The Fleet starts at the highest point in inner London which is the top of Hampstead Heath. It starts here because the clay hills are capped with sandy soil - specifically the Bagshot Formation and the Claygate Member - thus groundwater emerges from springs at the boundary between permeable and less permeable strata. There are several sources, all of which join up to form a western branch and an eastern branch which remain separate for the best part of two miles before joining together in Kentish Town. The highest point is just below Whitestone Pond, where a raindrop landing to the south will end up in the Westbourne, to the north in the Brent and to the east in the Fleet. And here we find the first of a dozen ponds, each originally a reservoir dammed to provide drinking water for folk further down the valley.
This is the Vale of Health, which sounds delightful but was originally a patch of boggy marshland called Hatchett's Bottom, rebranded when the reservoir was built in 1777. A throwback cluster of well-to-do villas nestles on the upper flank, the first of several places on this river walk that IYNBYRS (if you've never been you really should). Normally you'd see anglers dipping rods around the perimeter, but all fishing on Hampstead Heath was temporarily suspended last month due to a suspected outbreak of koi herpesvirus, a notifiable carp killer. The animal you're more likely see in the water is a happily-splashing canine, perhaps chasing a stick, this being the only Hampstead pond designated an official dog swimming area. And in the corner is a low brick-cased grille, currently dry above the water level but when overtopping occurs its sends the fledgling River Fleet on its tumbling way.
Everyone assumes the Fleet is a lost river but if you find the right bit, which is this woodland at the top of Hampstead Heath, you can gambol alongside the stream in an almost-natural state. Just below the Vale of Health Pond the slopes are steep and impenetrable with vegetation, also somewhat boggy, but veer off the main path a little further down and rivulets plainly trace a dip in the earthy banks. Despite all the rain we've had there's no flow at present, merely occasional damp mud, but the Fleet was more plainlyvisible in August 2005 in a shade of dirty chalybeate brown. A few minutes into the woods a separate tributary from the Viaduct Pond joins the fray (that's the pond you can cross on a high viaduct, once used for transporting wagonloads of bricks, which was dammed in 1846) and the muddy channel becomes a tad wider.
Since I was last here a 5m-high earth dam has been added amid the woodland, creating a 'catchpit' to fend off worst-case flooding scenarios, although you'd barely realise because the low hump's already been disguised by vegetation. Beyond this a wetland scrape briefly intrudes and then the Fleet makes its final appearance as a lazy linear stream because from here on it's all ponds and culverts. First up is the Mixed Bathing Pond, a deep facility for competent swimmers only, with a jaunty non-secretive vibe. It's currently 19°C in the water and £4.80 a dip. The next pair are Hampstead No.2 Pond and Hampstead No.1 Pond, thankfully neither named after unwelcome floating content, by which point we've reached the tip of the Heath near the shops at South End Green and it's time for the Fleet to vanish permanently underground. So let's switch tributaries.
My favourite Fleet source can be found just below the Henry Moore sculpture on the upper lawn at Kenwood House. A notch of woodland dips down from the main terrace, within which a dry path eventually merges into a boggy brown squelch. This is one of the springs to be found at the sand/clay borderline on the Heath, and perhaps the easiest to access. From here a shallow furrow meanders across the picnic lawn - a brilliant feature once you've worked out what it is - before trickling into a mighty pond at the foot of the bank. It's this large because a separate tributary feeds in from a peaty sphagnum bog in the West Meadow. The adjacent Thousand Pound Pond was also constructed in the late 18th century and features a sham bridge, a low bright arc designed to look decorative and convincing from the front but from round the back you can see it's just a propped up slice of balustraded planking.
From the boardwalk at the next crossing point I finally caught sight of a shallow watery channel through a screen of oak and holly, but generally anything that might count as a stream on this branch of the Fleet is securely fenced off. Instead what's coming up is a chain of six more ponds linked by culverted flow, each originally a reservoir and now used for a variety of different purposes. First comes the Stock Pond, one of the smallest and most natural-looking, which was hard to see twenty years ago but they've since removed 24 trees to make way for a strengthened dam. A survey for the City of London in 2013 confirmed that the Stock Pond was only capable of withstanding a 1-in 5-year flood, a consequence of its low capacity and relatively large catchment, whereas by contrast Vale of Health Pond could withstand a 1-in-1000.
50 years ago this month Hampstead Heath was hit by a 1-in-20000 year rainfall event, the infamous thunderstorm of 14th August 1975. 170mm of rain were recorded over a 24 hour period - that's almost seven inches - most of it between 5.30pm and 9pm. It's still the largest daily rainfall total ever recorded in London, indeed anywhere in southeast England, and because it hit an urban area caused considerable damage to property. According to news reports the storm dropped hailstones "the size of marbles", flooded roads, crushed cars, closed several stations, delayed the Proms and also led to the death of a pensioner, although that was from overexertion while moving furniture in a basement flat in Brondesbury. The flooding was extremely localised with 'only' 4 inches at Highgate, 1 inch at Highbury and barely a splash in Hackney, and we only have accurate measurements because the Hampstead Scientific Society happened to have a weather station in precisely the right location. It seems highly unlikely that such extreme rainfall would ever hit the same spot again, but the City of London have spent millions raising andstrengthening several dams on the upper Fleet just in case.
Next down the Highgate branch is the Ladies Bathing Pond, the most secluded of all. The sign outside still says Women Only, as it did 20 years ago, but now adds that "those who identify as women are welcome" and that a public consultation on future admission policy is being prepared. With three bathing ponds hereabouts - a Women's, a Men's and the aforementioned Mixed - let's hope the interfering gender-obsessives don't get their blinkered way. The Men's Bathing Pond is more open to view, especially from its newly raised dam, with betrunked swimmers visible on the jetties and bobbing heads looping in the admittedly distant waters. Inbetween come the Bird Sanctuary Pond and the Model Boating Pond, the latter one of the largest on the Heath if sadly devoid of young boys whipping toy yachts these days. The causeway to its central island is being removed this month to boost its credentials as a nature reserve.
Concluding the sequence of reservoirs is Highgate No. 1 Pond. It's neither the prettiest nor the most accessible, but it does have the greatest capacity of any of the Ponds, which is good news for residents downstream facing potential inundation. A large overspill culvert lurks on its southern edge, considerably larger than the micro-drain at the Vale of Health, marking the last time the Highgate branch will be seen above ground. It heads off unseen towards Swain's Lane and Dartmouth Park, while half a mile to the west the Hampstead branch is disappearing underground too. What lies between the two is the Parliament Hill, the famous viewpoint, and those admiring central London from its summit generally fail to realise that the River Fleet is what cuts away the land on either side. Look towards St Paul's and you can see the cranes in Blackfriars where those waters finally reach the Thames, but it'll be a month before my account finally reaches journey's end.
Hampstead Chain: Vale of Health (105m AOD) / Viaduct (90m) → Mixed Bathing (76m) → Hampstead No 2 (75m) → Hampstead No 1 (71m) Highgate Chain: Stock (82m) → Ladies Bathing (77m) → Bird Sanctuary (73m) → Model Boating (72m) → Men's Bathing (68m) → Highgate No 1 (64m)
Fleeting
