River changes

[Paul Arden]

Hi Guys,

I was wondering, how important is it for a river's health that it be allowed to develop naturally? I suppose the last 100 years has seen major restrictions imposed on most rivers (obviously some have been controlled for centuries). When a river changes course it leaves lakes behind and forms new channels. I assume this is all good stuff for the river, that is on the most part now lost?

The reason I'm thinking about this is because every morning I look at a major diversion on the Drava that has occurred since the end of the First World War. Only a few kilometres upstream however, there are major river defences to protect a railway line.

Cheers, Paul

[Harps]

I think that it is critical to the ecosystem in the river for the river to undergo the natural fluctuations that occur in the system.

In mobile rivers (freestone, alluvial, erosion/deposition) systems, floods allow gravels to be shifted, cleaning out embedded fine materials, opening the interstitial spaces that are important for many species egg development, as well as rolling gravels and coarse materials around to allow new ecosystems to develop on each surface- A succession cycle of the aquatic environment, much like forest or lake ecosystem successions.

In those same systems, droughts allow terrestrial vegetation to establish on the banks, confining the channel to one place, providing shade and allochthonous inputs (leaves, woody debris, etc). This changes the energy dynamics, which pushes a system to a different successional state- only to be “refreshed/reset” with a flood or river changing event.

In a system that has been modified by people to prevent floods or stabilize flows, there are no successional changes- just stagnentation. These rivers can be healthy, but the biodiversity will diminish as species fill the never changing ecological niches. Of course, when there is a disaster (major flood), the early successional species aren’t present in the system anymore to recolonize- resulting in a potential major loss of food to higher level species such as predatory fish.
Eventually the controlled system becomes the norm and species that persist (often species introduced by people) will become established and adapted to the conditions and we will see it as “normal”.

What is lost in controlled systems are the species that are adapted to the change, the new growth that depends on flooding, and sometimes, with the loss of scouring flows such as below many dams, there is no replenishment of instream gravels that provide the spawning substrates for many salmonids. I can name a number of rivers where the managers have been adding gravel (at great expense) to augment trout and salmon spawning areas. Another problem with the lack of gravel is that infrastructure (buried pipes, bridge footings, etc) are often “dug out” because there is no fresh gravel moving downstream to replace that which is lost.


Rivers that are inherently stable (low gradient or constant flow such as spring creeks) have a different successional response and ecosystem adapted to that stable environment. Often it is biological process that keep those river ecosystems functioning- trees falling in, islands forming, beavers, fish that stir up mud, wetland formation, mudslides, upstream erosions, etc, etc. I work in a very dynamic place and I’m not as familiar with stable systems.

[Paul Arden]

Thanks for your great reply, Harps. I figured it would be fundamental. I guess the only way to really see things is that the greater the resistance to change, the greater the change when it comes?

Cheers Paul

[Graeme H]

There is a flip-side to controlling the rivers though: if we fail to control the river banks after we've made massive changes to the vegetation in the catchment areas upstream, we run the risk of destroying the river due to the increased water flows produced by runoff increases. Changing forrested catchments into suburbs or farmland increases runoff into the river, so formerly placid river systems become torrents more frequently (i.e. frequent but small storm events produce the large flood events that previously only occured on large storm events).

The result it more errosion downstream and poorer water quality than without those controls in place.

Cheers,
Graeme

[Harps]

I guess the only way to really see things is that the greater the resistance to change, the greater the change when it comes?

Cheers Paul

That's what they learned in New Orleans when the flood protection structures failed...

Graeme makes a really good point though, especially in already flashy systems.
In the City of Calgary, there is a noticeable change in flood frequency peak (how fast the water comes down). The peak can change the average channel forming flows that the substrate and bank materials have to endure. Typically healthy banks can "absorb" the changes that a city causes, but of course that natural resilience is lost with human use of the riparian area (grazing, agriculture, urban development, etc).
Outside of re-creating permeable surfaces to capture run off or constructing storm retention systems, the best option to help the river would be to maintain or increase access to floodplains so that the peak can spread out in an area of lower flow (lower slope, lower depth= less erosion) and not cause in channel erosion which can lead to bed degradation (really crazy examples from some of the meadow streams in Michigan and Wisconsin). Bank armouring is rarely a good solution as it can lead to loss of diversity, localized increases in velocity (increases in erosion at the area), and increased impacts even farther downstream.
A good solution that has worked around the world (on smaller rivers) is to create a riffle complex (Newbury Riffles) with a crest frequency that matches the new stream hydrological conditions. Riffles in nature are great energy dissipators (and a function of friction cause sine waves- really cool stuff!) and constructed ones can act as great grade control structures that allow the re-establishment of bank vegetation.

The problem is when we try to manage one thing we through the balance off and have to manage more and more aspects which originally weren't considered.