Liveblogging the Deluge: Merced River in Yosemite Reaches Flood Stage

This picture is a cheat. I'm not in Yosemite, this picture is from two years ago

I was supposed to be in Yosemite right now. We had scheduled a field studies class today that would have sent us up the Merced River to Yosemite Valley and back, but nature has intruded with an unusual April atmospheric river storm that is setting some daily rainfall records across Central California. San Francisco, for instance, received rain equivalent to an average April in a single day. My backyard rain gauge in the Central Valley has recorded 2.2 inches since yesterday. In 26 years of measuring, there have only been three Aprils where we've received that much in the entire month.

I had already decided to postpone the trip by last Monday when the forecasts called for flooding on the Merced River in Yosemite Valley. In short, the river jumps its banks at the 10 foot level (about 6,000 cubic feet per second). At 12 feet (9,500 cfs), the flooding river covers some of the main roads in the valley. Today's storm is expected to result in a river that crests at 14 feet, or 14,000 cubic feet per second. Even if I had decided to go ahead with the field trip, we would not have reached our goal, because the park service closed Yosemite Valley yesterday. We're going to try and make the trip tomorrow to check out any flood damage, but if the valley floor is still closed, we'll try to see Hetch Hetchy instead.

Yosemite Valley is especially prone to flooding because it is high up in the mountains close to the headwaters of the Merced River, and when rain falls, it falls on mostly barren granite. The water flows into channels very quickly, and the channels gather into the Merced River and Tenaya Creek just as quickly. There are no reservoirs upstream for flood control (nor should there ever be).

If it drives home the point, Yosemite Valley has only been closed a few times in its history, most notably in 1997 during the unprecedented floods of that year, and during a few government shutdowns. It takes a major event to convince the park administration to shut down the iconic valley.

I hope to be back tomorrow with pictures of Yosemite Valley. We'll see what happens! I will update the flood hydrograph in the space below when the river flow peaks.

UPDATE: At 1:00 PM, the river has reached 10,800 cubic feet per second (12.85 feet). If I'm reading the data right (below), this is only the fourth time the flow has reached this level since 1996.

Here is the 1:00 PM report:

Also, here is a link to video of the main road in Yosemite Valley at the moment, courtesy of the Fresno Bee.

UPDATE 2: We've reached 12,100 cubic feet per second as of 5:00PM, and 13.68 feet (10 feet is flood  level), and the flood may be peaking. The discharge only increased by 100 cfs in the last hour.

FINAL UPDATE: It looks like the flood has peaked at 12,100 cfs, as it has been there for two hours. I expect it will now start dropping. I'm hoping the valley will reopen in time for our visit tomorrow. If not, I guess we'll check out Hetch Hetchy!

WELL, OKAY, ONE MORE UPDATE: Yosemite National Park has posted pictures of the flooding today: https://www.facebook.com/pg/YosemiteNPS/photos/?tab=album&album_id=1650167855031490

What is Lagtime, and Why Should You Care? Checking out Dry Creek

Dry Creek at 9AM this morning, just a few cubic feet per second

I wonder how many Dry Creeks there are in the country? I know of two of them within a thirty mile radius of my home, including this one just two miles to the north. The name itself imposes a sort of insignificance to the waterway. The Dry Creek of my town could not be mistaken for an important river. Instead of having headwaters in the high country of glaciers and granite like the nearby Tuolumne and Merced Rivers, it begins in the Sierra Nevada foothills in dry gullies west of Highway 49. It has no natural source of flow other than rainfall. It does run for much of the year due to of irrigation overflow from agricultural fields upstream. But it could never be mistaken for a major waterway.

Except during intense storms...

Dry Creek is one of the few waterways in the Sierra Nevada with no reservoirs or flood control measures. When big storms roll through, the water can rise quickly and overflow its banks. We had such a storm last night. Rain throughout California from Eureka to San Diego. I saw that some coastal areas received 7" or more. Less rain fell in the arid Central Valley, with about 0.70" in Modesto and 1.21" in the rain gauge in my backyard. But several inches fell upstream in the Dry Creek drainage, and I expected to see some high water today. I headed out this morning for a look and...nothing. I was briefly surprised, but then I remembered that I teach about this stuff. It's a perfect example of flood lagtime. Should you care? Yes, if you live anywhere near a river.

Dry Creek at 2,000 cubic feet per second this afternoon.

Rain doesn't fall into rivers, at least not much of it. If falls all over the landscape. In gently sloping landscapes like the Sierra Nevada foothills, it takes time for water to gather into rills and small channels so there is a delay in the rise of the river downstream. That's what lagtime is, the difference between the height of the storm and the height of the runoff measured at some point downstream. I checked on the NOAA website for the latest predictions and could see that the river was slated to begin rapidly rising in the late afternoon, peaking about 8PM at 4,000 cubic feet per second. Sure enough, when I left Modesto about 2:00PM, the river hadn't changed, but by the time I crossed it again 10 miles to the east, the channel was full and beginning to spill over into some of the surrounding fields. It was running at about 2,000 cubic feet per second.

Dry Creek at 4,000 cubic feet per second, as expected after nightfall tonight (March, 2011)

So, how can hydrographers predict floods with such accuracy? They have been monitoring all the major streams in the country for upwards of a century, so there is a huge database to draw from. They watch the pattern of the storm as it progresses, and compare it to those of the past. They can then forecast the onset of flooding, the height of the water, and the cessation of the flood for areas downstream. The greater the lagtime, the more time people downstream have to prepare for the deluge.

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

Accurate scientific data is fundamental to the health of our society. There is not a single place in our country (and indeed the world) that is free of geological hazards. Hydrologists analyze the storm data and report to government officials who then take action to protect the populace from flooding. Seismologists analyze fault activity in hopes of minimizing the damage from major earthquakes. Volcanologists monitor the dangers of active volcanoes. Climate scientists track the effects of global warming.

In a healthy society, the politicians and government officials accept the findings of those who know the dangers best, and act accordingly. I hope that the new administration will come to understand this. If they were to deny the existence of earthquakes or volcanoes (however strange that might seem; Bobby Jindal once famously complained about "volcano monitoring"), disaster would happen. And likewise, if those in government deny global warming, then it would be like a politician predicting that the rivers won't rise, despite the massive storm upstream.

A California Geologist Observes a Flood in Oregon: Hilarity Ensues?

OMG, a gigantic river! The Umpqua River below Roseburg, Oregon

I'm really more of a desert rat. I hail from a dry valley in California, I take my students on extended field trips to places like Death Valley and the Colorado Plateau. The rivers that I know would be considered something like large creeks in a place like Oregon or Washington. I just don't see stuff like this where I live.

I've been traveling up the California coast into Oregon, and landslide problems forced us inland from the coast onto Interstate 5, and with the long day and the extra 200 miles added to our itinerary, I just flaked out and stayed the night in Roseburg along the Umpqua River. As noted in yesterday's post, the river reached flood stage last night, but just barely, rising a foot above the minimum considered to be a flood. But the amount of water! At the peak, the discharge at Elkton a few miles downstream was 131,000 cubic feet per second!

Here's why this is so ridiculous to me. In 1997 the Tuolumne River, which flows near my home, experienced its greatest flood in recorded history, with a peak flow of about 70,000 cubic feet per second. It could be such a rarity at that level that another such flood has but a one in three hundred or four hundred chance of happening in any given year. In other words, quite probably not in my lifetime or yours. And that extraordinary flood was barely more than half what I saw today on our journey down the Umpqua River to the coast.

I just don't often see rivers this big, or filled to this level. So my two hour drive turned into a four hours as we stopped to take pictures and stare at the flooding river.

It was clear that trees (and the odd telephone pole or railroad tie) were ripped from banks upstream. There was a lot of debris in the river. Roseburg had received about two inches of rain the previous day, but the mountains upstream got much more than that. The ground was no doubt saturated from the rains of the previous week, so much of the precipitation ended up in the river.

A flood hydrograph, like the one above, is the tool that hydrologists use to analyze river flow and to predict possible flooding. It takes some time for rainwater and snowmelt to get into channels and flow downstream. By comparing the pattern, timing, and amount of precipitation in a given storm compared to storms of the past century (most U.S. rivers have been monitored for that long), they can predict hours in advance how high the floodwaters will reach, and for how long. For major rivers with huge drainage basins (like the Mississippi River), the warnings can be days ahead of the flood.

The Umpqua River flows into a large estuary at Reedsport, Oregon, and just upstream is a Roosevelt Elk viewing area. I would never have noticed such a thing on any other day, but the highway was actually a flood protection levee that prevented the river from flooding the meadows where the elk hang out. Driving along, I could see the Umpqua River just a foot or two below the level of the highway on the right, and the elk refuge on the left, about six or seven feet below river level! There was certainly some ponding as can be seen in the picture above, but if the highway weren't there, the entire scene above would be under at least four feet of water.

So this was a minor flood in the big scheme of things, barely rating mention in the local media. If you wonder what would really get their attention, how about the greatest flood ever recorded on the Umpqua River? It began on this very day fifty years ago, and the amount of water reached at least 260,000 cubic feet per second, about twice the flow of today's event. It's hard to even imagine.

So...your desert-living geoblogger got to get all fascinated about a river today. More rain is expected this week. If anything interesting happens, I'll let you know, and all my Pacific Northwest readers can quietly chuckle at my amateurish enthusiasm for BIG rivers.