Lecture 18: River ecosystems

Lecture 18: River Ecosystems

1. Physical settings of rivers

  • Unidirectional
  • Three axes
    • Lateral - material/nutrients are transported laterally from the channel to the riparian zone and vice versa
    • Vertical—water/nutrients travel from the surface to groundwater and vice versa
    • Upstream influences downstream “everybody lives downstream”

2. Stream ordering - a way of categorizing streams

  • 1st order - smallest year-round flowing stream, no other streams drain into it
  • 2nd order - when two 1st order streams meet
  • 3rd order - when two 2nd order streams meet
  • and so on.

3. General rules for stream order

  • There are bout 3-4 times as many 1st order streams as 2nd order, and 3-4 times as many 2nd order as 3rd order and so on
  • Lower order streams are generally less than half as long as the next high order streams
  • Lower order streams drain about 1/5 the area of next higher order streams

4. Describing streams by watershed

  • Hydrologic Unit Cataloging - HUC
  • Gives a unique number to each river and provides locational information

5. There is an important hierarchical structure to river organization

  • Rivers are made of repeated small units such as pools and riffles. Pools and riffles can be thought of as habitat units
  • Upstream units have an influence on downstream units but not vice versa—- because rivers are unidirectional

6. Fluvial Geomorphology:the shape of the river channel and the geological forces at work

  • Landscapes influence rivers and rivers influence landscapes in a dynamic way.
  • Factors affecting geomorphology: Slope, amount of water, and sediment
    • The interaction of: slope*amount of water (discharge)= power of the stream
    • This determines how much sediment will be carried

When a river meanders it adjusts it's slope. This is a primary way a river stays in dynamic equilibrium. In flat landscapes rivers cut outwards, rather than scour down to attain equilibrium. If on a map a river is straight it probably means it is in a steep landscape. When channelization of rivers occur the slope increases dramatically causing the river to attain a new equilibrium by eroding away the banks and stream bed. In this image you can see how as the river flows through different landscape it's sinuosity changes depending on the geological characteristics.

The power of the river influences the particle size and the amount of sediment that the river is able to carry

As the power is reduced, sediment is deposited
As the power is increase, sediment is picked up (the stream bed is scoured)

At the top of a hill:
Slope is steep but there isn’t much water
Middle of the hill:
Slope is still pretty steep and there is more water (higher order stream)
Bottom of the hill:
Slope is shallow but there is a lot of water

*Somewhere in the middle of the hill the river has the greatest power

As the power in a river reduces more sediment is deposited. Large sediment drops out first until finally fine sediment is deposited in slow moving areas

Lane’s Law

Sediment discharge (Qs) * sediment particle size (D50) ~ Stream discharge (Qw) * stream slope (S)

Stream discharge = m3/s
Sediment particle size is the average particle size
Stream slope is the steepness of the stream bed

Remember stream discharge * stream slope = power and power determines what size sediment is carried in the water

When this equation is balanced the stream channel will remain the same, when the equation is out of equilibrium the channel will change

For example:
If the flow increases but the slope does not, the channel will shift down (degradation) meaning sediment will be picked up and carried away
If the slope decreases and the flow remains the same, the channel will shift up (aggradation) meaning sediment will be deposited because the power of the stream is less than before

There are several factors and forces acting on sediment particles in a river:

  1. 1. Lift Force (works to move the sediment particle)
  2. 2. Shearing Force (works to move the sediment particle)
  3. 3. Gravitational Force (works to keep the sediment particle in place)
  4. 4. Frictional Force (works to keep the sediment particle in place)

There is a shear critical sorce that is defined as the threshold shear force which exceeds gravitational and frictional resistance and results in incipient motion. Depends on mass, surface area, and cohesion with adjacent particles.

  • if the shear force is greater than the critical shear force than the particle will be transported
  • if the shear force is less than the critical shear force than the particle will not be transported

Scenarios: What if

The channel is straightened but stream flow stays the same?
The slope will get steeper which will quicken the flow and sediment will be picked up.
A transfer of water into the basin increases Qw?
This will increase the power of the river and it will degrade the stream bed as it picks up sediment.
Water export decreases Qw?
The power of the river will decrease and sediment will be deposited.
A dam traps sediments, reducing Qs?
The stream bed will move up via aggradation through sediment deposition above the dam, below the dam the stream will be sediment hungry and the water will degrade the stream bed.
Restoration “re-meanders” a stream?
Meandering happens when there is a shallow slope which reduces the power of the river, therefore sediment will be deposited.


Energy derivation

Autochthonous energy originates within the stream system from algae and high plants

Allochthonous energy comes from outside the system such as leaves and invertebrates

Decaying organic matter can be in several forms:
DOM—dissolved organic matter
FPOM—fine particulate organic matter
CPOM—course particulate organic matter

These various forms of organic matter come from continuous processing


A leaf falls in to a stream (allochthonous energy) and is saturated with water→the soluble components of the leaf are leached resulting in DOM (tea demonstration)→microbial colonization softens the remaining leaf material while mineralizing the CHO to CO2→invertebrate colonization further processes the leaf→ continued microbial colonization and feeding by animals converts the remains of the leaf to fine particulate organic matter (FPOM) and feces

Microbial Loop: important in processing DOM and FPOM but again we don’t know all the interactions, its hard to tell how many trophic levels there are and how much energy is transferred from one level to the next (~10% per level) but generally
DOM*0.5=DC (dissolved carbon)

Trophic structures of streams are quite complex and it’s very hard to tell how many trophic levels actually exist. The interactions between algae, decaying organic matter and microbes (biofilm) are not very well understood. Simple trophic descriptions of algae—>macroinvertebrate—>planktivorous fish—>piscivorous fish may be far too simple.

Trophic levels:

Microbes—are generally counted as in-stream production but can use carbon sources from within the system or those that enter from outside the system, form a complex interaction with fungi and algae to produce biofilm (freshwater gunk that coats surfaces within streams)

Primary producers:
Algae—eaten by many herbivores, at a small physical scale there is a great diversity of algae
Macrophytes—few consumers eat these, energy is derived from these plants once the dye and become decaying organic matter

Primary consumers:
These are comprised of various insects and planktivorous fish and are separated in to guilds (groups of organisms that have similar feeding habits)

Grazers and scrapers→eat biofilm from surfaces in the river system and periphyton
Shredders→along with associated microbes eat leaves and CPOM
Collector gatherers→eat FPOM from the water column and streambed, often build silk webs to collect food

Secondary consumers:
Predators→eat various macroinvertebrates or other fish

River continuum concept:
Headwaters tend to be narrow and shaded (temperate regions) with a steep slope and most of the energy comes from outside the system (allochthonous)→ mid-order streams the river gets wider and more sun and more water making it a good place for algae (autochthonous)→in the lowlands rivers can be large and deep by this point carry much OM from upstream and floodplains and therefore light does not penetrate far making it less hospitable for algae

The river continuum concept (RCC) is a good way of thinking about rivers in areas such as Michigan where the rivers are typically shaded in the headwater areas but it is not as applicable for rivers in other places of the country and world. There are also other limitations to the RCC because there are many aspects that govern what you find in a river reach that are not taken into consideration by RCC. For instance, RCC does not address climate, topograph, geographic location, surficial geology, or groundwater input to name a few. All of these variables dictate what a particular river stretch will look like and are not taken into consideration by the RCC.

From the slide in lecture but also very good exam questions:

How would deforestation upstream affect downstream?
There would be much less energy in the form of organic matter feeding into the system which would negatively impact many organisms downstream
What is the effect of a dam on an order 6 river?
A dam would reduce the flow above the dam and particles would settle out, below the dam the volume of water would be large and the sediment load would be low making it a sediment hungry river which would then scour the stream bed

Would the ecosystem of a river change downstream of a major tributary input?
It depends on the slope, it the slope remains the same a large input of water will increase the power of the river and sediment will be picked up. Also, input of organic matter from upstream can affect the clarity of the river and thus affect the light entering the system which finally affects the growth of algae and macrophytes and alters the biotic composition of the river and could turn it from autochthonous to allochthonous
What are likely consequences of separating a river from its floodplain via levees?
This will affect the input of nutrients and particulate matter from the floodplain which can be important if the system is allochthonous.

Many rivers and their component biological species are at risk!
Freshwater shellfish and fishes are threatened
Only 42 rivers with more that 200km of free flow are left

There is much interest in determining the health of a river ecosystem…

Hypothetical Stress-Response Relationship
Looking at patterns in decline of species along a gradient (such as urbanization)
Is the decline steady (linear)
Is there a point of sharp decline (tipping point)
IBI index looks at species assemblages to assess river health
A numbering system to quantify stream health
Once a healthy stream is quantified it can be compared to unhealthy streams
The index considers tolerant and intolerant species—with the idea that when intolerant species disappear it is an indication that the river health is declining

EPT invertebrate metric: species that are considered intolerant
Ephemeroptera (mayflies), Plecoptera (stoneflies), Trichoptera (caddisflies)

If these species decline the river is likely to be in poor health

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