Tip:
Highlight text to annotate it
X
Okay, what I'd like to talk about today is some of the sources of physical complexity
in rivers and how their formed and why their presence is important, and what happens if
you lose that complexity. This first lecture is The Importance of Physical Complexity in
River Ecosystems. Basic objectives that I hope you'll get out
of this lecture are: to understand rivers as ecosystems rather than as simple conduits
for water and sediments moving downstream, and to understand what creates physical complexity
in rivers, why does it matter if they're different of similar to simple conduits, and understand
some of the implications of that physical complexity.
A little bit of background. Why are rivers important? Why do people care about rivers
and spend a lot of time and energy trying to manage rivers? First of all, they provide
a lot of what are known as ecosystem services. An example for rivers are clean drinking water,
fisheries whether recreational or commercial, flood control, and habitats for a wide variety
of plants and animals that live both on rivers and in valley bottoms. The ecosystem services
usually refers to things that we as humans depend on but may not be that aware of that
are provided by naturally functioning ecosystems. Rivers are also very important in terms of
biodiversity. There are species of plants and animals that live in the channel that
I'll refer to in this lecture as aquatic organisms, then there are those that live in the flood
plain adjacent to the channel that I'll call riparian. There are lots of studies that show
in a wide variety of environments from mountains to desert areas to the tropics, that there's
a much higher diversity of plants and animals in rivers and in the valley bottoms adjacent
to them than there are in most of the uplands. Rivers are also very important in terms of
nutrient dynamics. The primary nutrients that get a lot of attention are carbon and nitrogen.
The nutrients in this context, this refers to material that is vital to most living organisms.
Pretty much all plants and animals need some form of carbon and nitrogen. The way rivers
function and how they transport or store carbon and nitrogen is very important control on
the availability of those nutrients both locally and on the global scale. We'll talk a little
more later in this section. I'm going to focus particularly on headwater
rivers. You can ask the question, why are headwater rivers important? Headwater rivers
in this context are the small channels that don't have many or any channels tributary
to them. Although they're very small, some of these are things you can easily jump across,
when you start looking at the total length of rivers in a drainage network, whether that's
a very small network that's local or something like the Amazon, the world's biggest river,
something like 70-80% of the total length of channels in a network is composed of those
headwater rivers. They're a very important component of any drainage network. They're
also very closely linked to the adjacent uplands or terrestrial environment. Anything that
comes in to the river network, water sediment, nutrients, a variety of contaminants, much
of that comes in on these little first order rivers or the headwaters. Those smaller rivers
also are very responsive to changes. If the amount of water or sediment coming in is changed
for some reason, whether it's associated with human activities or some natural change, the
small headwater rivers are likely to respond to that. As you go downstream in a river network,
typically the valley bottom gets wider, you get more well-developed floodplains and those
are buffers. When you have a large amount of water coming through, say during a flood,
some of that water is likely to go out of the channel and across the adjacent valley
bottom. It moves more slowly through there. The flood peak, the highest point in the discharge
is attenuated or stretched out over time. Similarly, if you add sediment to a river,
some of that sediment is likely to go out of the channel banks or overbank and be deposited
on the floodplain. But these very small headwaters don't have much in the way of floodplains.
They're typically in steeper, narrower valleys. So, they lack some of that buffering of floodplains.
Because of that, they're much more responsive to change. Headwater rivers are also important
in increasing the biodiversity of a river network. The characteristic I was talking
about a moment ago, the diversity of species that live in the network. Headwater rivers
can have very different physical environments than big rivers. They can have areas with
shallow waters or warmer water, for example. That's important for both the younger stage
of certain organisms like fish, very small bodied fish can take refuge in the shallow
waters and predators can't get to them. There are a variety of plants and other insects
and aquatic and riparian species that only live in headwaters. They really increase the
diversity of the river network as a whole. Headwater rivers are also really important
because despite the fact that they perform all these functions, they don't really have
the legal protection that we have for larger rivers. In the United States, a lot of the
legal protection for rivers is governed by whether or not they're navigable. Very small
rivers, particularly ones you can jump over, are not navigable. They don't necessarily
have the same level of protection for clean water, for example, or for limiting the alteration
of the channel form. That's something that some people are trying to change but for now
some of those headwater rivers are kind of vulnerable and exposed, legally.
The other thing I want to provide by way of background is this idea of rivers as ecosystems.
The simplest view, if you're looking at the physical function of rivers is just a conduit
that water and sediment move downstream. But if you start thinking of rivers as ecosystems
that support plants and animals, they become more than physical conduits. One way to think
of this is what I call the six degrees of connection. If you look at any river as a
whole or any segment of river, it's actually very closely connected to the greater environment.
This slide sort of illustrates that. It's a picture of a river but if you start thinking
about what moves into and along that river, you can have not only downstream movement
of water, sediment, and organisms, you can also have upstream movement by organisms.
Think of some fish, salmon for example coming upstream to spawn. You can have a lateral
connection between the floodplain and the channel during high flows, water, sediment,
nutrients, organisms, are moving out of the channel into the adjacent valley bottom. As
the flood peak recedes, they're moving back into the channel. There's a lot of exchange
that way. You can have exchanges between flow in the channel and the shallow surfaces known
as the hyporheic zone. That's mostly water sediment and material that's dissolved in
the water, but there's also some aquatic insects that move back and forth between the surface
flow and that shallow subsurface. You can have a variety of things coming from the adjacent
uplands up into the channel, both the surface and the subsurface and groundwater. That variety
can be water, sediment, solutes in the water, sometimes contaminates, organisms. There's
two levels of lateral connection. Then there's a vertical connection between the channel
and the atmosphere. The obvious version of that is rain or snow falling directly on the
channel, but you can also have dry deposition of wind-blown silt for example, or a variety
of chemical constituents like nitrates and mercury, you can have things volatizing from
the river and going back into the atmosphere, you can have insects that emerge into the
atmosphere. There's a vertical connection both into the subsurface with the hyporheic
zone and subsurface but also a vertical connection to the atmosphere. If you keep in mind these
six degrees of connection, you can't effectively treat any river symptom in isolation. It is
very much associated with the greater environment. Sometimes that greater environment is the
whole world. As an example, this photo that you're looking at is from the western side
of Rocky Mountain National Park. It's the headwaters of the Colorado River. There have
been some studies in recent years indicating that the snowpack in this catchment is melting
more rapidly because of airborne dust that settles on the snow, makes it darker, makes
it melt faster. Some of that dust comes from the states just to the west of us, Utah or
the Great Basin. Some of it's coming all the way from the deserts of central Asia, it's
going all the way across the Pacific Ocean. If you start thinking of it that way, then
these river ecosystems are connected to the world as a whole.
Another component of thinking of rivers as ecosystems is of course the plants and animals
that live in the system. If we focus on that word system for a moment, the river would
be the channel itself but also the adjacent floodplain environment. Those two basic components
of the river environment would be habitat for both aquatic and riparian organisms. Again,
the plants and animals that live in the channel and in the adjacent valley bottom. The physical
characteristic of the channel or floodplain would govern things like the abundance of
habitat for those aquatic and riparian organisms and the diversity of habitat. The physical
characteristics also influence the disturbance regime. Disturbance regime in this context
refers to natural process or human processes that really alter the habitat. For example
in a river, a flood, drought, when you have very low flow or very warm water temperatures,
something that changes the sediment coming into the catchments, could be a wildfire or
clearance such as timber harvest. The types and the frequency and magnitude of those disturbances
through time are what compose the disturbance regime that I've got listed here. All of those
things, the channel and floodplain environments, aquatic and riparian organisms, the physical
characteristics combine to govern aquatic and riparian communities and we define those
phrases as the abundance of organisms, how many particular fish or Cottonwood trees are
there. Then, the diversity of both species of plants and animals and the diversity of
individuals. If you have a particular species, are they all adult organisms or are there
juveniles at different stages of development? A lot of times, ecologists will assess the
health of the community based on the diversity of the age structure. So, if you're looking
at cottonwood trees along the river, if they're all mature trees that are getting close to
the end of their lifespan, that suggests that they're not going to be replaced if there's
no seedlings when they're germinating. Ideally, a healthy community has both a diversity of
species and a diversity of ages of individuals within each species. One of the key things
with rivers is, I started with that unidirectional arrow and I just made it bidirectional because
the characteristics of the plants and animals present also influence the physical characteristics
of the river. As an example, if you have cottonwood trees growing along the river, or in particular
a dense thicket of cottonwood seedlings, they're creating frictional resistance along the banks
so when floodwaters go out of the channel and across the floodplain, they move more
slowly through that riparian vegetation. The roots of that riparian vegetation are also
helping to stabilize the stream banks and limit erosion. There's a feedback that goes
in both directions between these biological communities and the physical characteristics.
That's inherent to many ecosystems. I'm emphasizing it here to make this point that rivers are
most effectively viewed as ecosystems rather than as simple physical conduits.
Other components are important in understanding rivers as ecosystems. First is this idea of
complexity. This can be spatial complexity. As you go downstream along a river are there
bends, are there pools and ripples, are there changes in the cross-sectional geometry, are
there grain cells on the bed? Also variations through time. Most natural ecosystems have
some disturbance regime. You've got high flows, low flows, variations on the sediment coming
in. The other component of complexity is the idea that you have what's called a nonlinear
system. That just means that for a given input, such as water coming in, you can't necessarily
exactly predict the response of a movement. It's going to depend very much on site specific
conditions. As an example of this, I'm going to go through
a scenario from some of my own research on mountain headwater rivers in Colorado. If
you have a tree growing in the valley bottom and it falls over into the channel, some of
those trees that are growing on the bank have a portion of the tree trunk that's still resting
above the bank. Often it's still partially attached by a root ***. That's referred to
as a ramped piece. Those ramped pieces are more stable than something that's completely
in the channel. It's hard for the stream flow to move them and transport them downstream.
So they sit there as an obstacle to other wood that's coming down the channel. They
can form a logjam. The effects of that logjam depend very much on where it occurs. I've
got this described on this slide as a a threshold that's based on valley geometry. On one side
of this threshold, if that logjam occurs in very steep narrow valley that doesn't have
much of a floodplain, as you can see in the photo, you get a step that forms. There's
a little bit of backwater upstream of that step. But the backwater doesn't extend very
far upstream, maybe one to two times the channel width. When a big flood comes along, the water
depth increases pretty quickly upstream from the logjam. The force exerted from that water
increases quickly so the jam is probably a fairly transient feature. It may last for
only a few years. It's got a very limited effect.
If you're on the other side of this threshold in a wide shallow valley, you get the same
logjam forming but now when a larger flood comes along, there's this floodplain on either
side of the channel. The flood waters go over bank and the floodplain kind of acts as a
safety valve. As I mentioned before, they slow down, they've got shallower flow there
of lower velocity. But as they're going overbank they can locally erode the bank. That flow
across the floodplain can concentrate in slight depressions that create secondary channels.
You have this pattern of channels that branch and rejoin and kind of complex channel form,
what you see when you look in map view. Each of those secondary channels can have bank
erosions so you can be causing other trees to fall into the channel forming more logjams
on the secondary channels. What I'm illustrating in the upper part of this diagram is this
self-enhancing feedback where the logjam in the lower gradient section creates this complex
channel pattern with more wood recruitment, more logjams that further forces over bank
flow. The reason I use this to illustrate nonlinear effects is that the initial logjam,
even if it's the same size, can have very different effects depending on the channel
and valley geometry in which it forms. With that by way of background, if we think
of rivers as ecosystems, one of the inherent characteristics of many ecosystems is that
they have both physical and biological complexity. I want to focus for the rest of this lecture
on what creates physical complexity in rivers. The very first level, you can have complexity
in the stream bed as you move across the channel or go downstream there's different grain sizes,
maybe you have areas where there's finer sand and other places where there's cobbles or
boulders. There's complexity in the sediment. There's complexity associated with bedforms.
As you go down a river, you may alternate between pools and ripples or if it's a steeper
channel you may have vertical steps and pools below them. There's also complexity in the
streambed associated with wood in the channel. If it's a forced environment you can have
individual logs or the logjams I was just talking about. Each of those create heterogeneity.
Maybe another word for complexity in this context is heterogeneity. You don't just have
a consistent uniform channel with no variation as you go downstream or through time. The
bed is one source of that. The stream banks are another source. Most natural channels
are not completely uniform conduit with very straight banks. There are irregularities associated
with things like bends, there are smaller scale irregularities where there's little
invadements or protrusions in the banks associated with differences in the size of sediment forming
in the banks or where trees are growing or have fallen in. There's complexity or heterogeneity
associated with cross-sectional form. If you think of pools and ripples, ripples are typically
wider, shallower cross-sections. Pools are often deep and narrow. As flow goes down through
those complex changes in cross-sectional form you get differences in depth and velocity.
Finally, there's differences in the planform. Again, that's what you see when you look down,
say in an aerial photo or in a map view of a channel. The planform can be straight or
it can be sinuous. You can have a single channel, you can have multiple channels that branch
and reform. All of these sources of complexity create heterogeneity in a river.
As an example, okay, what does it matter if it's heterogeneous? This is a picture of a
headwater river again in Rocky Mountain National Park. What you're looking at is a big logjam
that spans the channel. There's a backwater associated with that. If you can see in the
foreground, there's fairly fine sediment and a lot of organic matter in that channel. Upstream
and downstream of this point there's mostly cobble and boulder size. You've created heterogeneity
in the bed sediment. There's differences in the velocity and the depth of the flow. If
you can see the darker water immediately upstream from the logjam, that's deeper and lower velocity.
The yellow arrows are indicating three different channels that branch off from this big obstacle
in this main channel. They go across the floodplain and eventually downstream they rejoin. But,
this one source of initial complexity in the form of the logjam has created heterogeneity
and variability in all of these other factors such as substrate, flow depth, velocity, and
the channel planform. You can compare that last photo to this one.
This is also a natural channel in Rocky Mountain National Park. This one has different sources
of complexity. It's a single channel, it's actually a fairly uniform width but you notice
there's a series of vertical steps with plunging flow over them, so there's heterogeneity in
the bedforms. Even though it's a fairly consistent width, the yellow oval that just appeared
indicates a little area on the margins where you're going to have a lower velocity, there
will be some finer sediment settling out so you'll have sand and gravel there as opposed
to big boulders there in the center of the channel. There are a variety of different
sources of physical complexity in natural rivers and headwaters.
Why is that important? What are the implications? First of all, if you've got things like differences
in the grain size in the bed, velocity, flow depth, that creates more diverse habitat and
typically more abundant habitat for aquatic organisms such as insects or the really small
ones like microbes and bacteria like fish. If you look at the whole valley bottom, you
also have more abundance and diversity across the floodplain.
As another example, this is an underwater photo that I took just upstream from the logjam
in Rocky Mountain National Park, the orange arrow is indicating the flow direction. The
logjam is to the left in this view. Most of the stream away from this log jam has very
large boulders in the bed. You can see one in the left foreground there. You notice what's
accumulating upstream from the logjam is this finer sand to gravel size material. This is
a really pretty diverse habitat for aquatic insects and fish. You can see there's places
where the wood is exposed and there are specific types of microbes that actually like to eat
wood, mainly they're going after the algae that grows on the wood. There's also cover
for the fish in the form of that log that's sticking above the water surface. There's
a variety of different flow depths of water velocities here.
Another example from the logjam, again. The logjam is to the left, there's a big boulder
that's forming the dark object at the back and you can see some bubbles from water that's
plunging over that big boulder. The surface of the water shows up as kind of silvery in
this photo because there's so many air bubbles on it. The water's being aerated. Again, you
can see the diversity of substrate. There's sand and gravel on the foreground, big boulders
in the middle part, wood at the back, and you can just see the silhouette of a trout
that's appreciating the abundance of diversity of habitat here. One implication is that you
provide more habitat for aquatic and riparian organisms.
Another implication is that physical complexity influences sensitivity and resilience. Sensitivity
is defined as the degree to which an ecosystem or a river in this example responds to disturbance.
If there's a big flood, does the channel change a lot or is it so, if it's formed in bedrock
and there's no sediment maybe not much happens. Resilience is defined as the time frame over
which the ecosystem returns to its original ecosystem prior to disturbance. For example,
if you have that big flood and there's a lot of erosion in the channel, the banks get wider,
the bed is eroded, does it stay that way or if it's a resilient system typically the channel
will return to its original condition over a period of maybe weeks or months or maybe
even years following this flood. The degree to which a system is sensitive in some respects
isn't as important as the degree to which it's resilient. If it does change in response
to disturbance the big question is whether it returns to its original condition. I've
got this list on this slide that looks like, sounds sort of biblical: fire, flood, and
drought. Those are all forms of disturbance in rivers. The physical complexity of a river
determines how sensitive it is to each of those disturbances and how quickly it returns
to initial conditions. The high flows or low flows of floods and droughts or the large
amounts of sediments coming in associated with wildfires can change naturally. Or they
can be changed as a result of human activities. We're using resources such as timber harvesting
the catchment or extracting water from the channel or we're indirectly causing change
through warming climate. As an example of how this works, this photo
is along the upper North Moraine Creek in Rocky Mountain National Park and it's what
ecologists call a beaver meadow. It's a portion of wide valley bottom, there's lots of beaver
dams in there, those are creating areas of ponded water that you can see in many branching
and rejoining channels. It's a very wet meadow. I should emphasize that there's a lot of physical
complexity associated with that. There's flowing water, there's ponded water, there's deep
and shallow water, willow thickets. It you remove that primary source of physical complexity
which is the beavers in their dams, then you lost a lot of the retention that's associated
with those beaver dams. You lose some of overbank flows, typically the water bank declines with
time, you go to a drier meadow environment. This photo is an example of a place where
that's happened. This is Upper Moraine Park in Rocky Mountain National Park. Just a few
decades ago there were a lot of beavers and beaver dams here. It was a very wet meadow.
But, the beavers have disappeared, their dams have fallen into disrepair so now it's become
this dry grassland. What you're seeing in this photo is an area that burned in 2012.
It was a fire that was started by an illegal campfire, it was in October of 2012 and it
burned Moraine Park. If the beaver dams and that wet valley bottom had been there, that
system would have been more resilient, it would have been too wet to burn. Sometimes
if you remove sources of physical complexity, you increase the sensitivity of the system
to disturbances, in this case fire, and you decrease the resilience. It's going to take
a long time for some of the recurring vegetation to grow here because now it's a much drier
environment than it was historically. That's another implication of physical complexity
in rivers. The third one is this idea of retention or
storage that I mentioned a minute ago. The more complex, physically complex, a river
is, the more slowly everything moves downstream, like water during the flood, because it's
going in the floodplains if it's stored by logjams or beaver dams, sediments, nutrients,
contaminants, whatever it is moving downstream, it's going to move more slowly in a very complex
system than in a very simple uniform conduit. The photo here is a logjam, the orange arrow
is indicating flow direction. Again, you can see a lot of sand stored upstream from that
logjam, there's also a lot of fine organic material there, twigs, pine needles, pine
cones, in most of these mountain rivers if you don't have something like a logjam all
of that material stays in transport and it moves downstream really fast. The importance
of retaining it is that if you store something like pine needles, even for a few hours during
the flood, then the biotic community can start to use those nutrients. Microbes and bacteria
and aquatic insects can start to ingest that, those pine needles, and convert them into
their tissues which can then be eaten by fish or by some of the birds that eat aquatic insects.
Complexity is really important in creating different levels of retention. The same sediment
goes into the floodplain it might be there for several thousands of years. That's what
I mean by different levels of retention. You can have retention that's minutes to hours
or you can have retention that's much longer. Even the short retention is important for
allowing opportunities for stream organisms to start using that material.
That's a little bit of an overview of what creates physical complexity in rivers and
why it's important. These are a few of the references that I cited during the course
of the talk. Thank you.