Bill Trush on Mono Basin stream restoration
An Interview
Editor’s note: In 1994 the State Water Resources Control Board ordered the Los Angeles Department of Water and Power to restore the four Mono Basin streams desiccated by fifty years of water diversions. A restoration plan was developed by a team of scientists, and Dr. Bill Trush is one of the scientists in charge of implementation in the coming years. Dr. Trush, who is a co-founder of McBain and Trush and director of the Institute for River Ecosystems at Humboldt State University, has spent time here in the Mono Basin researching the streams and will oversee their restoration for decades to come. We managed to interrupt his busy schedule long enough to ask him a few questions about his background and the future of Mono Basin streams.
Can you tell us a little about your background with stream restoration?
What’s most germane to the Mono Basin is the work we’ve done on the Tuolumne and the Trinity River, particularly the Tuolumne, where we’ve been designing flood plains. As a company [McBain and Trush], what we mostly do is analyze the downstream effects of dams. We do stream restoration, in that we try to get the floods and the natural flow regime to do the work for us. I’ve helped reconstruct flood plains and pull back levees on bigger rivers. What we really try to do is get a wide enough river corridor to let the river migrate and high enough flows to surpass geomorphic thresholds, such as mobilizing the channel bed surface.
Could you talk a little more about your philosophy of restoration? Is it from your professional work that you’ve reached these conclusions, through your education, or from some other source?
It’s hard to put a label on it. When I first got into Berkeley for a Ph.D., I was into theoretical population dynamics of invertebrates in streams. It was when I took a course from Luna Leopold that I realized I had no idea of how the ecosystem operated because I was just delving on the biological side. And so that was a real pivot point for me, when I started to look at the physical processes that would be involved in biological communities. Working under Luna gave me my start.
Why is restoration needed in the Mono Basin? Couldn’t you just leave things alone?
Sure, you could. If you leave it alone, you will have a stream ecosystem, and you can even throw the word "dynamic" in front of it. But it won’t be the kind of system that was there before, and that’s the crux of the issue. We’re going to try our best to bring the former system back.
What do you see as your role in Mono Basin stream restoration?
I see my major role as helping make the idea of adaptive management work. What we’ve done to date is we’ve relied on some scientific recommendations to tell us what to do, but we know that these are just our best guesses. So we’re relying on science in the next eight years to get better numbers and to be sure these numbers get turned into policy.
What are the critical stream processes you hope to see restored on the streams?
Well, the most glaring one is the lack of confinement on lower Lee Vining Creek. That’s number one. Confinement keeps the flows deep for a given discharge, which then mobilizes the bed. Right now we just have shallow flow. The process that will probably bring confinement back the quickest is the riparian vegetation establishment close to the channel, which is happening at a fairly rapid rate. The riparian vegetation is going to create enough roughness in the stream channel to create a lot of deposition, which will start bringing back that confinement. Of course we’re worried about the whole corridor and whether the middle and upper terraces will come back—that’s somewhat of a different issue really. But I think that the main issue is to see the confinement come back to Lee Vining Creek. And it’s going to take a while.
What would be one or two other things you hope to see?
The rewatering of more of the flood plain. To me, the lower Rush flood plain is mostly an issue of bringing back the higher elevational riparian vegetation. We’ve got pretty decent channel morphology in lower Rush. There’s a lot of nice pools forming down there. But when you walk by all those cottonwood stumps on the middle terraces you know something is still off.
Maybe a third is: when these channels all do start to reach some kind of equilibrium, whatever that word means, will we be able to maintain a multi-thread channel like what we’re seeing the creek do right now on lower Lee Vining? The Restoration Technical Committee scientists originally leaned more toward the idea that there was one main channel with other, secondary channels. But the way this January’s flood looks, there might be more than one primary channel, as Scott Stine has maintained. So maintenance of single or multi-thread channels, confinement, and the rejuvenation of the upper floodplain and terraces—I hope to see those three processes.
In general, could you talk about what we might expect to see in ten, twenty, fifty years?
I’m looking forward to seeing what sort of confinement will begin when we get the cottonwoods up to about one foot diameter. When the forest gets that mature in lower Lee Vining, I think we’ll start seeing an acceleration of the confinement.
That would be about . . . 20 years from now?
Yes, maybe fifteen years from now. We were asked at this year’s Water Board hearings if there are any geomorphic and ecological thresholds, and I think that accelerated confinement with riparian community maturation will be one of them. I don’t see any major change suddenly in lower Rush Creek. I see more flood plain expansion and rewatering. I see greater meandering occurring—it’ll be fast for a while and then start to slow down. We’ll see more point bar features.
When you say "fast" what do you mean?
I think the meander rate is pretty rapid now as far as cutting into certain areas. But as those outer banks erode, they also collapse and armor themselves. With a lot more flow now there’s going to be lush vegetation, so it will probably slow that process down. Historically it looked like the migration rate was rather slow—historically we saw deep, incised channels.
You’ve talked a little bit about the distinction between Rush and Lee Vining creeks. Can you say more?
I guess for Rush Creek I see a channel not terribly out of whack, with sediment bypass being looked into at the Parker and Walker diversion dams, so whatever little sediment supply Rush is getting will reach the main stem. I see it well on its way. Lee Vining Creek is still a basket case, and I just think it’s going to take a long time.
What underlies the problem on Lee Vining Creek?
My guess is that it’s steeper. And when the fires came through and wiped out a lot of the riparian vegetation, in addition to the desiccation, this was a second whammy. Then when the floods hit [in the 1960s and 1980s], they just took out the terraces. When I modeled flow in the valley-wide cross sections, I predicted 5,000 cfs was needed to inundate the middle terraces, which is way too much flow. What that’s really saying is there was a lot more terrace historically. So it’s going to take a long time to increase the roughness and promote deposition. Most of that roughness is going to come from riparian vegetation.
We haven’t mentioned Walker and Parker creeks. What would you say is their contribution; why would we even care about restoration on Walker and Parker?
I think we’re interested in them because they have a right, just like any other stream, to be restored. But they are also connected via sediment. In a creek that has most of its sediment being trapped in lakes, whatever sediment there is, is important—disproportionately so. We’re not getting a lot of channel migration and so we’re not getting sediment being recycled from terraces and flood plain and being dropped back into the channel. So Walker and Parker provide important sediment to Rush. We felt there wasn’t a need to spend a lot of money trying to quantify all the sediment contributions, but we realized that the tributaries are important sediment sources. Which is why sediment bypass on these creeks is being remedied.
What about peak flow?
Yes, Walker and Parker do contribute to peak flow. Collectively, on the bigger years they might get us over the threshold more years than we would otherwise. Collectively, they can kick in 100 or more cfs. The only problem is the timing of it. If we’re going to release floods on Rush Creek, they’re always released a little later than would occur naturally. So we may not get the full benefit of those flows for channel maintenance.
Let’s say it’s the year 2040. What do you think will be the biggest difference between the streams then and the streams as they are now?
You won’t be able to see them. I’m not saying the confinement will be back to pre-disturbance level, that’s going to take a long time. But I think we’ll be able to see a measurable confinement occurring. You’re going to have a dense canopy. It’s already happening on Rush and Lee Vining creeks.
What does monitoring really mean and what is its importance for restoration?
It’s going to shape future policy. It’s going to shape how we manage the creeks in the future. That’s the big difference from most monitoring programs—here monitoring and adaptive management has been provided in the language of the restoration agreement. We targeted stream processes as goals. For example, the channel bed should periodically be scoured, so our monitoring will assess if scour is being provided by the specified flow releases.
Disagreement has characterized both the Mono Lake struggle and the development of the restoration plans; as we get into adaptive management this will continue—would you have any comment on that?
I think adaptive management is a good way of reducing the stress caused by uncertainty. Everyone will know what we’re doing. We may not get results fast enough for some folks—I’m sure that will happen. But I see it working. Without adaptive management and monitoring, potential confrontation over flow recommendations will only be delayed to a future time.
Dr. Trush will be a panelist at the upcoming Fall Forum on stream restoration in Lee Vining.
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