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Upper Midwest Stream Restoration Symposium

I participated in this a few years ago and it was a great experience for practitioners, regulators, and academics of stream restoration.

Let me encourage you to submit an abstract to the 2015 Upper Midwest Stream Restoration Symposium (UMSRS) to be held Feb 8-11, 2015 in Dubuque, Iowa. Organized by the regional Partnership for River Restoration and Science in the Upper Midwest (PRRSUM), the UMSRS focuses on bringing together researchers and applied practitioners to advance the dialogue of river and stream restoration in the Midwest.

The oral abstract deadline for the conference is September 26, 2014. Learn more in the attached flyer and PRRSUM brochure. If you are unable to participate, please consider signing up for our mailing list to learn about other PRRSUM activities (only 1-2 emails a month!) at www.prrsum.org.

For more information: http://www.prrsum.org/

Floodwaters rising on the Red River

Cross posted at Highly Allochthonous

Fargo, North Dakota is coming out of its 3rd snowiest winter since 1885. Snow continued to fall into late March, and daytime temperatures have only been above freezing for few weeks. At night, it’s still below freezing, though starting tomorrow night the forecast calls for above freezing minimum temperatures. Soils are already saturated, and more rain is possible this weekend.

In short, it is perfect flood weather for the Red River that runs along the Minnesota-North Dakota border and into Canada. This is a place with the perfect geography for extensive flooding, and a long history of big spring floods.

Checking the water level on a bridge between Fargo and Moorhead. Photo from Minnesota Public Radio.

Checking the water level on a bridge between Fargo and Moorhead. Photo from Minnesota Public Radio.

Every town along the Red River has been devastated by a flood more than once. So they’ve all got emergency response plans in place for weather just like this. For example, Moorhead (Minnesota, across the river from Fargo) has a nifty GIS feature that shows how each foot of flood water affects each city block.

Residents are already filling sand-bags to build temporary levees. But with year after year of flooding, and with successful sand bag efforts the last two years, some residents might be taking this year’s flood predictions in a somewhat complacent fashion. But looking at the National Weather Service’s North Central River Forecast Center projections, there’s plenty of reason for concern all along the Red River.

As of 9 am Central time on 7 April 2011, most of the US portion of the Red River is already above flood stage, but water levels will continue to rise almost everywhere for at least the next week.

Flood stages as of 9 am 7 April 2011. Screen grab from NCRFC.

Current flood levels along the Red River and nearby drainages, as of 9 am, Thursday 7 April 2011. Orange circles indicate minor flooding, red indicates moderate flooding, purple indicates major flooding. Screenshot from the North Central River Forecast Center, using data supplied by the USGS.

The flood wave will move downstream – from south to north. In Wahpeton, a crest is expected today, with a second – equally high if not higher – crest next week. There the flood crest is likely to fall a few feet short of record water levels set in 1997.

Between Wahpeton and Fargo, tributaries to the Red River are having major flooding as well – in part because of backwater effects from the main river. If the Red River is flooding, there’s no place for water flowing down the tributaries to go. Instead they back up, causing even more widespread flooding.

In Fargo (ND) and Moorhead (MN) – which have a combined population of 200,000 people – the flood will not crest until late Sunday. Right now, the National Weather Service is predicting a crest of 39.5 feet, which 1.3 feet short of the record flooding of 2009. However, there some chance that the river will crest at 41 feet, or even higher if there is precipitation in the next few days. Currently, 80% of the city is protected by sand bags and levees to a height of 41 feet, but those may need to go even higher.

NWS Flood Forecast for Fargo, North Dakota (7 April 2011)

NWS Flood Forecast for Fargo, North Dakota (7 April 2011)

Two weeks ago, the National Weather Service issued a longer-term flood forecast for the Red River at Fargo. At that time they considered it a 10-50% percent chance that the river would reach 40 to 44.3 feet by mid-April. They provided a probability of exceedence curve for their modeled projections of this year’s flood season against the historical record of flooding, as shown below. To understand this graph, it helps to look at a few specific points. Right now, the river is at 35.32 feet. Based on the outlook from two weeks ago, it was virtually inevitable that the river would reach this level, with a probability greater than 98%, as shown by the black triangles. In contrast, 35.32 feet is reached less than 5% of the years in the historical record for Fargo, as shown by the blue circles. The current projected crest of 39.5 feet was given about a 50% chance of being exceeded as of two weeks ago, yet it has only be reached twice (1997, 2009) in 111 years of record. Two weeks ago, the National Weather Service was saying that there was a 25% chance the river could go above 42 feet, which is higher than the top of the sand bag levees now being prepared.

NWS Chance of exceeding river levels on the Red River at Fargo, conditional simulation based on current conditions as of March 24, 2011

NC River Forecast Center's 90 model showing the Red River at Fargo's chances of exceeding certain water levels, relative to the historical record.

The short term forecasts, like the one two above, have better skill than long term forecasts like the immediately above, but the long term forecasts are vital for emergency managers, city officials, and riverside land-owners in making early plans for the flood. The reason they’ve got all the sand and sand bags on hand in places like Fargo is because they knew there was a good chance a really big flood was coming. They’ve been talking about it since January.

Downstream (north) of Fargo-Moorhead lies Grand Forks, with about 100,000 people in its metropolitan area. Grand Forks was swamped by the flood of 1997, but the current forecasted peak stage this year is about 3.5 feet lower, though the crest won’t reach Grand Forks until late next week. For now, they are watching the water levels and making their preparations. Downstream further, lies Winnipeg, Manitoba. The flood crest won’t reach there until late April, but already the river is 17 feet above normal winter stage, and only 5 feet below the 2009 flood peak. Needless to say, they too are sand-bagging.

But for the next few days, the action focuses on the Fargo-Moorhead area. You can check out the updated data and forecasts or you can watch the flood play out in Moorhead with a live webcam pointed at the downtown waterfront:
http://www.justin.tv/widgets/live_embed_player.swfWatch live video from 702 Flood Cam – Moorhead on Justin.tv

A continental divide that runs through a valley

Now that’s pathological.

Parts of the Upper Midwest are disappearing under spring floods. The Red River of the North is at major flood stage, again, and the Minnesota River flood crest is moving downstream. It’s a pretty frequent occurrence in both of these river systems, and in part, flooding is a legacy of the glacial history of the area. The Red River flows to the north along the lake bed of Glacial Lake Agassiz, which is pathologically flat. The Minnesota River flows to the south along the channel of the Glacial River Warren, which was gouged out of the landscape by water draining from Lake Agassiz.

14,000 years ago there was direct connection between what is now the Red River basin and the Minnesota River basin. Today, there’s a continental divide – with the Red flowing toward Hudson Bay and the Minnesota flowing toward the Mississippi and Gulf of Mexico. But what a strange continental divide it is – for it runs through the former outlet of Lake Agassiz, in what is now known as Brown’s Valley or the Traverse Gap. This divide is not so much a high point in the landscape, but a just-not-quite-as-low area. The little community of Brown’s Valley sits between Lake Traverse (flows to the North, forming the headwaters of the Red) and Big Stone Lake (flows to the south, forming the headwaters of the Minnesota).

Here’s what it looks like on Google Earth. Note that I’ve set the terrain to 3x vertical exaggeration, so that you have some hope of seeing the subtle topography of this area.

Croppercapture12

And here’s a very, very cool oblique photo from Wikipedia. It shows the divide looking from north to south — mostly covered by floodwaters in 2007. It’s not every day you get to see a continental divide covered in water.

800px-browns_valley_flood_07

Why does the Red River of the North have so many floods?

Cross-posted at Highly Allochthonous

Communities along the Minnesota-North Dakota border are watching the water levels, listening to the weather forecasts, and preparing for another season of flooding. It must be a disconcertingly familiar routine, as this will be the third year in a row in which the Red River of the North reaches major flooding levels. But this isn’t merely a run of bad luck for residents in the Red River Valley, major floods are to be expected in a place with an unfortunate combination of extremely low relief and a river at the whim of snowmelt and ice jams.

The Red River of the North begins in Minnesota, near the border with North and South Dakota, and it flows northward through Fargo/Moorhead, Grand Forks, and Winnipeg before emptying into Lake Winnipeg, Manitoba. The landscape around the Red River is excruciatingly flat (Figure 1), for the Red River Valley isn’t a stream-formed feature at all, but is the remnant landscape of Glacial Lake Agassiz, which held meltwaters from the Laurentide Ice Sheet for more than 5000 years. The modern Red River has barely managed to incise into this flat, flat surface, because it slopes only very gently to the north (~17 cm/km). Instead, the river tightly meanders across the old lake bed, slowly carrying its water to the north. Topographically, this is a pretty bad setting for a flood, because floodwaters spread out over large areas and take a long time to drain away.

Topography of the US portion of the Red River Valley from SRTM data as displayed by NASA's Earth Observatoryredriver_srtm_palette

Figure 1. Topography of the US portion of the Red River Valley from SRTM data as displayed by NASA's Earth Observatory

The climate of the Red River watershed makes it prone to flooding during the spring, usually peaking in about mid-April. The area receives about 1 m of snow between October and May, and the river freezes over. In late March to early April, the temperatures generally rise above freezing, triggering the start of snowmelt. Temperatures warm soonest in the southern, upstream end of the watershed and they get above freezing the latest near the mouth of the river. This means that snowmelt drains into the river’s upper reaches while downstream the river is still frozen, impeding flow (Figure 2). As the ice goes out, jams can temporarily occur and dam or back up the river, exacerbating local flooding problems.

Red River near Oslo, Minnesota, 3 April 2009, photo by David Willis

Figure 2. Red River near Oslo, Minnesota, 3 April 2009. Here the main river channel is still clogged with ice, while surrounding farmland is underwater. Photo by David Willis of http://www.cropnet.com/.

Together the topography and climate of the Red River watershed are a recipe for large-scale flooding, and the historical record shows that floods are a frequent occurrence on the river. Usually, hydrologists talk about rivers in terms of their flow, or discharge, which is the volume of water per second that passes a point. But, when talking about floods like those on the Red River, it’s not so much volume that matters as how high the water rises (“stage”). The National Weather Service is responsible for flood prediction in the US, and they define flood stage as “the stage at which overflow of the natural streambanks begins to cause damage in the reach in which the elevation is measured.” If the water level continues to rise, “moderate flooding” occurs when “some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.” Further increases in water levels can bring a river to “major flooding“, when “extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.” That’s the sort of flooding that will happen in places along the Red River this spring, as it has many springs in the historical record (Figure 3).

Annual peak stage on the Red River at Grand Forks, North Dakota

Figure 3. Annual peak stage on the Red River at Grand Forks, North Dakota. Data replotted from the USGS, with local NWS flood stages shown.

Already, flood warnings are being issued for the Red River and its tributaries. As I’ll discuss in my next post, the long-range forecast for this spring’s floods on the Red is looking pretty grim. But as the communities along the river brace for the on-coming flood, it is important to remember that the geology and climate of the region make repeated major floods inevitable.

Conference presentation: Effects of river management & sediment supply on island evolution in Pool 6 of the Upper Mississippi River, southeast Minnesota

Watershed Hydrogeology Lab graduate student Brock Freyer has spent the last two years learning deeply about the hydrology, geomorphology, and sedimentology of the Upper Mississippi River System, as well as learning to use some sophisticated GIS techniques for 3-D analysis of topographic data. This week he is presenting the results of his work: “Effects of river management & sediment supply on island evolution in Pool 6 of the Upper Mississippi River, southeast Minnesota” at the Upper Midwest Stream Restoration Symposium. Brock is speaking in a session on Large River Restoration. Brock will be defending his M.S. thesis sometime in late spring.

Inspiration in ancient rocks and simple physics

If you ask my mom how I got started in geology, she’d tell you that it began with her taking 3-year-old me to see landslides coming off steep hillslopes during the spring thaw. That makes a nice story, but its not the real reason I got sucked into geology. Truth be told, I picked geology because it was the field of science my parents knew nothing about.

In my hometown public school system, the smart kids were herded towards doing in-depth middle school science fair projects. There was a wonderful teacher who helped us find projects and mentors, and taught us the art of visual displays and public teaching. As the child of two scientists, I was a natural fit for the program. There was only one problem: I didn’t want to do anything with which my parents could help. That was my mild form of early teenage rebellion. With my parents’ expertise in biology, chemistry and computer science, I felt I only had one choice: physics. But physics had too much math for my taste. (Little did I know just how mathy geology can be.)

Then a family friend suggested a geology project, I took it and ran with it, and the rest is history. My family friend was a resident of the Bayfield Peninsula, which juts up into Lake Superior from northern Wisconsin. Our friend was a sailor and nature enthusiast, and he pointed out that all of the rock cliffs along the lakeshore had right-angle fractures. He wanted to know why.

Figure 1. Shoreline at Big Bay State Park, Madeline Island, Wisconsin.

Figure 1. Shoreline at Big Bay State Park, Madeline Island, Wisconsin. Photo by Anne Jefferson, July 2007.

That question was the inspiration for my first real science fair project was “Fracture characteristics and geologic history of the Chequamegon Sandstone (Bayfield Group, Late Precambrian).” I collected dozens of stones from the rocky beaches of Madeline Island, where the Chequamegon Sandstone is exposed. I measured the angles between all sides of the stones, and tried to correlate them with grain size, induration and other characteristics. I made my first and last thin sections and I sieved samples using the same sort of Ro-Tap machine I now teach students to use. I also learned about things like properties of non-crystalline materials, the North American Mid-continental rift sytem, paleocurrents, and Pleistocene glaciations.

Figure 2. More shoreline made of Chequamegon Sandstone in Big Bay State Park, Madeline Island, Wisconsin. Glacial scour marks are visible on some of the rock surfaces.

Figure 2. More shoreline made of Chequamegon Sandstone in Big Bay State Park, Madeline Island, Wisconsin. Glacial scour marks are visible on some of the rock surfaces. Photo by Anne Jefferson, July 2007.

I don’t think my conclusions were particularly startling to people who knew anything about rocks. The rocks generally broke along their bed planes, and then at 90 degrees from their bedding, with more than 50% of the rocks exhibiting fractures between 80 and 100 degrees from bedding. Secondary modes were 60 and 120 degrees from bedding. More tightly indurated rocks had a higher propensity to have obtuse fracture angles.

Figure 3. The young scientist at work.

Figure 3. The young scientist at work. Photo by Carol Jefferson, August 1991.

That first project led to a second project, a year later: “Strength, porosity and fractures in the Chequamegon, Mount Simon, and Eau Claire Formations,” in which I contrasted the materials properties of two building stones and an aquifer. Then the Mississippi River floods of 1993 pretty permanently steered my interest from ancient rocks and materials properties towards the more dynamic modern landscape. I’ve never again worked on rocks within an order of magnitude as old as my first rocks, and these days I’m more apt to think about the water flowing over and through rocks than the rocks themselves. But sometimes I’m in the field, and my eyes will be drawn to an outcrop, boulder, or piece of float. And I still find myself silently inspired by the amount of geologic history that rock has experienced to end up in the stream bed, hillslope or lakeshore obeying simple laws of physics.

Figure 4. Perpendicular joints in the Chequamegon Sandstone at Big Bay State Park, Madeline Island, Wisconsin.

Figure 4. The adult scientist still inspired by those perpendicular joints in the Chequamegon Sandstone at Big Bay State Park, Madeline Island, Wisconsin. Photo by James Jefferson Jarvis, July 2007.

Why you can get '500 year floods' two years in a row

Cross-posted at Highly Allochthonous. Any further discussion will be found there.

For the past week, the flooding in the Upper Midwest has been all over the news, as rivers have reached record levels and thousands of people have been evacuated across several states. A couple of science bloggers have been personally affected, and we hope that they, their families, and their labs continue to be safe and dry.

Floods are a personal fascination for me, as I can trace my interest in hydrology directly to the 1993 Mississippi River floods that affected my hometown in Minnesota. However, flood recurrence intervals are also one of my professional pet peeves. I make sure that students in my classes never walk away with the misconception that a 500-year flood can only happen once every 500 years. If you finish reading this post, you’ll be disabused of the notion as well.

The most important point is that a “X-year flood” is a poorly-chosen way of expressing the probability of a flood of a given magnitude happening in a given year. A 500 year flood, has a probability of 1/500, or 0.2% of happening in any given year. Just like when you flip a coin the probability of getting heads is always 50% on the next flip, even if you happen to get heads three times in a row. In the same way, if a river has a 500-year flood in 2008, there is the same probability of having such a big event in 2009. That’s bad news for those flood victims with a poor understanding of probability. Fortunately, a quick scan of this round of media coverage has revealed very few reporters getting it wrong (and some news outlets even taking time to get it right).

Flood probabilities are based on historical records of stream discharge. Let’s use the Iowa River at Marengo, Iowa as an example. It reached a record discharge of 46,600 cubic feet per second* (1320 m3/s) on 12 June. That flow was estimated to have a 500 year recurrence interval, based on 51 years of peak flow records. Here’s a graph of the peak flow record for the site:

Typically flood probabilities are based on a time series of the highest instantaneous discharge measured during a given water year (1 October to 30 September). These data are then fitted to a statistical distribution, often this one. These distributions then allow the estimation of the probability (or recurrence interval) of a flood of a given magnitude. Taking the peak flow time series from the USGS website and using the distribution above, I also get a ~500 year recurrence interval (0.2% probability) for the flood of 2008. But there’s a big problem here…I’m estimating a 500-year flood based on only 51-years of record. So I’m going beyond my data by a factor of 10!

When you are extrapolating beyond your data by an order of magnitude, the highest points in the dataset start to have a lot of leverage. Let’s imagine that there’s another big flood on the Iowa River next year and we do the same analysis. Now our dataset has 52 points, with the highest being the flood of 2008. When that point is included in the analysis, a discharge of 46,600 cubic feet per second* (1320 m3/s) has a recurrence interval of <150 years (>0.6%). It’s still a darn big flow, but it doesn’t sound quite so biblical anymore.

OK, so our predictions of the probability (recurrence interval) of big floods can be really wrong just because of the limited nature of historical data (the situation is better in some other parts of the world). But there are a number of other possible confounding factors. First, erosion or deposition of the channel and surrounding regions over time can change the height of the flood relative to the volume of the flood. And flood height is what those people manning the sandbags really care about. Second, changes in the watershed can affect how much and how quickly precipitation makes it to the river. Urbanization and the adding of impervious surface is one cause of increasing flood peaks, but in Iowa, a more likely culprit is agricultural. Between the 1780s and 1980s, more than 95% of Iowa’s wetlands had been drained. (Most of this drainage occurred prior to the 1930s in Iowa, so it is unlikely to affect the example above.) Conversely, flood control dams (like Coralville Dam on the Iowa River) can suppress flood peaks downstream. Another potential culprit is climate change, though it is nearly impossible to attribute the occurrence or magnitude of any one event to changing climate.

All right, with all of that under your belt, the next time you hear someone say something like “They said 1993 was a 500-year flood. How can we be having another one only 15 years later?” you can patiently explain to him that recurrence intervals are only shorthand for probabilities. The hydrology professors of the world will thank you.

*Still the standard units for reporting discharge in the U.S.