channel downward (as happened at Don Pedro in 1997) which could then lead to higher flows and flooding downstream. It sounds like the operators increased flow on the other damaged spillway, so that the flow over the auxiliary spillway is down to 2 inches, rather than the 1 1/2-2 feet. That takes a lot of pressure off the auxiliary spillway, and offers hopes for some kind of repairs before the next storm hits.