Using 70 years of data, researchers led by an Oregon State University professor found that landslides in the Pacific Northwest are influenced more by clear-cutting and roadwork than by heavy rain. Catalina Segura is an associate professor in OSU’s department of forest engineering, resources and management. She joins us with the details.
The following transcript was created by a computer and edited by a volunteer:
Dave Miller: A new study found that clear cutting and road building on forest land can have a bigger impact on the frequency and severity of landslides than heavy rain. The study was based on 70 years of data from an experimental forest in Lane County and it was led by Catalina Segura. She is an associate professor in OSU’s department of forest engineering resources and management, and she joins us with more. Welcome to the show.
Catalina Segura: Thank you for having me.
Miller: What prompted this study?
Segura: I think the fact that we had all this long term data available and we were in a situation where we could do a retrospective analysis that really took into account 70 years of data. So I think the study also allowed us to summarize data that had not been published, and collected over the years. And it was just a nice opportunity to get a longer view about the issues around mass wasting and forest.
Miller: You were able to look at the effects of logging and road building on forest land because of this data you’re talking about; this particular history in the Lookout Creek watershed. Can you give us a sense for the distinct periods there that have taken place since 1950, in the last 70 years.
Segura: Yes, absolutely. We actually divided the study in five different periods and these periods were related both to land management but also to the occurrence of large storms. And so the first period is the initial forestry period, like between 1950 and the beginning of the 60s, where a lot of the forestry operations took place. And the second period corresponds to the time where we had some of the largest storms and flooding in the region, in 1964-1965.
Then we had an inner period of around 30 years, where forestry continued a decreasing trend of intensity, and until the next period that we defined, in 1996, where we actually had the flood of record in a region that was the biggest storm we’ve had.
And finally, the last period is a post-ODOD period we call it, 25 years since 1996, where our forestry operations have basically ceased in this watershed.
Miller: So what did you find in terms of the connections between clear cuts or road building on the one hand, and the frequency or severity of landslides on the other?
Segura: What we found is that in the initial period, where we had intense forest practices going on - and I want to emphasize this was a historic practice, the way things used to be done before we changed our rules - we explore the landscape experience a large number of debris flows and landslides that occur particularly in the areas that were associated with the construction of roads and the clear cutting of the forest. What is interesting also is that during that period they were not really very large storms. Yet, we had very, very high production of sediment.
Miller: How do you explain that? What is it about roads or clear cuts that, even in the absence of really big storms, led to big debris flows or landslides?
Segura: So I think what happens is there’s a couple of things: when a clear cut occurs, especially back then when after a clear cut you also slash burn whatever was left, the terrain is left very vulnerable to mass wasting and all the water that used to be allocated to above the transpiration by the trees is not available to be run off. And so naturally the amount of water available in the rivers increases. But also what is important here, as well, is that a lot of these forestry operations took place in areas that were very steep and very friable, meaning they’re very susceptible to movement. So the entry to this watershed started from the bottom up and in the bottom, the rock that we have is a lot more susceptible to movement.
So there was a combination of factors, the fact that the practice [we used at] the time used to cut everything and burn everything, but also that the cutting occurred in places that were very vulnerable. At the same time, the roads were built with less kind of management practices that would prevent a lot of the delivery of water towards the streams without any control. And so I think it’s a combination of things: the steep terrain, the intensity of the forestry and the lack of management practices around road building.
Miller: You’ve been pretty clear that what you’re talking about here, in terms of the logging activities, which now happened a long time ago, that you’re calling them historic. How much have these logging activities changed with respect to the kinds of effects on debris flow or landslides? I mean, how much of the way logging is conducted now could still lead to these same negative consequences?
Segura: That’s a very good question, and unfortunately, I don’t have a straight answer for you because I think it really depends. We’ll be able to answer that question more clearly once we’ve had another large flow event, and once we’ve investigated this kind of processes in industrial land, in private land. This study was done in federal land where all logging stopped basically by 1990. And so even though we had a relatively large event in 2011, there was no freezing clear cut then.
I think our scientists and managers [need to] prepare for when the next big storm occurs to really make detailed inventories, both in federal and private land, considering also the intrinsic vulnerability of the landscape to really understand what we’re doing. It’s like the exam that our management is going to take. But a big flood is really what it’s gonna tell us whether or not what we’re doing is making a difference in how much of a difference it is making. And so that’s where the value of long term data is very important, right? Because we have to continue to make observations and be ready when the next flood event occurs, which we don’t know when it will be, to really be able to contextualize the new management techniques with the historic practices.
Miller: We’ve been talking here about logging or road building connected to logging. But how do forest fires figure into this?
Segura: That’s another very good question. And I think one that is very relevant to the time we live right now with the increased size of the fires we’re experiencing in the region. I think it depends on where you are. And what happens after the fire is that you lose the vegetation cover just as you would do with a clear cut and the vulnerability of the landscape to produce a lot of sediment depends on how long it takes for the root system of the trees that are not there anymore to decompose and and stop providing support to the ground. And so that varies with fire intensity and that varies with the size of the root system. But we can probably expect that a couple of years after these fires that we’ve been having, we might see an increase in the production of sediment if we get last flow events.
If we think about the recent 2020 fires and the recent fires this last year, the landscape is gonna be very vulnerable to mass movement, probably a couple of years after the onset of the fire, when those roots of the trees are no longer there to provide any stability of the soil. And again, if we couple that situation with a big storm, then it would not be surprising if we see some production of sediment that is kind of higher than what we used to have because fires are getting larger.
Miller: Catalina Segura. Thanks very much.
Segura: Thank you for having me.
Miller: Catalina Segura is associate professor in the department of forest engineering and management at Oregon State University.
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