The impact of stream support on the hydrology and macrophytes of the upper Bristol Avon

Abstract

Low river flow due to water abstraction has impacted the macrophyte communities on several rivers in England. Stream Support is a method of alleviating low flows by augmenting water supply to the headwater channels of rivers when flow falls below a trigger level. This project has studied that the effect Stream Support has had on the hydrology of the upper Bristol Avon, and the consequent impact these changes have had on the macrophyte communities of the river. The Winterbourne classification methodology was used to investigate hydrological effects on macrophyte communities since the early 1990s. Stream Support was found to have successfully targeted low flow conditions with the annual Q95 measure of low flow closely associated with augmentation (Tetbury: R 1⁄4 71%, p , 0.001; Sherston: R 1⁄4 78%, p, 0.001). Decade long Q95 (discharge exceeded 95% of the time) measures of low flow on the river were shown to have risen in 1998–2007 by 60% above measures for 1978–1987 and 1988–1997. Aquatic species such as Ranunculus penicillatus pseudofluitans had increased coverage since the 1990s by as much as 40%, but wetland species such as Myosotis scorpioides and Mentha aquatica had marginally lower coverage. These changes may be partly attributable to a reduction in low flow conditions. Stream Support may not be sustainable if global warming leads to annually occurring prolonged periods of summer drought.