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The Hogsmill in July 2018

The weather may have made life uncomfortable for many people and beasts. But the Hogsmill river has

provided a small oasis of green amongst the increasingly brown landscape; and many insects, including some

tiny beetles, have been soldiering on close to the river enjoying the flowers in the blistering heat!

“This ragwort is jolly good!” (photo thanks to Pamela) ”I much prefer a drop of dandelion and burdock!”

This newsletter summarises the results of River Monitoring Initiative (RMI) sampling on the Hogsmill in July,

together with other pollution monitoring and River-related activities and events.

The RMI is a national scheme for monitoring the health of rivers. Volunteers undertake regular surveys using

a standard net sampling technique to count the number of certain “water quality sensitive” invertebrates.

An overall “score” is then calculated. A sharp fall or a drop below a “trigger” level could indicate pollution.

This can then be reported to the Environment Agency (EA) to enable further investigation.

Misconnected wastewater pipes and cross-connected sewers are a major source of pollution in the Hogsmill.

Under a Pollution Patrol scheme organised by the South East Rivers Trust (SERT), volunteers undertake

regular assessments of the outfalls thought most susceptible to pollution, the results being reported to EA

and Thames Water (TW) to help steer remedial work.

If you see pollution along the river or indications of possible pollution, such as dying fish, please call the EA

Hotline: 0800 80 70 60; and if possible take a photo.

The Hogsmill in July

Two months with negligible rain have had a significant impact on the water in the Hogsmill. The flow looks

to have been particularly affected, though this is hard to assess accurately. But so far the impact on water

depth seems to have been less than might have been feared; and has varied between stretches of the river.

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The average water level at the Worcester Park gauging station fell by about 15 cm between the end of May

and mid-July. But it was still slightly higher than in early July last year; and it was given a large temporary

boost at the end of July when a burst water main in Chessington sent a surge of water down the Bonesgate.

At Ewell the level has also fallen this month, but it is still around that at the start of the year when SES Water

began increasing the rate of augmentation. The recorded level at Kingston has risen slightly since early June,

presumably because of increased discharges of treated water from the Sewage Treatment Works.

At our RMI sites the reductions in depth generally appear small: 2cm or less since May at those where we

measure levels. At some, notable the Storm Tanks and Bonesgate, the main effect of dry weather seems to

have been to narrow the channel. But at the others there is still a reasonable width and depth of water.

Riverbed and surface weed have become more extensive in recent weeks, but generally the water has

remained clear; and the scum and cloudy water that appeared widely last summer has so far been absent.

Ewell Storm Tanks (photo thanks to Toby) Bonesgate

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Met

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Average Daily River Level

Worcester Park

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Average Daily River Level

Ewell Kingston

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Berrylands Middle Mill Green Lanes Ewell (photos thanks to Bill, Phil and Pamela)

There have been a few pollution incidents this month. A blockage in the system caused sewage to divert

through the A3 “concrete box” outfall. This has been fixed and the river aerated, though the outfall itself

still looks horrible and a dead fish was found. The burst main led to a surge of water across farmland and

into the Bonesgate, which turned yellow-brown; further dead fish were found soon after, though it is not

clear if this was related. A sewage spill is also reported near the Mills in Ewell, but we don’t have details.

A3 Concrete Box Hogsmill at Old Malden after the “burst” Dead fish in the Hogsmill (photo thanks to Alison)

The effect of the dry spell on “hotspot” outfalls has been mixed. Some have looked unusually benign,

perhaps because the normal flows of surface water have dried up “trapping” any pollution further down the

pipes. Others have looked particularly bad, such as those by the New Malden Railway Bridge and the south

side of the A3. But Sheephouse Way, which has not only had a recent “spurt” of pollution but has also had

the dirty water from the water main burst flowing past, is July’s “horrid outfall”.

Malden Way South Sheephouse Way

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Hogsmill RMI in July

6 sites were surveyed between the 13th and 21st of July. All scores were in the “middle-upper” “6” to “8”

range, comfortably above the “trigger”; and the overall average score was higher than last month and above

that in most months in the past 2 years.

The results need to be qualified: some counts have fallen to the bottom of scoring bands; and narrower

channels may be concentrating invertebrates, making it more likely that they are caught. Other water quality

indicators are mixed: we haven’t seen a large rise in species, like hoglice, that thrive in poor water, but the

number of dead fish is worrying. So far though, and perhaps helped by water company augmentation, the

impact of the weather looks less than might have been expected, but summer still has some way to go!

Some notable features:

• Lowers scores than last month at both the Green Lanes sites with a massive drop in olive numbers.

Although the water coming from the upper Hogsmill still looks very clear, weed is increasingly

covering the water making sampling difficult and perhaps also impacting on critter numbers;

• An 8” at Ewell Storm Tanks, maintaining the improved results seen here since the spring. This is

surprising as the site appears the most vulnerable to reduced water in the river. Perhaps the extra

augmentation of Bourne Hall Lake has helped, though the need to adjust the sampling location to

adapt to the “shrinking river” may be a factor; and

• improvements in scores at the other 3 sites compared to June, though these mainly reflect quite

small changes in counts and unusually high cased caddis numbers at Berrylands and Bonesgate.

The detailed results are below:

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GL Upstm GL Downstm Ewell Stm Tanks Bonesgate Berrylands Middle Mill

Total Score Hogsmill July 2018

Olives Caseless caddis Cased caddis Gammarus

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Within the overall scores, there were some large changes in counts for some species, as well as some unusual

and in one case very worrying “other catches” in the nets:

• Olive numbers fell sharply at most sites to little more than a quarter of last month’s overall total,

though this is probably partly seasonal as there was a similar fall last summer. By contrast, gammarus

numbers recovered from June’s drop; and were back to well above the long-term average;

• More than 10 cased caddis were found at 5 different sites this month, a first for the Hogsmill, with

both tiny “sticks” and large ones with elaborate cases being found;

• We found impressive specimens of alderfly larvae and water scorpions at Green Lanes;

• The worrying catch was of a juvenile signal crayfish at the Storm Tanks; another one has been seen

at the Berrylands Sewage Treatment Works. As well as wiping out native crayfish, they burrow into

river banks causing erosion and sediment and are a voracious predator of fish, frogs and

invertebrates, so hopefully something can be done to stop them spreading along the Hogsmill.

“My case is greener than yours!” “Shall we dance?” “A bad signal at Ewell” (photo thanks to Toby)

Summer along the Hogsmill corridor

The meadows along the Hogsmill corridor that were particularly green and colourful in the spring have now

turned to faded brown with small patches of summer flowers just about hanging on. But in and alongside

the river there is still an abundance of green, as water plants have thrived in the hot weather.

”Dry Longmeadow!” “Watersedge oasis!” “Is the Hogsmill somewhere here?”

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The sun has also brought out the insects. July has been a good month for butterflies: large numbers of

gatekeepers and meadow browns, along with blues and whites, have been enjoying the flowers; others such

as ringlets, coppers, brown argus and marbled whites have been flitting around the meadows; and brown

and purple hairstreaks have put in an early appearance. Other colourful little critters have also been out.

“Two’s company!” “Isn’t there room for a little copper?” “.. or for a little blue?

“I’m really brown!” “… and I’m really purple” “but what am I?” (photos thanks to Robb)

Several species of dragonfly and damselfly have been seen. On a walk along the Tolworth Brook and the

Hogsmill led by Tony from London Wildlife Trust, we were treated to the antics of demoiselles, darters,

hawkers and more, as well as some “blue activity” on the new pond in the Berrylands Nature Reserve.

Unfortunately, the summer has also brought out a less appealing critter: the litter bug. But during London

Rivers Week, SERT organised a group of volunteers to clear a stretch of the river in Kingston that had been

badly fly-tipped. They removed a motorbike, furniture, a shopping trolley, bicycles and many smaller items.

“Lay your eggs under that leaf, dear” “I’d hoped it would be a Harley…” (photo thanks to Jess)

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Plenty of activities are planned along the Hogsmill corridor over the next few weeks. If you fancy some

exercise and refreshment this weekend, come and join the Malden Manor Community Group for a “Fun Run

or Walk” around the meadows and woods besides the Hogsmill in Old Malden followed by a picnic. This is

being held from 11 a.m. to 2 p.m. on August 4th: details are on the Group’s website: maldenmanor.co.uk. If

you’re lucky you might catch a glimpse of our beautiful exotic visitor from the East who often flies around

Six Acre Meadow; and you never know who might drop in for the picnic!

“I’m a red-billed blue magpie!” “Will there be any honey sandwiches?”

If you’re fascinated by bats, Alison will be leading a bat walk along the Hogsmill on August 7th starting at 9

p.m. from the bridges by the Bonesgate confluence in Tolworth. If you can’t make that, Elliot is organising a

walk at the Berrylands Nature Reserve starting at 8 p.m. on August 15th. Alison and Lucy will also be leading

a walk through the ancient droves of Tolworth Court Farm on August 12th starting at 11 a.m. again meeting

at the bridges by the confluence. Details of these walks are on FB: facebook.com/Tolworth treasure and on

facebook.com/BerrylandsNatureReserve

As part of this year’s Thames Tidefest, TW will be holding an Open Day at the Hogsmill Nature Reserve from

11 a.m. to 3 p.m. on September 2nd, with birdwatching, pond-dipping, wildlife talks and much more. Details

of this and other Tidefest events are on their website: thamestidefest.net

Comments on this newsletter and suggestions and contributions for future editions would be welcome.

Please send these to Peter Short: rpetershort@hotmail.com