The fallacy of the pool-riffle sequence

There aren’t THAT many controversies in the world of fluvial geomorphology. Generally you get things that are accepted and things that aren’t. Secretly there’s politics and cliques and different groups that do work in different areas, but it’s no worse than any other scientific discipline and the critique that does occur is necessary and ultimately welcome. It’s certainly nothing like climate science, where the recent criticism from the media regarding  the Met Office’s performance has become quite unsavoury.

Where we do fall down is in terms of public understanding. For example, what people think of as an archetypal English river, a la Constable’s ‘Scene on a Navigable River’ (above, which was based on Flatford Mill on the Stour – where a fish and eel pass has recently been installed!), is different to what an archetypal English river would be.

For starters there’s no such thing as a typical English river. Spatial variations in climate, geology and historic land-use mean we have a wide variety of natural and unnatural river planforms, flow regimes and sediment types. Most generally you could say there is a gradient of river types, from the high-energy, relatively steep, bedload sediment transport-dominated systems of northern England to the comparatively low-energy, shallow long-profile, suspended sediment-dominated systems of the south. You’ve got the chalk streams, which are relics of bygone processes. You have wooded rivers such as those in the New Forest. There are the highly-responsive catchments in places like Boscastle, East Yorkshire and Cumbria. Some of our rivers go through predominately rural areas, such as the River Wensum in Norfolk; others like the River Ravensbourne in Catford travel almost exclusively through the urban environment.

And it is our diversity of river types that underpins the pool-riffle fallacy.

*ENTER CONTROVERSY*

Pool-riffle sequences are characteristic of gravel bed rivers and gravel bed rivers alone. You do not find pool-riffle sequences at the mouth of large alluvial river’s such as the Mekong or the Rio Parana (see the video below). By the same token you would not naturally find a pool-riffle in a silt-dominated stream in East Anglia. The fallacy is the notion that these features are a generic river restoration tool that can be applied to any stream to help it approach something like its former state.

Now this isn’t to say that pool-riffles sequences can’t be used to improve the condition of a watercourse in which they would not naturally occur. The deep pool with its silty substrate and slow flows, the riffle with its gravel substrate and fast flows, and the transition in between will all provide a habitat for one thing or another. But let’s make sure people understand this notion of ‘river restoration’. Beyond the semantics is an important point to do with the way scientists  policymakers and the public perceive our rivers and how they should work. What you’re striving for may not be natural at all, even if you have good intentions.