The Hogsmill in August 2020
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The Hogsmill in August 2020 There’s been a touch of the tropics along the Hogsmill corridor this month: heat, humidity and storms, with spectacular skies in the sun but dramatic surges of water down the river and over the banks after the rain. Tolworth Court Farm Ewell (photo thanks to Brian) The extreme conditions must have been a shock to wildlife: river invertebrate “scores” fell sharply in our surveys this month; and insects struggled as plants wilted in the heat, leading to “close-sharing” and even the “dreaded” balsam proving valuable. But some critters just puffed up their feathers and enjoyed the sun! This newsletter reports on volunteer activities to monitor and restore the habitat along the Hogsmill river, working with the South East Rivers Trust and local groups, and alongside the Environment Agency (EA) and local water companies, mainly Thames Water (TW). The Hogsmill in August With negligible rain up to mid-August, the water level in the Hogsmill again fell steadily. In the upper reaches, with aquifer levels still relatively high, it appears to have stayed just above recent summer lows. But downstream, with slow flows down the tributaries, there were large stretches with uncovered gravel banks or stagnant low water. Also the heat led to unusually high water temperatures, probably explaining large patches of weed and algae along the river. This combination is of concern for water quality and river critters, though at least some of our regular fish “colonies” appeared unscathed. 1 Groundwater Level at Chipstead Average Monthly River Level 105 0.5 0.4 100 0.3 0.2 95 Metres 0.1 Level (mAOD) Level 90 0.0 85 Jul-18 Jul-19 Jul-20 Jan-19 Jan-20 Sep-19 Sep-18 Ewell Bourne Hall Nov-19 Nov-18 Mar-19 Mar-20 May-19 May-20 Old Malden Lane Middle Mill River Club Conditions changed dramatically on August 13th when a localised storm dropped over 20 mm of rain in an hour in Ewell. This led to a large short-lived surge of “black” water down the river: its level rose by over ½ metre in an hour but a couple of hours later over half had been reversed and two days later it was almost back to its pre-surge level. But the storm left a legacy of big shifts in contours of the riverbed, damage to bankside vegetation and trees dislodged and left tilting over the river. Depth at Worcester Park 1.7 1.6 1.5 1.4 1.3 Metres 1.2 1.1 1.0 Time of day 13th August Green Lanes Ewell Green Lane Bridge New Malden 2 It has been another bad month for pollution. Some “hotspot” outfalls had what now seem to be regular but usually short-lived “bouts” of pollution, and some others had new TW booms around them so may also have had pollution “episodes”. The storm appears to have caused more overflows at the Storm Tanks. Rose Walk A3 South Ewell Storm Tanks (photo thanks to Brian) But this month’s “horrid outfall” is Sheephouse Way, which in one week experienced a “triple-whammy” of pollution from sewage, “black” water from the storm and silt from another burst water main. 8th August 13th August 15th August If you see pollution along the river or indications of possible pollution, such as dying fish, please call the Environment Agency Hotline: 0800 80 70 60, and ideally take a photo. Hogsmill RMI in August The River Monitoring Initiative (RMI) is a national scheme that uses counts of “water quality sensitive” invertebrates to assess river health. Our 6 main-river sites were surveyed between 12th and 15th August. Results were poor everywhere. Scores fell at all sites compared to July: by 2 points on average. The overall average was the lowest since 2018. The “strange and erratic” water conditions this month are likely to have been the cause but, as our surveys straddled the “surge”, precise analysis is hard. 3 were sampled before the storm in low water levels and 3 were surveyed soon after and, while levels had fallen back, the impacts of the storm were clear with shifts in gravel beds and weed and “dirtier” water than usual, all of which could have affected counts: 3 Pre-storm Bonesgate Middle Mill Ewell Storm Tanks (photo thanks to Sivi) Post-storm Berrylands Green Lanes Ewell (photos thanks to Pamela) • there were breaches of our “trigger score” at Bonesgate and Ewell Storm Tanks. Both were sampled before the storm in water levels well down on July. While the water looked clear and there was a reasonable flow, its temperature was 20o C at both, which was well above normal and could have had an effect on numbers. It is possible that there could also still have been a lingering effect from the pollution incidents in the previous couple of months; • the Green Lanes Ewell sites were surveyed after the storm and its impact on the “contours and vegetation” of the river was very obvious. But while the scores were down they only dropped by “1 point” and the changes in counts were quite small, apart from olives. Fish numbers were also unusually high upstream; probably they’d been swept downstream by the strong storm flow; • scores were down by more - “2-3” points - at the 2 downstream sites, again mainly from lower olive counts. But this was from what were unusually high scores in July and the scores were still above the historical average. At Middle Mill, where the water level looked particularly low, sampling had to be done in water resembling “pea soup”, which could have led to some undercounting; • the stand-out feature this month was the collapse in olive numbers: well down at all sites and with those found mostly tiny or very sluggish. While counts have tended to be relatively low in the summer, perhaps a life-cycle effect, the total of barely 100 for all 6 sites was less than half of the previous low. By contrast, gammarus numbers were up overall compared to July and high historically, albeit mainly from high counts at the 2 downstream sites. 4 Hogsmill RMI Scores August2020 12.0 10.0 8.0 6.0 4.0 RMI Score 2.0 0.0 Jul 20 Jul 20 Jul 20 Jul 20 Jul 20 Jul 20 Aug 20 Aug Aug 19 Aug 19 Aug 20 Aug 19 Aug 20 Aug 19 Aug 20 Aug 19 Aug 20 Aug 19 Aug 20 Aug Average Average Average Average Average Average GL Upstm GL Dwnstm Storm Tanks Bonesgate Berrylands Middle Mill Olives Caseless caddis Cased caddis Gammarus Late summer colour along the Hogsmill It’s been a strange and short summer along the Hogsmill. A wet winter and a dry spell afterwards seem to have upset nature’s clock. Some species emerged two weeks or more earlier than usual, but had relatively short lifespans as plants shrivelled quickly with the lack of rain. Even in early August there were signs of summer ending: abundant autumn berries and harvesting of meadows a month earlier than usual. But there’s still been plenty that’s colourful to enjoy along the Hogsmill corridor in August, despite what seems not to have been a particularly good year for much wildlife. There was a range of butterflies to see, including hard-to-spot hairstreaks and other colourful species, and damsel and dragonflies that had seemed scarce earlier on re-emerged in the hot weather, along with other brightly-coloured critters. 5 (photos thnks to Robb and Pamela) The predominant colour in the riverside meadows has been brown this month, as grass and plants have faded early. But this has highlighted the bright colours of the plants that have managed to flower and of the berries that also seem early but look to be heading for a good crop this year. 6 .