Extreme Flows
Whilst floods and droughts occur seasonally, extremes of either have a profound effect on the life in a river. An extreme flood leads to huge movements of gravel, displacing and killing invertebrates and washing out fish eggs, and can cause landslips from the surrounding valley. Whilst in the medium term the gravel and rocks these floods introduce provide the future homes for the rivers’ inhabitants, in the short term they can smother and kill all life in a tributary.
Extreme droughts can be even more damaging. High water temperatures and low oxygen levels can seriously stress all life and in extremis, the river can dry up killing all those who live there. In time, the streams recover but recent changes to the way we manage the land, combined with the effects of climate change, have meant that these extreme events are increasing in frequency. More intense rainfall throughout the year leads to an increase in flooding whilst the extended hot dry periods of the summer mean that 5 of the worst droughts in the last century have occurred in the last 15 years. 1976 remains the worst on record for the Wye. A combination of low flows and the deoxygenating effects of dying rannunculus caused a massive fish kill in June that year.
It is very easy to blame the climate but the way in which we have managed the river catchments has made our rivers more prone to theses events. Water now runs off faster, leading to higher flood peaks but leading to low recharge rates for the natural aquifers. In the summer droughts, when the natural base flows from the aquifer should support the life in the streams, these flows are weaker and run out more quickly. We can do something about this by increasing the permeability of catchments through managing our land sympathetically.
Yet summer is the period of greatest demand for water. In theory, better water management including better control of abstraction provides at least part of the solution, but no plan can ever succeed against unlimited demand and ineffective abstraction controls.
