In commencing this section it will be well to devote a few lines to the proper meaning of the frequently misunderstood term “watershed.” It means the line of water-division, or water-parting; that is to say the line along a range of hills from which the streams flow in one direction on one side, and in the opposite direction on the other. The ridge-tiles on a roof form an excellent illustration of a watershed.

Every local stream is separated from the one nearest to it by a watershed; while a river-system, such as that of the Thames, is separated by a much more important watershed from the other river-systems which take their origin near its source. Such river-systems, each enclosed by a single watershed which it shares with its neighbours, are known as drainage-areas, or catchment-basins.

Practically the whole of Hertfordshire lies in the Thames drainage-area. At first sight it would be natural to suppose that the summit of the line of chalk downs forming the continuation of the Chiltern Hills in the neighbourhood of Tring and continuing thence to Dunstable would constitute the watershed between two river-systems. But this is not the case, for the Thames cuts through the Chiltern Hills between Wallingford and Pangbourne, and thus receives the drainage of both the northern and the southern flanks of that range. The watershed formed by the continuation of the Chilterns in the Tring district is therefore one of second-rate importance.

On the other hand, in the chalk hills near Hitchin we have a watershed of first class, or primary rank, for it divides the Thames catchment-area from that of the Great or Bedfordshire Ouse, which flows into the North Sea miles away from the estuary of the Thames. Only four comparatively small streams flowing into the Ouse basin lie for part or the whole of their course within the limits of the county. The first of these is the Pirre, or Purwell, a small brook which rises in the parish of Ippolits, and, after passing Much Wymondley, flows into the Hiz near Ickleford. The Hiz itself rises in a spring at Wellhead, a short distance southward of Hitchin (formerly also called Hiz), and after receiving the Purwell, flows to Ickleford, where it leaves the county, being joined in Bedfordshire by the Ivel, which rises not far from Baldock, passing Biggleswade to join the Ouse. Below Biggleswade it flows through Tempsford, where it unites with the Ouse. The last of the four streams belonging to the Ouse system is the Rhee (a Saxon term signifying a water-course or river), which springs strongly from the chalk a short distance west of Ashwell, and after passing Accrington Bridge and crossing the Ermine Street, eventually falls into the Cam.

The whole of the remaining rivers of the county belong to the Thames catchment-area. With the exception of the Thame, to be mentioned immediately in a separate paragraph, these form two main systems, namely that of the Colne draining the western and that of the Lea the northern and eastern part of the county; the watershed between these two systems running in a north-westerly and south-easterly direction between St Albans and Hatfield, and thence to the north of Chipping Barnet.

The Thame is almost entirely a Buckinghamshire river, but it rises in our county and runs on the north side of the great watershed formed by the continuation of the Chiltern Hills; this watershed constituting a broad, nearly waterless belt separating the catchment-area of the upper Thame from that of the Lea. The Thame itself springs from three heads in the parish of Tring; the first of these rising near the vicarage, the second at a spot called Dundell, and the third in a spring known as Bulbourne. The three become united at New Mill, whence, after passing through Puttenham, the Thame flows by Aylesbury, in Buckinghamshire, and from there continues its course till it eventually joins the Thames near the village of Dorchester, a considerable distance below Oxford.

Of the tributaries of the Colne system, the most important is the Ver (or Verlume), which rises some distance to the east of Cheverell’s Green, on the Watling Street, and passes through the village of Flamstead, and thence near the line of the high road to Redbourn, where it is joined by an intermittent stream, or “bourne,” known as the Wenmer, or Womer. The latter crosses the road from Harpenden to Redbourn at the foot of a steep descent; and in the old days its appearance as a running stream was believed to forbode a death, or “some extremity of dangerous import.” From Redbourn the Ver continues its course by way of Shafford Mill, at which place it crosses the high road, to St Albans, where it passes between the present city and the ruined walls of ancient Verulam, to which latter it is generally believed to have given its name. Thence its course is continued through the old nunnery of Sopwell (of which it supplied the extensive fish-ponds), and from that point it flows through Park Street to join the Colne near Colney Street; the latter stream giving the name to the united rivers, although the Ver is considerably the larger of the two constituents. The Colne itself rises in the neighbourhood of Tittenhanger, between St Albans and Hatfield, and passes through London Colney on its way to join the Ver. Near Watford the Colne receives an important tributary in the shape of the Gade, which issues from the chalk in the meadows of Great Gaddesden (to which and Gadebridge it gives the name), and after passing through Hemel Hempstead and Nash Mills, is joined at Two Waters by the Bulbourne. The latter rises at Tring very close to the Bulbourne source of the Thame (the watershed being here very narrow), and runs by way of the Frith, Dagnalls, Aldbury Meads, Dudswell Bottom, and North Church to the north-east side of Berkhampstead, where its volume is increased by two important springs, and thence to Two Waters. There is also an intermittent stream known as the Hertfordshire Bourne, which, when running, flows into the Bulbourne about halfway between Berkhampstead and Boxmoor. It is reported—and apparently correctly—to flow on the average once in every seven years; the point from which it flows may be higher up or lower down in the valley according to the amount of water discharged.

Below Two Waters the Gade (as the united stream is now called) passes through Kings Langley, Hunton Bridge, and Cassiobury Park to join the Colne between that park and Watford. After skirting the north side of Moor Park immediately below the last-named town, the Colne receives the Chess (giving the name to Chesham, in Buckinghamshire), which passes through Sarratt in our own county. The Colne then reaches Rickmansworth, where it forms the boundary between Buckinghamshire and Middlesex, and thence flowing by way of Uxbridge and Colnbrook, discharges itself into the Thames above Staines, after a course of about thirty-five miles.

We pass next to the basin of the Lea or Luy, the Logodunum or Logrodunum of the ancient Britons, and the largest river in the county. The Lea itself takes origin in a marsh at Leagrave, or Luigrave, a little north of Luton (= Lea-town), in Bedfordshire, and after passing through Luton Hoo, in that county, where it is expanded into a large artificial lake, enters Hertfordshire at East Hyde, in the valley north of Harpenden, and flows thence by way of Wheathampstead and Brocket (where it again expands into another artificial lake) to Hatfield, whence its course is continued by way of Essendon and the north side of Bayfordbury Park to Hertford. Just before entering the county town the Lea receives the Mimram, or Marran, which rises in the parish of Kings Walden, to the north-west of the Bury, and flows to the eastward of Kimpton Hoo, and thence by way of Codicote, Welwyn, Digswell (with added supplies from local springs), Tewin Water, and Hertingfordbury to its junction with the Lea close to Hertford. In the upper part of its course the Mimram receives the small brook known as the Kime, from which Kimpton and Kimpton Hoo take their names.

In Hertford the Lea is divided into two channels, one of which runs through the eastern portion of the town to cross Great Hertham common, while the other, which is navigable, passes along the western side to join its fellow at the aforesaid common.

At about one-third the distance between Hertford and Ware the Lea is largely augmented by the waters of the Beane (Benefician) and the Rib, which join close to their union with the main river. The Beane rises from a ditch in the parish of Ardeley (between Stevenage and Buntingford), and thence flows in a southward direction by way of Walkern, Aston, Frogmore, Watton, and Stapleford, passing on its way through Woodhall Park, at the entrance to which it is reinforced by several strong springs. Between Watton and Stapleford it also receives a small brook flowing from Bragbury End.

Starting in an easterly direction from Ardeley or Walkern, we shall cross the low watershed dividing the valley of the Beane from that of the Rib; which latter takes its rise near Reed, and after crossing the Ermine Street at the south end of Buntingford, flows by way of West Mill and New Bridge to Braughing, where it is joined by the Quin, which issues from a spring at Barkway, and passing by Hormead and Quinbury (to which it lends its name), reaches Braughing Priory. Below Braughing the Rib, as it is now called, flows by way of Hammels, Standon, Barwick, Thundridge, to cross the Ermine Street at Wade’s Mill, and thence vi Ware Westmill and Ware Park to join the Lea with the Beane.

From Luton to Hatfield the course of the Lea pursues a generally south-easterly direction; at Hatfield it becomes for a short distance due east, and then trends to the north-east through Hertford to Ware. Here it takes a sudden bend, so that the remainder of its course through the county and the adjacent portion of Essex is almost due south. A direct line from Luton to the Lea near Waltham Abbey measures about 40 miles; but, owing to its great north-easterly bend, the course of the Lea between these two points is something like 45 miles.