It's a River Again! Winter on the Tuolumne

All the pictures in this post are from the Tuolumne River Parkway Trail in Waterford.

All in all, it's been a good year (so far). One can judge the quality of a year on the basis of many things, and in this instance I'm talking about water. So much of the time, my part of California is at an extreme in one direction or another. Last year it was a drought up until some late storms in March that salvaged the water year. The year previous was one of floods and threatened floods, unlike anything seen in twenty years, but at least it filled the state's reservoirs after an astounding five-year drought. This year finds us in the sweet spot, kind of in-between, but a bit above average.

The rainy season started a bit slowly, just enough to make one worry a little bit about drought. No storms in September. A single small storm in October. But then in late November the pace picked up with just over 3 inches in my rain gauge, and December added more than 2 inches. The same with January, nearly 3 inches, and now February has already produced 2.5 inches with more storms coming this week. Of course my backyard is not the measure of water conditions in the state. The news reports are full of stories on the incredible snowpack that has built up in the Sierra Nevada in the last few weeks, with single storms producing six feet of snow or more.

The critical Sierra Nevada snowpack sits at between 109% and 135% of normal (measuring from north to south), with a statewide average of 123%. If no more snow fell, our season would end at 84% of normal. It's a comfortable place to be.

But no water planner is ever comfortable. With so much snow in the mountains, the reservoir water masters always have to worry about the possibility of a big atmospheric river storm, the kind that combines the extreme low pressure of an Arctic storm with a stream of extremely humid air out of the tropics. Like the one that could happen this week. In a worst case scenario, such a storm could cause rain at high elevations, melting much of the snowpack and raising the specter of flooding downstream.

And that is why the Tuolumne River came alive this week. For many months, the river has remained at an unnaturally low flow of about 200 cubic feet per second, a minimal amount. There are large reservoirs upstream, especially Hetch Hetchy and Don Pedro, and the operative mode is to save as much water as possible. During the recent storms, inflow to Don Pedro reached as high as 8,000 cubic feet per second, but outflow remained at 200 cfs.

Last week the river was dramatically higher, flowing at 2,000 cubic feet per second. The water masters are clearing out some storage space in Don Pedro in case of a flood emergency. It looks like they intend to go as high as 3,500 cfs in the next few days. Certainly not a flood (it would have to reach 9,000 cubic feet per second for that), but enough to clear the channel of invasive water hyacinth, and enough to make one feel the river is closer to a natural seasonal condition.

It's a nice time of year to walk the Tuolumne.

After the Deluge: Dam! We Almost Lost a Dam!

But for the vagaries of the weather, we would never have seen it. Running field studies courses in the winter and spring always involves an element of chance, but in the dry year that we've had, it seemed safe enough to schedule a trip to Yosemite National Park in early April. But as the previous two posts have shown, an epic atmospheric river storm, a Pineapple Express, pummeled the area last Saturday. We postponed our trip to Sunday, and had a fine day, but we reversed the direction of our trip, because we still weren't sure if Yosemite Valley would open up in time for us to visit. If the valley remained closed, we wanted the choice of going to Hetch Hetchy Valley instead, and to allow that we needed to go by way of Highway 120 through Big Oak Flat and Groveland, instead of Highway 140 through Mariposa.

I had heard that Moccasin Reservoir almost failed a few weeks earlier, but didn't give it much thought while I was bustling about trying to think of alternate field trip stops. We had left the Red Hills Area of Critical Environmental Concern and were approaching the Priest Grade when I remembered the dam episode. During an intense storm on March 22, floodwaters overwhelmed the spillway of the small earthen dam on Moccasin Creek. Moccasin Dam is about 700 feet across and about 60 feet high, holding back about 554 acre-feet of water when full.

There is one very strange fact about the dam. Even though it blocks Moccasin Creek, it doesn't hold water from Moccasin Creek! In instead contains Tuolumne River water that has been diverted into the Hetch Hetchy system and pumped through a hydroelectric plant on lower Moccasin Creek. The dam is a small forebay that feeds into the much larger Don Pedro Reservoir and Lake just a mile downstream. Moccasin Creek was heavily mined during the Gold Rush, and the sediments may be contaminated by mercury and other toxic metals related to the gold extraction process. The water of Moccasin Creek is diverted around the reservoir.

As we drove by, I commented on the radios that the flood had happened, but I saw that a lot of the damage was visible from the highway and we screeched to a stop to have a look. It had clearly been a serious storm. The flood had ripped out trees, roads, and telephone poles. The dam itself looked undamaged, but the spillway downstream had been severely eroded. Few people were threatened by the flooding, but roads were closed. If the dam had failed, the effects would have been limited because Don Pedro Lake, only a mile downstream, would have easily absorbed the extra water.

Maybe the saddest aspect of the flood was the near destruction of the fish hatchery. As can be seen in the picture below by Mark Brooks and aired on several news outlets, the entire hatchery was flooded, and all of the fish either killed by turbulent mud, or carried downstream to Don Pedro (when does fishing season start?).

Geology and weather are capricious beasts. One of the lessons of the earth sciences is that no place anywhere on this planet is free of the possibility of natural disasters. Some places are more dangerous than others, but there is always a big advantage to understanding the possible hazards where one lives and works. One may think they may never happen in one's lifetime, and they might not, but that's a dangerous gamble to make.

Photo by Mark Brooks

Liveblogging the Deluge: The First Day of Fall and a River Finally Returns to Abnormal

I was mistaken. Nearly two months ago, I suggested in a post that the abnormally high flows on the Tuolumne River were finally subsiding, and that the great flood of 2017 might finally be ending. This happened because I strolled along the river on a day when the output from Don Pedro Reservoir finally fell below 5,000 cubic feet per second, to around 1,500 cfs. That lasted for an entire day or two. The inflow at Don Pedro was still several thousand cfs, and the effort to build up some emergency flood storage space in the reservoir meant that they ramped up flows to 5,000 cfs, and that level continued for another seven weeks.

Source: https://waterdata.usgs.gov/ca/nwis/uv?11289650

I walked the river again today and saw something I haven't seen in nine months: a river at very close to minimum mandated flow. A quick check of the river gauge on the USGS site confirmed that the river was flowing at a mere 339 cubic feet per second. A look at the table above shows how the flow has been declining over the last week.

The CDEC site for Don Pedro Reservoir is even more informative, showing high flows throughout August and early September. Inflow to the reservoir finally dropped below 1,000 cfs about the 12th of September, and the outflow was then also lowered (about 1,500 cfs is diverted for irrigation which accounts for the discrepancy between output from Don Pedro and the river flow below LaGrange Dam where the diversions take place).

Source: http://cdec.water.ca.gov/cgi-progs/queryDaily?s=DNP&d=22-Sep-2017+20:14

Don Pedro Reservoir remains well above normal in water storage after an extremely wet year of precipitation. It presently holds 1.7 million acre-feet, about 84% of lake capacity, and 126% of normal for this time of year.

The floodplain is a different place now, compared to what it was a year ago. The biggest change is the vegetation. Hundreds, maybe thousands of trees and shrubs have been uprooted and washed away downstream. There are huge piles of branches and tree trunks across the riverbed. Other areas along the streambed have been washed clean of any vegetation at all, leaving vast tracts of barren gravel and pebbles. The invasive and damaging water hyacinth is gone, carried downstream in the many months of flooding. One can only hope that the seeds were carried away as well, although they probably weren't. I'll be pulling any hyacinth out of the river that I see in coming months.

The river is not back to normal, because "normal" disappeared more than a century and a half ago when gold was discovered upstream. The original channel and floodplain was upended in the search for the elusive metal. Trees and shrubs were removed, and every bit of gravel was run through sieves, longtoms, or dredges. After the search for gold ended, quarrying for gravel and sand took over, further altering the floodplain, ultimately forming numerous ponds fed by groundwater seepage.

These days, efforts are being made by organizations like the Tuolumne River Trust to bring about some sort of natural balance on this badly scarred landscape. Adjustments to the channel can help expose the gravel bars that salmon need to spawn, and one can hope that the fall run will be more successful than those of the last five years during the drought. Only the Merced River has a more southerly run of Chinook Salmon. At one time 130,000 salmon cruised the Tuolumne. In the last decade, the run has exceeded 1,000 only once (1,926 in 2013).

Walking the Tuolumne River Parkway Trail in Waterford. This is my backyard.

In the meantime, the flood of 2017 seems to finally have subsided. The river is back to abnormal, and we now await the new storm season. After a monster season in 2017, one has to wonder what the coming year holds in store for the river.

Liveblogging the Deluge: Is the Big-Boned Lady Singing? The Aftermath of the 2017 Flood on the Tuolumne River

The Tuolumne in August of 2015. This was a sick river overgrown with invasive hyacinth. Flow is about 200 cfs.

Goodness sakes, are we still talking about that flood? Well, yes we are. It isn't quite done, although events this week are signaling the end, at least in some respects. In another respect, the five-year drought that we thought the floods put an end to is still with us.

I've had the privilege of seeing two great floods in my time living along the Tuolumne River where it flows into California's Great Valley. The first was the incredible flood of 1997 when the river overwhelmed Don Pedro Reservoir and rampaged through my town at around 70,000 cubic feet per second. The second was this year. The river never topped 15,000 cubic feet per second, but the flood continued for kind of a long time...it lasted for more than six months! The watermaster at Don Pedro Reservoir upstream had to do a delicate dance of balancing the inflow of storm water and snowmelt with a lake that was at more than 95% of capacity. The snow this year was around 200% of normal, and even today, on the first day of August, the inflow is still a respectable 4,000 cubic feet per second. Normal would be a few hundred cfs. I am reasonably sure that I'll never see an event like this again in my life.

The Tuolumne River in the same spot in January 2017, at about 12,000 cubic feet per second

These six months have been a stark example of geology in action. There have been real changes in the channel of the Tuolumne River that are only just beginning to become visible. At the beginning of July, the river was still running close to flood level, at 9,000 cfs, although irrigation canals were taking some of the water. In the last week, the flow dropped from around 7,000 cfs to around 1,500 cfs. Consider this: 1,500 cfs is about a seventh of flood level, where the river stayed for six months, but is about seven times the average flow of the river at this time of year!

After five years of horrific drought, the river channel was in trouble. Without the flushing action of at least a moderate flood, silt had covered many potential nesting sites for salmon and other fish, and invasive hyacinth threatened to choke the channel (the hyacinth crowds out other life and prevents light from penetrating the water; in some places the river was covered entirely by the floating mats).

The Tuolumne River this morning, at about 1,500 cfs. All of the hyacinth and many trees and willows have been swept away. 

As the river channel starts to emerge from the floodwaters, we can see that trees and willow thickets have been swept away from some areas, leaving a floodplain of barren river cobbles (above). The hyacinth is gone (although I bet seeds are hiding in the soils along the river). In other areas, the floodplain is a tangled mass of trunks, branches and root balls.

The flood took away a lot of habitat for the wide variety of animals that normally live on the floodplain, and took it for a long time (I'm wondering if homeowners on the bluff above had problems this year with raccoons and the like). It will be interesting to see how and when they come back.

For farmers and anyone who uses water, the results are spectacular. Compare where California's major reservoirs were in January of 2015 during the height of the drought to where they are today. It's almost like we have an embarrassment of riches, but not really...

There is one aspect of the drought that cannot be shown on these maps, but which will affect Californians for decades. It's the groundwater. During the drought when surface water was not available, agricultural interests went underground to meet their needs. Paradoxically, almonds became a hot crop, and despite the dry conditions, tens of thousands of acres were planted with the water-hungry trees, and the orchards were almost exclusively irrigated from new wells. After a number of years without subsidence, some areas of the Central Valley began to sink again as the water was pulled out.

The high river flows will contribution a little to the recharge of the groundwater reservoir, but for the most part the water is irreplaceable, and we went through a lot of it. It's analogous to living off of a savings account without making any deposits. In the long run it is unsustainable.

http://cdec.water.ca.gov/cgi-progs/products/rescond.pdf

The problem now, of course, is trying to plan for the future health of our groundwater basins, but with the end of the drought people are thinking of other things. There are important decisions that need to be made about the future use of our groundwater resources, and when citizens aren't paying attention, the decisions end up being made by those who stand to profit from the use of the water. They won't be acting for the common good.

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.

Liveblogging the Deluge: Big Changes on the Tuolumne River

Source: http://www.capradio.org/articles/2017/02/20/don-pedro-reservoir-spillway-opens-for-first-time-since-97/

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 (https://waterdata.usgs.gov/ca/nwis/uv?11289650)

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.

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.

Source: http://cdec.water.ca.gov/cdecapp/resapp/getResGraphsMain.action

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: https://waterdata.usgs.gov/nwis/uv?site_no=11287500

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. It...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.