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Biological Drivers: Restoring the biological processes that maintain Stage 0 in Rocky Mountain headwaters streams

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The importance of biological drivers in stream restoration. Examples of process-based restoration on beaver-mediated streams in the Rocky Mountain headwaters.
Biological Drivers
Restoring the biological processes that maintain
Stage 0 in Rocky Mountain headwaters streams
Riparian Reconnect
Jessica Doran & Mark Beardsley
Biotic drivers are living—or once living—parts of the system that influence the form and
function of a stream. In many systems, plants and animals interact with abiotic factors—
geology, hydrology, climate—as fundamental drivers in developing stream characteristics.
In systems where biological process are active, we see self-sustaining, complex stream systems
that have high ecological value.
Biology is important!
At the risk of giving away the punch line too early, if you take one thing away from today’s
presentation, I hope it is that biology is important. And while we have all always had some
appreciation for this, recent research, observations, and practice are expanding our
appreciation of just how important biology can be.
In today’s presentation, I will talk about how biotic drivers influence stream form and function
and about the importance of restoring biology, and not just hydrology and geomorphology to a
stream system.
Geology and Hydrology
An important shift is happening in stream assessment and restoration. In the past, biology has
been viewed as what will result if abiotic factors in streams are ‘right’. The idea is that the
driving factors of geology and hydrology are responsible for the form and function of the system
and that biology is simply supported by that. Biology is the ‘cherry on top’.
This is a very physics-centric approach which is appealing because it suggests we can reduce to
a naturally complex situation to sets of equations. But ignoring biology or treating it as a
secondary factor reduces our ability to understand the system.
Geology and Hydrology
Stream Evolution Triangle (Castro and
Thorne 2019)
(with stream types from Rosgen, 1994)
Janine Castro and Colin Thorne are among many scientists integrating the role of biology into
our existing understanding of streams. They developed the Stream Evolution Triangle which
accounts for the influences of geology, hydrology, and biology as drivers of fluvial forms and
processes. In this model, biology is included on an equal basis with geology and hydrology.
In Janine’s words “The SET reframes physics-based fluvial geomorphology to acknowledge and
explicitly account for the power of biology as a process driver.
Stream Evolution Triangle (Castro and Thorne 2019 in review)
(with stream types from Rosgen, 1994)
I imagine most of us are familiar with the soil texture triangle, which was the inspiration for the
SET. This model helps us consider the proportional influence of each of these high-level drivers.
These drivers are interactive and different types of streams may be dominated by any one driver
or a mix of any combination of the three.
Along the sides of the triangle are stream power, erosion resistance and biotic interaction.
The SET can be using with existing stream classification systems and helps us build our
understanding of why systems function the way they do and extrapolate possible past and
future conditions. Here we see Rosgen’s stream types overlaid in the SEM.
Stream Evolution Triangle (Castro and Thorne 2019)
(with stream types from Cluer and Thorne 2014)
The same can be done with Cluer and Thorne’s stream types from their Stream Evolution Model
Fully Alluvial Partially Alluvial Colluvial Bedrock
Stream Evolution Triangle (Castro and Thorne 2019)
(with stream types from Schumm 1985)
And here are Schumm’s stream types. So, whichever classification system is preferred, we can
see the relative influence of each driving force and the resultant stream type.
Regardless of the classification system, we are seeing a pattern. Biology is critical in supporting
the multithread or stage 0 or anastomosing geomorphic state.
While this model still has biology at the top—like the sundae—it is now explicitly considered as
a driver of fluvial processes and not just product of abiotic factors.
Beaver dams
My friends and I work in the Rocky Mountains and these are the types of biotic drivers that are
present in our area. In forested areas, the input of large wood influences stream form. In the
open parks, flow through slope and fen wetlands is interrupted by hydrophytic wetland
vegetation. In unconfined valleys, the most common biotic driver in the rocky mountain
headwaters are (or were) beavers.
This is where the majority of my experience is, and for the rest of the presentation I will focus
on beaver streams as an example of biologically controlled fluvial systems.
We are lucky to have sections of stream that still have active beaver zones. Being able to study
and observe streams in real time has played an invaluable role in my understanding and
appreciation for these systems.
There is a body of evidence showing that these types of streams dominated the landscape pre
Let’s look again at the SET with beaver systems in mind
Fully Alluvial Partially Alluvial Colluvial Bedrock
Stream Evolution Triangle (Castro and Thorne 2019 in review)
(with stream types from Schumm 1985)
So here we have the SET with examples of two types of streams. One with intact biology, one
without. Now let’s look closer at how the plants and animals are acting on the system to change
to form.
Biotic drivers
OK, even though I proudly identify myself as an ecologist, I still have moments of physics envy.
And this is one of those moments. Consider the stream on the left with the native biology
intact. The energy input from the active beavers and intact riparian vegetation keep the system
in a stable energy state.
Now, consider the stream without its native biology. The willows have been cleared, beavers
are gone, and the riparian area has been transformed to a hay meadow. Without the energetic
input from biotic drivers, this stream has adapted and settled into an alternate stable state.
Biotic drivers
in the level
of biotic
In the absence of biotic drivers, streams move from a complex state to a more simplified state.
The role of biotic interaction decreases, and the roles of geology and hydrology are increased.
This shift from complex to simple channels is ecologically important in several ways.
The ecological benefits in complex systems can largely be directly related to the level of
connectivity between the stream and riparian area. If these are highly connected, it is hard to
tell where one ends and the other begins.
Habitat and
ecosystem benefits
Polvi and Wohl 2013
Beyond the ecological benefits provided, these systems are persistent and resilient. This graphic
is from Polvi and Wohl’s 2013 article Biotic Drivers of Stream Planform: Implications for
Understanding the Past and Restoring the Future
By analyzing sediment deposits in one Colorado mountain valley, Lina Polvi and Ellen Wohl
discovered that Stage-0 beaver systems have likely been the dominant form for most of the
10,000 years since the ice age glaciers melted away. Beaver systems can be very persistent.
Only recently has that changed—after humans came and modified them.
These systems are dynamic and have evolved by responding to shifts in local and climatic
changes. The habitat and hydrologic benefits are present at a variety of stream flows and, as
long as the main components of the system remain intact, the stream can adapt to changing
The unknowns of climate change can be intimidating to plan for. But when we reduce the
trends to the very basics, we know patterns of high and low flows are shifting and that the
occurrence of extreme events are increasing. Streams with functioning native biology will be
better equipped to handle these changes. They are resilient and adaptive.
So, we can see that beaver systems were common stream types pre disturbance, and that there
are ecological and societal benefits to working to reestablish these systems. How do we get the
biological machine back up and running?
Scientists sure have a funny way of finding complex ways to explain things. Nevertheless, I still
like this diagram because it depicts the positive feedback loop maintaining the persistent,
complex, connected Stage-0 beaver meadow stream. That is the upper loop.
There is also a feedback loop when the abundance of beavers and dams drops below some
threshold. Decreased overbank flow occurrence results in drier floodplains, diminished
vegetation, increased erosion, consolidation of flood energy, and increased stream power. And
so it goes, on and on.
In these systems, Stage Zero restoration petty much boils down to beaver dams. But how do we
know where and how to intervene to start the biologic machine that will support them?
Let’s try starting with riparian trees… but we need the overbank flow and backwater to support
the vegetation. Maybe we start with building dams to restore the hydrologic connection?
Beaver stream assessment
1. Space
Appropriate setting?
Compatible land use?
2. Beavers
Currently present?
Suitable habitat? Any conflicts?
Connectivity to a metapopulation?
3. Vegetation
Sufficient woody vegetation?
Is vegetation regenerating?
4. Hydro-geo
Sufficient flow regime?
Incised or entrenched? What stage?
Has it been manipulated (channelized, levees, etc.)?
Beaver dams
Beavers Hydro-geo
One way to depict these parts is as a 3-legged stool. This helps keep the interconnected nature
of all these critical components at the forefront.
Depending on what is missing, you can design your treatments accordingly—being cognizant
that one treatment may need to be supported by another treatment until the process takes
over as a feedback loop.
Space - setting
Site selection is the first step. Low gradient, unconfined valleys are the places where beaver
systems occur. As depicted in the SET, biologic drivers dominant when stream power is low.
Beads on a string
Ellen Wohl recently described connected reaches of confined and unconfined valleys as beads
on a string. The beads—unconfined valleys—are depositional, support wide riparian areas with
high levels of hydrologic connectivity. Perfect for beavers. Referring to the relative levels of
ecosystem benefits provided presented by Cluer and Thorne, these stream reaches provide
excellent opportunity for ecological lift.
Stream and riparian
Historical footprint
Space - land use
The space requirement is important to systems influenced by biology. These systems need
room to move and adapt over time. And when we combine the highly dynamic nature of these
streams with the high hydrologic connectivity between land and water, the traditional
boundaries of what is channel and what is riparian start to blur.
This inspiring a shift in how we define the boundaries of streams is particularly interesting in
locations were signatures of the historical footprint of beaver systems are still present on the
Presence and connection to metapopulation
The ecological value of beavers is a relatively recent interest for the restoration community.
Historically they have been harvested for fur or managed out as a nuisance species. Now that
we are interested in supporting beaver populations, we are realizing it is not as easy as
This is an aerial of an active beaver site in 2011. The blue polygons are beaver ponds. There
was a population crash in 2015 and this is the resulting change in habitat.
It is a little hard to see those 2 small polygons, so here is a chart of the comparison
Population crash Population crash
The blue graph is beaver pond area in acres/valley mile and the green graph is beaver pond area
as a percent of riparian area.
As of 2018, the beavers still had not reoccupied the site even though the requisite space,
hydrology and vegetation are intact. Initially, this surprised us, but…
When you look at the location of the nearest neighboring population, it is almost 6 miles away.
Beaver population dynamics have been massively influenced by habitat fragmentation.
It is important to evaluate the possibility for natural recruitment of beavers and realize that
suitable habitat does not mean that beavers will colonize on their own.
Sufficient woody vegetation with regeneration?
Sufficient woody vegetation can be a tough factor to evaluate. I have not found any definitive
research on how much willow is needed to support stable beaver populations. Sometimes
when your site looks like this, it is pretty obviously lacking.
Vegetation treatments at this site included transplanting adult willows with machines. This was
successful and the survival rate was high. The treatment immediately and dramatically changed
the landscape. 8 years later, in 2018 we noticed some water stressed and dead willows,
particularly affecting willows further from the stream.
This is an example of the tricky balance of restoring native vegetation into an altered hydrologic
condition. Without the characteristic overbank flows to support the willows, survival and
regeneration may be limited.
Riparian Reconnect
Sufficient flow regime and connectivity to floodplain?
Finally, we get to start treating the stream!
Hydrology and geology are grouped together because it is the relationship between these two
factors that is important. The flow regime and shape of the channel will be biggest influences
on overbank flow frequency and aquatic habitat for the beavers.
In this case, there was ample space in a low gradient, unconfined valley. There was an upstream
neighboring beaver population without any corridor impediments. The riparian area still had
varied age classes of willows although there was evidence of drought stress and poor
recruitment. This was the prime site for building in-channel structures to mimic the functions of
beaver dams.
Riparian Reconnect
Riparian Reconnect
This is the upper-most structure built—closest in proximity to the neighboring beaver
population. Within weeks of construction, beavers had started adding material to the structure.
We did a high-water visit a couple weeks ago and can barely recognize the location of the initial
Where should my cross section go?
Long pro?
Green line vegetation?
Should I measure bank erosion?
Developing tools to quantify complexity
Deep and shallow water table levels
Aquatic and terrestrial habitats
Exciting time for monitoring geeks—traditional approaches don’t work. This could be a talk of
its own but would be great to touch on the evolution of monitoring approach and importance of
being creative.
As with any monitoring, the parameters you measure should reflect the goals of the project.
Since the goal posts have been moved, be prepared to develop new methods to capture the
important parts of your project.
Biology is important!
While redefining the role of biology in streams is new, the general appreciation for biology in
fluvial valleys is not. Revegetation plans are an established part of most restoration plans. As
we have heard today from John and Jon, we have good techniques for planting. The desire to
improve conditions for wildlife is often a stated goal of projects.
What is new is the scale we are aiming for. The goal posts have changed and now we want
biology that can influence stream form and function.
Over the last 10 years experts in academia and practice are developing methods for recognizing
and applying the power of biology. Expanding the concept of biotic drivers is a big change with
big payoffs.
There are many ways to accomplish this and I bet we will see new creative approaches
developing. We just need to keep in mind that biology is important and keep working toward
recognizing and restoring biologic drivers.
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