June 2017 - in this watery context , flashy means responsive up-down, on-off flows along rivers draining largely impermeable basins; not bling or strutting funky stuff. The impermeability come naturally - a hard rock basin with crystalline rock (the English Lake District), or older compacted sedimentary rock (the Pennines), clay in lowland areas, notably Wealden Clay in SE England on which the Mole, Medway, Arun and other south flowing Sussex rivers rise- or hard urbanisation involving paving, tarmac, concrete and culverting. Such rivers rise quickly (in under 12 hours) after rain, then drop back quickly once the rain stops, as seen in northern England this spring.
Usually people only notice that they are in a flashy catchment when rivers rise up and flood. Usually during the winter half of the year when the surface soils are more likely to be saturated. As it was in northern Britain in December 2015, and southern England and the English Midlands two years earlier. On such rivers, less intense summer rain can make a difference. Monday 5th June was one of the wetter days of the summer so far, with heavy rain southern England overnight 5th /6th June. Tuesday 6th was showery on the south coast, though there may have been more rain inland. On the Tuesday, I walked along the coast between Langstone and Emsworth. As when I visited this area at the beginning of May, the fresher, cloudier weather was a relief, punctuating otherwise prevailing dry weather. As I sketched on the shore, I reveled in the busy, fast moving skies, as showers passed over Portsmouth and Hayling Island, but just missed me. Clear skies and sunshine contrasted with dark clouds.
The following day, 7th June, I went to the Box Hill area, taking the train up the Arun valley to Horsham, then across the West Sussex watershed to Dorking, Denbies Vineyard and the Mole beneath Box Hill. With the tide picking up after the neaps, the Arun looked livelier than it had done from the train to London the previous weekend. I was surprised to see the current still strong at Pulborough; the effect of the rainfall on top of the tide, perhaps. The corresponding EA Rivers and Sea Levels page did indeed show a rise superimposed on the tidal curve for the gauged river levels at Pulborough.
The Mole at Castle Mill, Dorking, meanwhile showed a seemingly modest but significant rise, from almost bumping along the bottom at around 10cm deep to a peak of 39cm deep on the Wednesday morning. Enough to cover the Stepping Stones beneath Box Hill and turn the water a turbid brown. Normally in the summer it is shallow and fairly clear running here. By the time I headed home, though, the flow was quickly dropping back, the weather dry all day on Wednesday. The CEH / National River Flow Archive hydrographs for Dorking and further upstream at Kinnersley Manor show a perpetually rollercostering line. However, particularly when low, effluent makes up a high proportion of this flow. The Mole is among the most flashy rivers in England, naturally in part due to the impermeable Weald Clay near its source, but also because its upper catchment is very urbanised, with Crawley and Gatwick Airport. As well as rapid run-off from tarmac etc, urbanisation means more people, more people mean more effluent. Mostly treated sewage effluent, including the sewage works beneath on the SW side of Box Hill. During the 1990s, there water were pollution problems from run-off at Gatwick, where the Mole is banished in a culvert beneath the runway. I believe this has improved now and the water quality along the Mole is generally good (or at least it was 8 years ago, according to the most recently available data from the EA). According to one website I came across, at least one wild river swimmer was happy to swim in the lower reaches of the Mole in 2015. As far as I know and despite some very dry spells since, the last time the Mole ran dry was during the hot, dry summer of 1976 which hit rivers throughout Britain and Europe. I suspect the effluent contributions account for that.
This time of year in 2012, during the wet early summer, both the Mole and the Arun flooded. After the 2013 Christmas storm (called Dirk on the Continent, I think), the Mole was one of the first rivers to react to the heavy rain which came with the gales. On Christmas Eve and through to Christmas morning, the EA had a Severe Flood Warning (Danger to Life) at Leatherhead. Upstream, Burford Bridge Hotel beneath Box Hill was flooded. This involved about 75mm rain in 24 hours. Much less than in Cumbria during Storm Desmond two years later (over 300mm in 24 hours at Honister Pass), but this was more than enough to cause the worst flooding along the Mole for 45 years. The photos from Burford Bridge and around show the river totally out of its usual gentle character, with bridges damaged, cars overturned. There was further, though less damaging flooding in January and early February 2014. By then the ground was well saturated, prompting flooding even after more modest rainfall totals (40mm and under). As this year, though spring came early and it was largely dry and very warm. Unlike now, groundwater levels were high after the soggy winter. Being so flashy, though, the Mole was very quick to fall back. I recall the CEH said that its flow was "Notably Low" during March (monthly hydrological summary, March 2014). Around the same time, some parts of Hampshire still had groundwater flooding.
This time round (June 2017) responsive rivers in northern England responded to the rainfall. The Severn, as always slower to respond, picked up during the following week.
The middle part of June was very hot, particularly 19th-21st June. June 21st was the hottest June day in Britain since June 1976. My energy levels fell with the river levels, all the more with warm nights and the heat building through the day. The heatwave broke on 22nd June. Thankfully then cooler. More rain in Hampshire on 27th - 28th. The grey skies were a relief for my light-sensitive eyes, though admittedly a downer a mood otherwise. The changing weather, as always reflects the changing course of the jetstream, which has been meandering again quite a bit lately.
This time round (June 2017) responsive rivers in northern England responded to the rainfall. The Severn, as always slower to respond, picked up during the following week.
The middle part of June was very hot, particularly 19th-21st June. June 21st was the hottest June day in Britain since June 1976. My energy levels fell with the river levels, all the more with warm nights and the heat building through the day. The heatwave broke on 22nd June. Thankfully then cooler. More rain in Hampshire on 27th - 28th. The grey skies were a relief for my light-sensitive eyes, though admittedly a downer a mood otherwise. The changing weather, as always reflects the changing course of the jetstream, which has been meandering again quite a bit lately.
More information on the Mole
CEH / NRFA
Mole, Castle Mill, Dorking
Mole, Castle Mill, Dorking
Wild Swimming blog
http://outdoorswimmer.com/blogs/why-finding-a-snake-in-your-open-water-swimming-spot-is-a-good-thing
Reigate Grammar School - Flooding on the River Mole, with lots of other interesting stuff about the river more generally
My River Mole blogs 2013-14
