Googling “Natural England chalk stream map” or “chalk stream priority habitat map” or “map of chalk streams” has proved frustrating lately because a plethora of web pages, portals and digital oubliettes lie strewn about the ether. I have been told my blog is littered with broken links and so I tried a few times over the last few days to find out what’s where and rebuild.

I suggest that it is best to start with the narrative of how the map was put together. This captures the difficulties and uncertainties involved when computer mapping software picks up drains and ditches and not winterbournes, for example.

In some ways this mapping exercise would be more accurate if done in the old school way of actually drawing lines on a page, without relying on the turning on and off of layers of “dumb” data, including too much and too little. This digital form of mapping, for example, almost completely ignores the lost paleo channels that might only be discernible by looking at ancient maps, parish boundaries or satellite images. See my illustration of a Norfolk chalk stream above: only the darker blue lines of a man-made mill-leat would show up on any DRN derived mapping: the lighter blue lines show the natural river course. We want to restore these channels, where we can: it would be a great shame if some authority or edict down the line protected a drain because the map says it’s a chalk stream and precluded the restoration of a paleo channel, because the map says it isn’t.

Natural England has handled this difficulty by showing high and low certainty layers. The high certainty layer, the dark blue lines, pick up pretty much all of the named chalk streams we know of and is essentially the “official” map of the chalk streams.

The low certainty layer picks up any waterbody of any sort within a given orbit of outcropping chalk or a chalk stream. As the narrative explains “some of these will be natural chalk streams, some completely artificial drainage channels, and some channels fed completely by non-chalk water (e.g. from impermeable soils overlaying the chalk)”.

Use mindfully, therefore.

The next job in this mapping process will consist of incrementally ground-truthing both layers and picking out or pulling in erroneous inclusions or exclusions.

Mapping lost channels will also be an important exercise, as will mapping winterbournes.

Including names, base flow indexes, type (Type A pure chalk, Type B mixed geology etc), catchment size, average annual aquifer recharge and abstraction rates etc. will all add to the functionality.

• click here to view the chalk stream priority habitat map with both high and low certainty layers

• click here to read the Natural England narrative of the process of mapping the chalk streams