May 5, 2013

Sedimentation and no Ranunculus in the lower River Wye at Lydbrook 2013

Water levels have been slowly falling and at the beginning of May are now 2″ and more below summer level. Clear water and bright conditions have allowed a full inspection of the stone and gravel  river bed through the Lydbrook shallows at the bottom end of Courtfield and Wyebank. As suspected the bed has significant sedimentation and hardly any ranunculus. From my experience I believe it is the only year in a decade or more when  ranunculus growth at this point in time has been limited to a few patches here and there. I believe  it is probably the result of protracted turbidity since May 2012 up to early 2013. More worrying is the extensive growth of blanket weed showing over much of the river bed combined also with extensive covering of blanket weed stifled by sedimentation.

Ranunculus whilst important in the habitat of the lower Wye for providing shelter for fish species, is vital for maintaining water levels. As flows drop through the late spring and early summer ranunculus grows and creates an expanding biomass, which acts as a natural dam, thereby raising and maintaining water levels, whilst maintaining daily oxygen levels. Without the ranunculus there is nothing to prevent the important shallows from rapidly drying up.

There must be a concern that with an enriched river bed, as demonstrated by the blanket weed, and a rapidly falling river that we may be facing a high risk of excessively low water, high algal blooms and de-oxygenation, if the weather moves back to low rainfall and high temperatures. There is probably no single greater issue on the river Wye at the present time than the appalling loss of soil from the land, being carried directly into the river with every flood, with the diffuse pollution it brings, combined also with storm discharge policies allowing untreated sewage and chemicals to flow freely into the river.