Recovery of headwater stream flora following the 1989-1992 groundwater drought

Over a three year period in spring, summer and autumn from 1993 to 1995 118 sites on 24 headwater reaches of groundwater streams were investigated. Ha lf were also surveyed at least once the previous year. Example rivers were chosen from catchments where there are no abstractions, where there are sma ll or large historical ones, and where reductions or variations in abstract ions were likely to occur during the study period. Key objectives of the study were to identify the effects of atypical low fl ows and bed drying on aquatic and wetland plants following an exceptionally long period of poor groundwater recharge between 1989-92. The extended dro ught, followed by two years of very good groundwater recharge, provided a u nique opportunity to determine the rate, and extent, of recovery. Such know ledge is crucial for conservation bodies, water regulators and water utilit ies to help them make judgements on the degree of impacts groundwater abstr actions have/could have on river flows, and separating these from natural c auses. The macrophyte survey data have been used to develop a classification syste m for headwater streams fed by groundwater. Thirteen different community ty pes were recognised, these giving a clear insight into the flora expected i n groundwater streams based primarily on flow periodicity and channel form. Determining the behaviour of individual species helps in environmental ass essment of proposed new abstractions, and allows accurate predictions on wh ich ones might decline, be lost or invade. It is equally valuable for predi cting the benefits of alleviation strategies based on target flows. Lesser water-parsnip, brook water-crowfoot, blunt-fruited water-starwort an d whorl-grass were identified as the most characteristic species of perenni al reaches of headwater chalk streams. Pond water-crowfoot only dominates i n reaches where flow fails for short periods in late autumn. Marsh foxtail is a classic indicator of winter wet/summer dry flow channels. The River Ver was one of the study rivers, for which environmental impacts from abstractions were well accepted. During the period of survey the River benefited from a low-flow alleviation programme, based on ceasing abstract ions from a headwater bore hole. Dramatic changes occurred to the flora in the river following this, these being greater, and sustained for longer, th an in any of the other 23 rivers surveyed. The research has clearly shown that there are very distinctive communities associated with streams in the upper reaches of groundwater catchments, and these can be correlated with different flow and physical habitat character istics. Some communities are very stable, and changed little during and aft er the drought; others are highly sensitive, and changed dramatically after flows returned to 'normal' at the end of the drought, or when impacting ab stractions were curtailed. In most systems flora returned to predicted stab le states within two years of normal recharge, even at locations where ther e had been no flow for several years. Copyright (C) 1999 John Wiley & Sons, Ltd.