Our First Dam Podcast: Dam Removal
We get educated on the whats and whys of dam removal by geomorphologists Jim O'Connor and Jon Major. BONUS: Watch a very cool time lapse video of Oregon's Marmot Dam being breached--click ‘Show Details’ below and scroll to the bottom!
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Hello and welcome. You're listening to the USGS CoreCast, episode 24 for December 2007. I'm Steve Sobieszczyk.
Well, December is almost over and so is the year 2007. Before the year ends, we wanted to give our loyal listeners at least one more taste of the USGS CoreCast. And rather than do one of the year-end recap type of shows, we decided to stick to our normal schedule and give you fresh new CoreCastic content. Today's show will focus on the impact of dam removal on the downstream ecology and fluvial geomorphology of rivers.
Now there are two things I'd like to make clear before we begin. First, the topic of dams and dam removal is somewhat controversial. To keep this episode light-hearted, yet informative, we're going to strictly focus on one dam in particular. And that is the Marmot Dam out here in Oregon.
The second thing I wanted to mention is we are fully aware that "dam" is a double entendre. Believe it or not, we are professionals here, so we'll do our best to keep this dam podcast professional [laugh].
So, let me give you a little background on this Marmot Dam we're talking about. The Marmot Dam was located along the Sandy River, which is a major tributary to the Columbia River, east of Portland, Oregon.
The dam stood originally at 47 feet high. It was constructed between 1906 to 1913 and produced about 22 megawatts of power. The actual decommission and destruction cost about $17 million. The dam was removed primarily because it wasn't economically feasible any more, it was an old dam, new restrictions, retrofitting, all that . . . it really wasn't cost effective to keep it.
Also, by removing it they were able to restore an old salmon run up the Sandy River, as well as offer new recreation possibilities for kayakers, canoers, and fisherman. The good news is, now for the first time in almost 100 years, the Sandy River runs of salmon can now go unimpeded from the base of Mount Hood, through the Sandy, through the Columbia to the Pacific Ocean.
That's probably enough introduction, so without further adieu, I would like to introduce today's guests. I'm joined by two very well-respected geomorphologists: Jim O'Connor and Jon Major.
Jim has been studying fluvial geomorphology for about 20 years now, including research on flooding and river systems. Jon is a geomorphologist, as well, although his research is primarily looking at large-scale sediment transport associated with volcanic eruptions. I'd like to thank you both for being here.
Thank you Steve.
Well thanks, Steve, it's a pleasure to be here.
First off, Jim, would you like to describe a little bit about what the conditions were like, and what concerns were prior to the Marmot Dam coming out?
So, in the years leading up to the decision as to whether or not to take Marmot Dam out, there were several concerns. One of the major issues, of course, was the sediment that had accumulated behind Marmot Dam. The 50-foot dam was completely filled with sediment behind it, with perhaps a million cubic yards. And the concern was what would happen after the dam was taken out and that sediment started to work its way down the river system.
Can you walk us through the chronology of the actual dam removal process?
The first real discussions about removing this dam began in 1999, when Portland General Electric was faced with a decision about whether it was in their best interest to try and retrofit the dam to improve fish passage. In 2002 the final decision was made that the dam would come out. In the end, the dam was taken out in a couple different steps.
In July of this year, a cofferdam was built about 100 yards upstream from the concrete structure, and that allowed the river to be diverted around the original Marmot Dam so that they could excavate some of the sediment adjacent to the dam and then blow up and take out the concrete structure.
Now this diversion could only handle about 2,000 cubic feet per second of water going around the dam, so they recognized that at the first large flow of fall that their diversion would not fully function and that the earthen dam would breach.
Just to add to that a little bit, Steve . . . in anticipation of this, sort of, threshold discharge that PGE was looking for in order to kind of initiate the breach of this cofferdam, us at the USGS and colleagues down at universities and with other Federal agencies . . . we really tried to put together a plan of how to watch and how to take advantage of this breach for scientific purposes.
So, for example, we set up a network of cameras around the reservoir to try and capture the breach from different angles. We had people manning a station several hundred yards downstream of the dam, where they were going to measure the water discharge and the sediment discharge as it came by.
We had another team that was upstream of the dam to measure the water discharge and sediment discharge coming down the Sandy River above the dam to give us some insights on the background level that was coming in, so we could compare what was coming out with what was coming in.
We're really looking forward to assimilating all the results that we've been getting to date, but we put a lot of effort and planning into this, we're really trying to take advantage of this grand experiment that nature . . . nature and man have kind of set up and executed for us.
Now that we kind of have a feeling for the preparation for studying the dam removal, let's fast-forward to the actual breach. How long did it take for the river to breach the cofferdam and how did the trapped sediment respond?
The flow breached down through the dam very quickly: In half an hour or so the river incised through the dam. And the nick point that formed upstream moved upstream quite rapidly. And the volume of sediment that was evacuated from the reservoir, I think, exceeded everybody's expectations.
Downstream, all that sediment ended up raising the river bed. At our cableway site, which was just a couple of tenths of a mile downstream from the breach, the bed came up 14 or so feet over the course of the weekend. And the depositional wedge that resulted extended nearly half a mile downstream.
And what's really been interesting is going back and looking at the photography we have. The photography really does show that probably within 4 hours of the breach, certainly within 5 or 6, the dam was essentially gone. So the river rapidly eroded the dam away, and all that sediment that was behind the dam was also being very rapidly eroded.
At this point it started to deposit a lot of that sediment just immediately downstream of the dam, but it was also sweeping a lot of that sediment downstream. And the people we had at the gaging station, a few hundred yards downstream, measured quite a nice spike in the sediment load and the water discharge that came sweeping by that station.
And my last question—how long will it take for the sediment to make its way through the system, and for the river to actually return to a pre-dam equilibrium?
That's the $64,000 question, and it's exactly what makes this experiment so exciting. We really don't know. We're continuing to go out there and measure: We're measuring bed load and suspended-load transport and we're going back out there in the summer and resurvey the cross-sections. But that's the question that everybody is wondering about and its one that's certainly going to be relevant to future dam removals as they occur.
Jim sort of took the words out of my mouth. That really is the $64,000 question. It's the one that is foremost on everyone's mind. From my perspective, I can offer perhaps a little bit of insight based on some of the work we've done at Mount St. Helens looking at how those river systems up there have kind of flushed out and processed a lot of the sediment that has been deposited in the river valleys as a result of the large eruption in 1980.
Those channels had a lot more sediment deposited in them than what is stored behind this dam. But in a few of those channels it was surprising how rapidly they actually move sediment out of the system. They moved tremendous amounts of sediment out of their systems within 3 to 4 years.
But there are still channels in the upstream areas where there was a lot of sediment deposited—where the amount of sediment being transported down those systems is still anywhere from 10 to 100 times above what we would consider typical background levels.
So, my guess is for a system like this, I wouldn't be surprised if we still see lingering effects of this sediment that was stored behind the dam a couple years down the road.
Okay, thank you guys for joining me. That's all the questions I have for today.
Oh, you're welcome, Steve.
As I mentioned before, if you'd like to learn more about the Marmot Dam removal, head over to the Marmot Dam Web site, the USGS Marmot Dam Web site, or the PGE Sandy River Watershed Web site, all of which are listed in our show notes.
If you're more interested in general discussion about what the politics, policies, research, and discussion are involved concerning dams and their removal, the University of California has a nice Web site: It's a Clearinghouse for Dam Removal Information.
Lastly, if you think you have what it takes to decide the fate of a dam, check out the PBS Dam Challenge, where you decide the best solution to a dam problem.
Well, that's all the time we have for today's show. As usual, if you want to reach us, you can do so via e-mail: Our address is email@example.com. We hope you all have a have a safe and happy New Years, and we'll see you next year.
The CoreCast is a product of the U.S. Geological Survey, Department of the Interior. Until next time, I'm Steven Sobieszczyk, rock on! Owww!
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"Elec_guit_cleanfunk_riff6" by Soundgram Post
Mentioned in this segment and additional resources:
- USGS Marmot Dam Project
- Marmot Dam Website
- PGE Fact Sheet on Sandy Watershed
- USGS Oregon Water Science Center Studies
- USGS Cascade Volcano Observatory
- USGS Dams and Rivers Publication
- Clearinghouse for Dam Removal Information
- Association of State Dam Safety Officials
- American Rivers
- PBS: The Dam Challenge
BONUS: Time Lapse Video: Marmot Dam (Windows Media Video)
About the dam breach video and other info:
The USGS put together time-lapse video of the breach of the Marmot Dam, on the Sandy River in Oregon. Hydrology experts from the USGS Oregon Water Science Center and the USGS Cascades Volcano Observatory are studying this removal, the largest planned removal in the Pacific Northwest thus far.
Real-time USGS streamgages and other high-tech instruments are monitoring erosion, sediment transport, water quality, temperature changes, and turbidity of the water. This work is done in partnership with Portland General Electric (PGE), the owner of the dam, and other State and Federal agencies and university colleagues.
Removal of the Marmot Dam began in July 2007, and the final breach that unshackled the river occurred on October 19, 2007. As part of its decommissioning of the 100-year-old Bull Run Hydroelectric Project, PGE also plans to remove a smaller dam on the Little Sandy River in 2008.
The Sandy River and its tributary historically provided important habitat for fish and other aquatic species to migrate from salt water to fresh water and for the out-migration of juvenile fish, such as salmon, to the Pacific.
The reservoir behind Marmot Dam contained approximately 1,000,000 cubic yards of sediment in a wedge extending roughly 2 km upstream from the 47-foot concrete dam. The USGS will monitor the effects of the dam removal as the river returns to a normal flow and water quality.
The USGS's work, along with that of partnering agencies, will also provide valuable information for the removal of the Little Sandy Dam and other similar projects elsewhere in the future. More information on this project can be found at the Oregon Water Science Center's Web site.
Stephanie Hanna, 206-818-7411 or firstname.lastname@example.org