May 2021 – An update

Despite appearances and the best part of a month of rain and high winds, I have managed a couple of ringing sessions at SML. They were so unproductive that I struggled to motivate myself to update the blog. Five hours on the 7th May resulted in just 12 birds, only four of which were new and another five hours yesterday, (27th May), was only slightly better, producing seven new birds and fifteen re-traps.

A high proportion of re-traps is to be expected during the breeding season, especially as the spring migration seems to have come to an end. The best of these was a Reed Warbler first ringed as a juvenile on 9 August 2014 and, even better, a second Reed Warbler, originally ringed as a juvenile on 7 August 2010, almost 10 years and nine months ago, meaning that the bird has made an astonishing twenty-two trips between the UK and Africa. There is still some way to go to beat the BTO record age for the species of 12 years 11 months 21 days.

In between net rounds, I continued to gather evidence to support my case against the excessive dredging of the drainage ditch on the northern side of the reserve. Historically, an increase in the area of reed at the eastern end of the Ley was much publicised, although this was actually the direct result of fencing the perimeter and the exclusion of livestock and grazing rather than management of the water levels. The loss of almost 14,000m2 of reed, since the year 2,000, due to drying out of the reedbed has, understandably, generated no publicity at all from the reserve manager. One result of the lowering of the water table has been the uncontrolled spread of Hemlock Water Dropwort at the expense of reed. The photo below compares the middle Ley in 2015, with about 50% reed, with the situation yesterday and clearly illustrates the total loss of reed in the area. The yellow line of reed in the right-hand picture marks the demarcation zone between the dry eastern part of the reserve and the wettest part of the central Ley, (at about 650m from the eastern boundary.

Looking SW across the Ley in 2015 (left) and 2021 (right)

If I’m honest, after the photos have been scaled to fit on the screen, it requires a bit of imagination to see the difference between the two images. However, thanks to Google Earth, the following aerial shot better illustrates the point I am trying to make. Taken on 30th May 2020, a time of year when the Hemlock Water Dropwort has already reached between 1-1.5m in height and is starting to produce flower buds, whilst the reeds have only reached about 30cm, the paler, silvery green areas show the extent of the dropwort in the dryest region between the public footpath and Marsh Ride, rapidly decreasing in density to the west as it reaches those areas, which are regularly inundated during flooding events. Unsurprisingly, it is particularly abundant along both sides of South Milton Stream, where the elevated and drier banks produced during path construction, together with the spreading of nutrient-rich spoil and fragmentation of its tubers during ditching operations has allowed the species to proliferate.

The extent of Hemlock Water Dropwort (paler green areas) on May 30th 2020 (Image from Google Earth)

In recent years, in collaboration with English Nature, we have been allowing the western end of the drainage ditch to silt up naturally and there has been some improvement, particularly in the central 500m. Exceptional rainfall and a persistent high sandbar in has enabled sediment to deposit in the lower reaches, where the flow is impeded by water impounded behind the sandbar.

The reserve manager has always had a free hand at SML and his work proposals have often been approved without question. Whilst his experience as a groundsman has been useful in managing the grassland and hedgerows around the perimeter of the reserve, his lack of understanding of the principals of hydrology and a preference for “big” engineering projects has meant that almost every intervention involving drainage has had a negative effect on the water table and impacted on the quality and extent of the reedbed. As Nick Baker commented, in his Sept 2019, BBC Inside Out program featuring SML, Hemlock Water Dropwort has been able to outcompete reed over much of the central reserve. Stands of Greater Reedmace have also disappeared as the water table has fallen. The previous dredging was both unnecessary and excessive but the manager seems unwilling to accept this or that there have been adverse impacts.

I am concerned that, once again, the reserve manager is proposing to dredge this channel later this year and I have been lobbying Devon Birds’ Council to ensure this doesn’t happen. The figures below show the surface elevation along the length of the reserve from Mill Lane in the east to the footbridge on the South West Coastal Path in the west. The shaded pink area in the first figure illustrates just how much the water table was lowered following dredging in 1994.

The height of the water table at SML relative to the surface, pre and post dredging in 1994

However, the shaded brown area in the second figure shows the depth of the ditch as measured this month, following a gradual, slow improvement over the last couple of years. The steeper gradient at c. 800m coincides with the location of the boardwalk across the reedbed and marks the point where the flow downstream meets water impounded when the sandbar is intact, allowing sediment to build up. It is likely that the gradient below this point is actually shallower than illustrated but the boundary path deviates to about 15m away from the ditch below the boardwalk and I considered it unsafe to attempt to measure the depth when operating on my own.

Realistically, it is unlikely that there will be much improvement upstream of the public footpath at 400m as dredging along the eastern length of ditch caused a fourfold increase in gradient and the consequent higher flow rate prevents sedimentation from occurring.

The height of the water table at SML relative to the surface in May 2021

Not dredging the ditch will allow sedimentation to continue and the shallowing stream should restore the natural floodplain in the centre of the reserve at the expense of Hemlock Water Dropwort and to the benefit of the reedbed and its associated ecosystem. At the very least, doing nothing and allowing nature to take its course, costs nothing and, if it all goes pear-shaped, can easily be reversed by a man in a hi-vis jacket and a very big digger!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s