Tip:
Highlight text to annotate it
X
In this second lecture, I'm going to be talking about some of the implications of losing physical
complexity in rivers. The first lecture emphasized the sources of physical complexity and why
it was important. Now we're going to talk about what happens if you lose physical complexity.
The basic objectives for this section I think would be to understand how human activities
have altered the type and the magnitude of rivers' physical complexity, understand the
implications of the loss of that complexity for river form and the processes that create
function in rivers, and explore a case study from the headwaters in the Colorado Rocky
Mountains.
First of all, how do our activities alter river form and function? There are two basic
levels at which we influence rivers. The first one I'm calling indirect effects. We're not
doing anything right in the river channel itself, but we're still altering rivers. That's
mainly through changing the land cover. Land cover generally refers to vegetation. If you
have timber harvest for example, or you're changing the native vegetation to crops or
changing the native vegetation through grazing, you're urbanizing the catchment of the river
or some portion of the catchment, all of these can change the land cover. The other basic
form of indirect effect is through changing climate which we have been doing for a period
of time now but have become much more aware of it recently. If you change the precipitation
and temperature of a given river's catchment or drainage basin, then that in turn filters
through all of the components of a basin: it changes the vegetation, the rate at which
bedrock weathers into sediment and soils, stability of the hill slopes. Precipitation
and vegetation are the first stage but then it filters down through all of these other
levels to influence water and sediment coming into the river. Really, whether it's changes
in land cover or changes in climate, the effects on rivers are that you change the material
going into the rivers. That's primarily water and sediment but also nutrients that are used
by plants and animals and contaminants that can influence biological communities.
The other basic way that we alter river form and function is by directly changing, doing
things in the channels or valley bottoms themselves. A big one is altering the flow of water and
sediment through the river. Dams are one of the primary sources of that around the world.
We also take water from points in the channel and move it to other channels or other drainage
networks, through diversions for example. Another big effect is to alter the form of
the channel. Channelization usually you're making a multi-thread, like a braided or branching
channel into a single channel or you're making a meandering sinuous channel straighter. Dredging
usually you're trying to increase the cross-sectional area of the channel so that floods for example
don't go over bank or there's a minimum depth of water that's maintained throughout the
navigation. Bank stabilization you're trying to prevent the banks from eroding. One way
or another we're altering the form of the channel. I emphasized connectivity in the
first lecture between the river ecosystem and the greater environment through the six
degrees of connection. We can also alter connectivity if we built levees or if we build these transportation
corridors which become like roads or railroads next to the river. That typically dramatically
limits the connectivity between the channel and the adjacent valley bottom and floodplain.
Dams of course limit the connectivity of water and sediment moving downstream and organisms
such as fish that live upstream. The net effect of all of these activities is usually that
we tend to reduce physical complexity. If you think of a sinuous river that's been dredged,
it has stabilized banks, it's much more unified and simplified. Although there are human activities
that increase connectivity, most of the things we do decrease connectivity so you end up
with these fairly simplified, disconnected river corridors.
What are the implications? What does it matter if we change complexity and particularly if
we lose complexity in rivers? Get back to the things I was emphasizing in the first
lecture that complexity creates. If you lose complexity, you tend to lose the abundance
of diversity and habitat on which plants and animals depend. You have fewer organisms,
you're decreasing the abundance, and you also typically lose biodiversity whether it's reduction
of the number of species present in the channel or floodplain or a reduction in the diversity
of the individuals in those species. I used the example of cottonwoods in the first lecture.
If you have cottonwoods that primarily germinate immediately after a flood, if you put a dam
in and you reduce the number of floods or the size of the floods, you're probably going
to reduce the opportunities for the cottonwoods to germinate as individuals so you end up
with just an older forest. Also, you'll often lose resilience. One of the things that allows
an ecosystem to recover following a big disturbance such as a flood is that there are places where
organisms can get out of the way of the flood. There's secondary channels or parts of the
floodplain where fish for example can take refuge during a very high flow and swift velocity.
If you lose complexity and go to something that's more like a simple uniform conduit
then you lose those shelter spots and it's harder for organisms to recolonize an area
after a flood for example. You lose some resilience to disturbance. If you're losing all these
other thins like abundance, biodiversity, and resilience, you're also losing the ecosystem
services that I talked about in the first lecture, things like clean drinking water,
navigation, flood control, fisheries. I also emphasized retention or storage in the first
lecture. Again, if you lose physical complexity you lose retention. Things like water, sediment,
nutrients, contaminants just stay in transport, they don't get stored along the river corridor,
they, nutrients in particular, there's less opportunity for stream and organisms to take
them up. That is one of the reasons you lose biodiversity of species and individuals.
Okay, a specific example of this from the headwaters of the Colorado Rockies, in specific,
Rocky Mountain National Park. Old growth forests tend to produce a large number of trees per
area of ground, a dense forest, and big trees. They have a very large diameter. If you combined
old growth forest and wide valley bottom as you see in this photo there's a lot of downed
wood on the floodplain here. This was taken before the mountain pine beetle started to
affect these headwater floodplain forests. There's a channel at the far rear right in
this photo and there's some secondary channels that you really can't see in the middle. There's
lots of wood as a result of the old growth forest. It's accumulating across that wide
valley bottom and floodplain. It's creating logjams that are big enough to span the length
of that entire channel. They're very effective at bringing those backwater effects and storing
those sediments and nutrients as I talked about in the first lecture. You end up with
these obstructions in the channel that force flow across the floodplains. You get overbank
floods. Some of that water infiltrates the floodplains so you have a high water table.
That supports some of the wetland species that wouldn't be present otherwise. Those
floods across the valley bottom are typically shallower and slower moving than the floods
in the main channels. Some of the sediment and nutrients that are being transported drop
out of suspension and are being stored in the floodplain. You get the secondary channels
that I talked about in the first lecture that branch and rejoin and there's different habitat
in those and more abundance and diversity in those, both in the channel and floodplain
in this scenario.
Just an illustration of that, this is looking upstream in the main channel. You can see
the step appearance. There are logjams that are storing a lot of sediment and creating
a plunge pool at the base. They're very closely spaced. What you can't see in this photo is
that there are smaller secondary channels that branch away from the main channel and
then rejoin it across other portions of the floodplain.
Similar types of scenario happens if beavers are present. Beavers of course build dams.
Because of that ecologist call them both ecosystem engineers and keystone species. Ecosystem
engineer refers to any plant or animal that alters the surrounding environment enough
to create habitat for other species. In the case of beavers, they build the dams. The
dams are just like the logjams I was talking about a moment ago. They create an obstruction
in the channel, a backwater, and that forces water across the floodplain. Some of the water
infiltrates and creates a really high water table. That creates habitat for wetland plants:
willows, sages, and rushes, for example at those high water tables. The beaver is engineering
an ecosystem for those plants. Beaver are also an example that ecologists call a keystone
species. If you think of an arch, the keystone is the central portion of the arch. If you
move that keystone the whole arch collapses. Beavers are creating all this habitat for
the other species, if the beavers are removed and the dam falls into disrepair then much
of the habitat they create vanishes. The species that were dependent on that habitat leave
the area as well. Analogous to that example I just gave, if you have beaver building dams
in a wide valley bottom which you see in that lower photo, it's the same process I talked
about for logjams. You have more overbank floods, some of that water infiltrates, you
have a high water table, you're storing sediments and nutrients across the valley, you have
secondary channels, you have a lot of abundance and diversity of habitat for both plants and
animals in the channel and valley bottom and you have more biotic diversity.
This environment as I referred to in the first lecture is also known as beaver meadows. There
are lots of studies by ecologists that show that there is a really heterogeneous habitat.
There's a couple of photos here illustrating that. You can see in the photo on the left
that there's areas of ponded water, there's water flowing over the dam in the picture
on the right, there are portions of old beaver dams that are now vegetated so they're slightly
drier and they're supporting different animals or vegetation that are in the ponded water.
There's a lot of heterogeneity and complexity. Again, ecologists have found much greater
diversity of species, sometimes known as richness of species with pretty much everything they've
looked at in beaver meadows from bacteria and microbes up through insects, plants, mammals,
fish, water fowl, there are a lot of organisms that use these wet meadows. They may not live
there year round, they may be species that live primarily in the upland but they come
down and use these wetland valley bottoms periodically. Certainly, the sediment that
is forced onto the floodplain and stored on floodplain environments and often beaver dams
has a lot of organic material in it. There are nutrients such as carbon, nitrogen, and
phosphorus in particular that are dissolved in the water. They're also in particulate
form in the sediments and they're being retained in those environments. They're not moving
downstream nearly as quickly as they would if you had just a very simple uniform channel.
Getting back to this idea of the implications of the loss of complexity. If beavers disappear
from an area for whatever reason, then eventually, their dams fall into disrepair and the ponds
begin to drain. There's a couple of photos here also from Rocky Mountain National Park
showing different stages in that process. If you don't have the ponded water and the
overbank flow, then eventually the water table in the valley bottom declines and it becomes
a drier environment. Once it's a drier environment you begin to get animals such as voles, small
burrowing mammals, that eat something called ectomychorrhiza fungi. These are fungi that
are present in drier soil. The voles eat the fungi but they also disperse the fungal spores
through their feces. This type of fungi is critical for the survival of certain types
of conifers such as spruce and fir. If the valley bottom is flooded the voles aren't
going to go in there because they can't burrow and the bacteria and ectomychorrhiza fungi
can't survive. As the valley dries out, the voles disperse these fungi spores so the fungi
becomes more common in the soil. That facilitates the colonization of the valley bottom by conifers
as you can see in the photo on the right here. Behind that valley that's shrinking and drying
with time, there's some spruce and fir coming into the valley. Basically, you go more towards
a woodland, dry environment as the valley dries and the environment changes. Sometimes
these are referred to as drier grasslands or woodlands rather than the wet beaver meadows.
Prior to very intensive human manipulation of forests and rivers, both in this environment
in Rocky Mountain National Park and worldwide, old-growth forests were much more widespread
and beavers were much more common across Eurasia and North America. There are two species of
beavers, Castor canadensis in North America and Castor fiber in Europe. Historically,
beaver were present from Siberia over to Spain and down to southern Europe and up to the
Arctic regions. If you have those sources of complexity such as old-growth forests and
beavers, dams and logjams are more common, beaver dams and logjams, so this headwater,
multi-thread channels that were closely connected to the adjacent floodplain were also more
common. These river corridors and valley bottoms really had a lot more physical complexity
and much more ability to retain water sediments and nutrients.
Another way to look at this, if you take away beaver and old-growth forests, the basic configuration
of a valley doesn't change. You still have a floodplain present. What changes is how
that floodplain is connected to the channel and the retention that's associated with that
floodplain. On the left here is a very simple drawing in a plan view or a map view, what
you'd see if you were looking at a map. The blue squiggly line are secondary channels
that are branching and rejoining the main channel. These green blobs are supposed to
be willow. On the right is a side view of that main channel. The brown lines are a beaver
dam and the secondary channel is off to the side. When beaver dams are present they're
forcing overbank flow in the secondary channels, you've got water, sediment, nutrients spreading
out across the floodplains being stored there with the water infiltrating. If the beavers
disappear and that's happened for a variety of reasons from direct trapping to other causes,
again the valley geometry does not change. What happens is that the beaver dams fall
into disrepair, you lose the overbank flood. You go to a scenario more like this where
each of those little symbols of three lines are grass clumps and the triangles are conifers.
The beaver dam falls into disrepair, floods are more likely to be contained in that single
main channel and they're going to move downstream more rapidly. The velocity is going to be
higher, they're going to be more erosive so that single main channel becomes larger and
deeper with time, it can cut down with size. The adjacent floodplain dries out because
you don't have the overbank floods, the water table drops, some of the carbon and nitrogen
is stored in that soil is oxidized or lost to the atmosphere and you're not getting the
increment of additional storage each year during riverbank floods.
To put this in a little more quantitative terms, I give you a very specific example.
This is based on work we did in the headwater rivers of the Rocky Mountains. We looked at
how much carbon was actually stored in the floodplains. The y-axis here is mega-grams
of carbon per unit area of floodplain. The brown bars are carbon stored in wood, dead
trees basically for old-growth forests, those trees that have fallen over and take a long
time to decay on the floodplain. The green bars are the living vegetation. The white
bars that are below the x-axis are carbon stored in sediment, organic matter in the
floodplain soils. These first four bars are all valley segments that were very wide, low-gradient,
and had either old-growth forests or beaver dams. If you look at the size, the vertical
extent of those bars, there's a lot more carbon stored in those types of valley segments than
either where you have a more confined valley segment or younger forest or no beaver dams.
Another way to express this, those broad, low-gradient valley segments constitute less
than a quarter of the total river length. If you look at how extensive they are up and
down stream on the river network as a whole, they're not very common, but when we extrapolate
it from our measurement sites based on that proportion of the river network that those
types of valley segments occupy, they constitute less than a quarter of the total river length
but they actually store three quarters of the carbon stored in the valley bottom. These
wide low-gradient sections are disproportionally important in storing carbon. Again, I'm using
carbon just as an example here. You could I think you'd have a similar story if you
looked at sediment or nitrogen or some of the other nutrients. Point is, if you have
physical complexity and you have this high level of connection between this channel and
the adjacent floodplain, these spatially limited segments are very important in creating retention
and ultimately resilience in these river networks.
What's happened historically? What has changed complexity in these rivers? Worldwide there's
been a dramatic loss of old-growth forests. In the world as a whole, humans have reduced
forest cover by about half of what it was prior to the development of agriculture. Globally
we've mostly eliminated old-growth forests. Old-growth forests are important because they
produce those really large trees and abundant wood that can make logjams and create this
complexity and retention. IF we look at beaver, ecologists estimate something like 6.12 million
beaver in North America at present, but historically there's a big span on the estimates but a
lot more. A minimum about four times as much and maybe even more. 50 -125 million animals.
This is hard for us to imagine now, but beaver would have been present in every environment
that had a forest from the forest in a place like Arizona to New Mexico up to the fringes
of the Arctic and Alaska and Northern Canada. If we look just at Rocky Mountain National
Park, one of the things I've done there is map the distribution of abandoned beaver dams.
There were a lot more beavers there in the past than are present today. An example is
this place that's specifically called Upper Beaver Meadows, that's the proper place name
for it. It doesn't have any beaver today. We used ground-penetrating radar, which is
shown in this photo here to image the depth and the type of sediments that have been deposited
in this valley since the glaciers melted 15,000 years ago. What we found is anywhere from
a third to a half of the sediment deposited since the glaciers retreated is associated
with beaver dams and beaver ponds. There's this long term effect where beavers have been
responsible for a lot of retention and complexity and productivity in these river systems and
it's changed dramatically very recently.
What does it matter? This is where the title for these lectures, both of these lectures,
comes from. I know refer to these rivers that have lost physical complexity and retention
as leaky rivers because they leak water, sediment, nutrients, everything downstream. That material
is transported at a much greater rate than it would have been if they'd had the physical
complexity. If we look at this globally, look at rivers in the context of the global carbon
cycle, carbon is coming from terrestrial environments, from forests and grasslands, and comes into
the freshwater networks of rivers and lakes and eventually goes out to the oceans. But
what comes in from the terrestrial environment is a much larger amount than what gets out
to the oceans. Something is happening in the freshwater network. This is a very schematic
illustration of that. The numbers of that are pentagrams of carbon per year, the average
global fluxes. A pentagram is a very large unit but it's the relative numbers that are
important for our purposes. We have 2.7 units of carbon coming off of the land globally.
It goes into rivers, lakes, wetlands, the freshwater network. About 1.2 of that is returned
to the atmosphere. That's through respiration of living organisms, plants and animals releasing
carbon into the atmosphere. Another .6 goes into the geosphere which is storing sediments.
This could be storage for a few hundred years in a small headwater floodplain or it could
be storage for a few thousand years in a floodplain in the Amazon or a big river delta. The remaining
.9 units goes out to the ocean. If you look at this very simple diagram for a moment and
think about the implications, the details of how carbon is partitioned, whether it goes
into the atmosphere, the geosphere, or out into the ocean, depends a lot on the complexity
and the retention and the processes that are going on in the global river networks. One
of the things that bio-geo-chemists and biologists and ecologists are starting to realize is
that as we lose physical complexity in rivers, we're more likely to get a lot of the carbon
and other nutrients coming into the rivers going straight out into the ocean, which can
create its own problems, things like nutrification or an excess of nutrients in ocean environments
can create what are called anoxic zones where there's a lack of oxygen and you have dead
zones, places where fish and many organisms can't survive for example. If you remember
the first lecture, 70-80% of the total length of most river networks is headwater rivers.
The whole carbon partitioning between the atmosphere, geosphere, and ocean really starts
with the headwaters where that carbon is coming off that terrestrial environment and entering
the river network.
These are the references that I've been citing on each of the slides as we've gone along.
I think what I'd like to leave you with is this idea that first of all rivers are not
just simple physical conduits for water and sediment. They function as fairly complex
ecosystems. Physical complexity is very closely tied to the biological complexity as expressed
in things like biodiversity. If you lose physical complexity for whatever reason, there are
some very important implications in terms of how the river functions as an ecosystem.
For example, in terms of how many organisms and what variety of organisms it supports,
climates and biodiversity. Also, how material moves through the river network, whether that
material is water or sediment or carbon. I think it's important that we understand these
implications because if we view the increased transport of the material such as carbon as
negative, then we have to think about ways to enhance or alter physical complexity or
storage in rivers and limit some of that downstream transport of carbon.
Thank you.