About watersheds in general
Rain falling on either side of a mountain ridge naturally tends to run off in different directions into different streams. In some cases these each flow into different rivers, and in even more particular cases, these rivers empty into different seas. The line along the very top of the ridge which divides the waters flowing in one direction from those flowing in the other is a watershed.
Watersheds can be identified on any land-mass where the geography of the land causes rainfall in different locations to drain into distinct bodies of water.
A watershed is not by nature a single line, but a topographical tree of ridgelines dividing every portion of land according to its run-off. Such a watershed could in effect be plotted between any two points on the coast.
Related to the concept of watersheds is that of catchment areas. A catchment area for a particular river is bounded by a watershed line which defines the total land area over which fallen rain ends up in that river.
A note on terminology
On this site I use the meanings normally used in British English.
In some other parts of the world, notably the United States, "watershed" is typically used to describe not the ridge, but the catchment area. The ridge itself is known as the "drainage divide".
Choice of Dunnet Head and Leathercote Point
In the case of Great Britain, the two major bodies of water surrounding it are the Atlantic Ocean and the North Sea. For the purposes of determining a single major watershed line, the other lesser named waterbodies must be grouped with either one or the other. In particular, the Channel, the Irish Sea and the various other smaller seas and channels on the west coast are taken to be arms of the Atlantic Ocean. (In the case of the Channel, it's obviously significantly sized on its own, but can clearly better be considered an arm of the Atlantic Ocean than as a part of the North Sea.)
Therefore this watershed route runs between the two coastal points where the Atlantic Ocean meets the North Sea. These are defined by the International Hydrographic Organization as Dunnet Head, on the north coast of Scotland, near John o'Groats, and Leathercote Point, on the south-east coast of England, at St Margaret's Bay near Dover. (Ref: Limits of Oceans and Seas (pdf))
Alternative candidate points would include Duncansby Head (John o'Groats) and Cape Wrath, at either end of the north coast of Scotland, and Land's End in Cornwall.
Determining the route
I made use of a number of resources to determine the route.
These include: Ordnance Survey 1:25,000 and 1:50,000 mapping, books by Dave Hewitt, Peter Wright and Andrew Bibby, maps from the Environment Agency and the Scottish Environment Protection Agency, Malcolm Wylie's blog, local photographs and information from Geograph, Google Maps Street View and numerous other maps and details found on the web.
Further details can be found on the resources page.
Practical issues of route following
Paths and roads along the line of the route
The route is not at all official. There is no signposting, and often no right of way.
In some places paths or roads do run along the course of the watershed. Ridgeways frequently make effective routes for pedestrians and for vehicles. In other places unmade tracks may be found following the route. In yet others, I expect to have to find a route myself.
How closely can it be followed
In areas of open country (particularly in Scotland and the Pennine moors), there's mostly a right of access. Elsewhere, where it crosses farmland or built-up areas, there may be no easy way to follow the true course of the watershed. However, in most places, a suitable path or road can be found within, say, a kilometre or two of the route. I'll aim to follow the line as well as possible without being too overly pedantic about reaching every last possible location on the route. (For example, one could walk up every last road and path that crosses the line of the watershed, just in order to stand as many parts of it as possible, before having to return to continue along the nearest right of way running in the correct direction. Given the other sources of imprecision and difficulty following the exact route, this seems like overkill.)
The one major exception is the MOD Warcop training area in the Cumbrian Pennines. Although some people have managed to get permission to walk across it, the complexities of arranging this for an appropriate date, and visiting the base in advance for paperwork, make it impractical for me. I intend to follow a (somewhat large) detour to the east, partially following the Pennine Way.
Theoretically, the watershed twists and turns not only due to every spur on every hill, but also due to every rock, hummock and peat hag. For practical reasons, it's normally only possible to determine the line to a certain level of precision. I will attempt as far as possible to follow the high ground and avoid crossing running water, but where, for example, the route crosses a boggy col (as it frequently does!) I won't be too obsessive about finding the absolutely correct line.
Another issue is where the course of the watershed has been changed by human activity. There are many reservoirs and quarries on the hilltops, as well as culverts, canals and other channels diverting the actual course of the water. The natural line of the watershed is ideally the one to follow, but there are cases when this has been so altered that it becomes impossible to locate or impractical to follow.
Furthermore I think it's only sensible to assume that further information or opinions may later emerge which suggest a different line for some part of the route. These are unlikely to significantly change the route outside very localised areas, so I'll be satisfied aiming for a best-effort approach with all the information currently available.
Settlements along the route
The route is for the large part rural, frequently remote. In the Scottish highlands, very few settlements are even approached. The watershed frequently avoids large settlements, many of which have, of course, been established in locations with a fresh water supply. The way the route curves around the valleys wherein lie towns and cities like Glasgow, Manchester, Stoke-on-Trent, Coventry, Cheltenham, Basingstoke and Crawley is evident.
However, there are a few major exceptions. Most significantly, the centre of Wolverhampton is directly on the route. Dudley and other parts of the Birmingham conurbation are also passed through. Further south-east, Crowborough is notable for actually containing a Marilyn within the built-up area of the town, which lies on the route. Elsewhere in densely populated (and lowly-elevated) southern England, the route is rarely far from civilisation.
Previous watershed walks
As far as I can tell, nobody has previously completed nor attempted this particular route. Nevertheless it may have been done before but either not documented or not become widely known.
However, a number of similar walks have been undertaken, mainly listed on the Wikipedia page for the Scottish watershed. Notably among these, Dave Hewitt appears to have been the first to walk the Scottish watershed to Cape Wrath, Malcolm Wylie walked from Duncansby Head to Land's End over the course of fourteen years, and Peter Wright walked (and thoroughly documented) the route from Peel Fell to Duncansby Head in sections during one year (see Ribbon of Wildness).