How Sensitive is Your Chalk Stream?

Above: the River Nar in Norfolk: a “protected” chalk stream not very sensitive to abstraction … apparently.

Did you know that in order to assess if a river’s flows support Good Ecological Status (GES) using the Environmental Flow Indicator (EFI) every river is now put in one of three Abstraction Sensitivity Bands (ASBs)?

RIVER TYPEQ30Q50Q70Q95
ASB3 HIGH SENSITIVITY24%20%15%10%
ASB2 MODERATE SENSITIVITY26%24%20%15%
ASB1 LOW SENSITIVITY30%26%24%20%

The % figures in the table above indicate the reduction from natural flows deemed to be acceptable as a result of abstraction for the given river still to support GES. The Q number refers to the % of the time a given flow is exceeded: so Q30 equals relatively high flows and Q95 low flows. Flows in a high sensitivity river (ASB3) should deviate by no more than 10% below natural low flows.

The ASB banding is based on an assessment of three components: physical including gradient, catchment size, rainfall, and base-flow; macroinvertebrates; and fish communities.

One would expect, therefore, that all chalk streams would fall within the same ASB banding given that they are, almost definitively, of a type when it comes to the natural physical characteristics, which then define the macroinvertebrate and fish communities. In the original flow target matrix devised by UKTAG (from which the EFI evolved), all chalk streams fell within a single river type, divided between headwaters and downstream reaches.

So, I was surprised when I took a look at the ASB table to find that my local chalk stream, the River Nar, a protected site (SSSI) and arguably East Anglia’s finest chalk stream was in ASB1, the least sensitive to abstraction. This seems an anomaly to me.

But the River Nar is not the only river to have fallen into ASB1 in what actually appears to be a rather random assessment process. Can it really make sense that within a single chalk stream catchment we find rivers that are physically much alike spread across three different Abstraction Sensitivity Bands? The Wissey is one such case. But elsewhere, on the Frome say, we have the Wraxall and the Hooke (physically similar rivers) in two different bands. Likewise the Sydling and Cerne. If the assessment for ASB really is based on physical, macroinvertebrate and fish community assessments then this doesn’t make sense.

Below is a link to a table I have made of the ASBs for every chalk stream in England. Check it out to see what ASB your local chalk stream has been given. You can read the table by looking for the catchment your river is in. For example Thames / Colne etc. It starts at the River Bride in the southwest of Dorset and proceeds north east to end at the Gypsey Race in Yorkshire.

I suspect that those chalk streams in ASB1 are there by honest mistake: they are all in Kent, Norfolk, Lincolnshire and Yorkshire, in lower gradient catchments where the chalk reaches of rivers glide into fen-like rivers and often the waterbodies are lumped together. So, perhaps someone somewhere thinks the Nar is a cyprinid river because the waterbody division between the upper and lower river is in the Fens? Maybe. Even so, there are also a lot of chalk streams in ASB2 as well and I wonder why that is. In my opinion all chalk streams should be in Abstraction Sensitivity Band 3

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