A Field Guide to British Rivers. George Heritage
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Название: A Field Guide to British Rivers
Автор: George Heritage
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781118488027
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Newbold (1998) estimated there were originally some 2 000 000 ha of lowland floodplain in the United Kingdom; this had been reduced by 86% to 274 000 ha by the turn of the century. This degradation was aided by post–World War II land drainage grants which saw 84 000 ha drained with no measurable increase in agricultural yield (Purseglove 1988). Further insight into wider modification to floodplain areas was reported by Heritage et al. (2016) in their analysis of floodplain connectivity and land use on eight SSSI rivers in England and Wales. They found that even these high‐value watercourses have been significantly impacted by current and former engineering and management of the river and valley bottom. Floodplains along all eight watercourses exhibited a loss of geomorphic functionality and natural habitat due to farming. Publication of the 2015 land cover mapping for England allowed Entwistle et al. (2019a) to further investigate floodplain habitat change in England, analysing floodplain habitat composition over time to chart recent historic degradation. Data from 1990 showed that intensive agriculture occupied around 38% of floodplain zones expanding to 53% by 2000 before the rate of expansion slowed slightly to cover 62% in 2007. Between 2007 and 2015, this statistic remained relatively static (64%) with some suggestion that arable areas were being transformed to pasture. Wetland areas in the form of fen, marsh, swamp, and bog are key indicators of natural floodplain functioning, and these have been devastated over recent historic time with data sets indicating that these fundamental floodplain units have been all but lost. Upland and lowland areas are both severely impacted with a near ubiquitous loss of natural floodplain functioning.
Figure 1.1 Water Framework Directive status summary for UK Rivers (2008–2015).
Such statistics relating to river and floodplain degradation are repeated across other temperate areas. In Europe, fifteen years after the Water Framework Directive came into force, achieving its objectives remains a challenge; in 2012, the European Commission predicted that 47% of EU surface waters would not attain Good Ecological Status by the first cycle census date of 2015 (European Commission 2012). During the first WFD cycle, which operated from 2009 to 2015, the number of surface water bodies in “Good” state only increased by 10% (van Rijswick and Backes 2015). Interestingly, of the natural systems failing to achieve good ecological status, the primary reason in 40% of cases was hydromorphological pressure. The UK government statistics associated with the assessment of hydromorphological degradation suggest that the primary reason for in‐channel issues is engineering modification (Figure 1.2). These figures deserve further consideration as they only reflect what has been audited. Diffuse catchment pressures are also significantly impacting watercourse form and function but were simply not considered in the Water Framework Directive assessment process.
These statistics, although stark, should not come as a surprise. From a snap‐shot baseline sample of more than 5600 RHS reference sites across the United Kingdom and Isle of Man, Purseglove (1988) noted the following key points about the physical state of the 85 000 km of rivers and streams:
1 Very few pristine lowland channels flowing through semi‐natural landscapes remain;
2 Only 13.6% of lowland sites in England and Wales, 28% in Scotland, and 10.1% in Northern Ireland have an entirely unaltered channel;
3 3.7% of lowland sites in England and Wales, 1.7% in Scotland, and 5% in Northern Ireland can be classified as having severely modified channels;
4 Land drainage, flood defence, intensive agriculture, and urban development have significantly altered the channel shape and river landscape of many sites in the lowlands;
5 Extensive reinforcement and re‐sectioning of riverbanks, and channel impoundment can cause a significant reduction in habitat diversity;
6 Only 4.2% of sites below 50 m above sea level, with adjacent floodplain, have extensive wetlands, reflecting historical land drainage and current land‐use pressures affecting many river corridors.
Broad estimates are that some 80–90% of Europe’s river floodplains are now cultivated intensively compared to figures of 46% for North America (excluding northern Canada and Alaska) and 11% for African Rivers (Tockner and Stanford 2002). As such floodplain condition and functionality has been reported as being in a critical situation across Europe (Wenger et al. 1990; Klimo and Hager 2001). Tockner and Stanford (2002) note that in Europe and North America, up to 90% of floodplains are already “cultivated” and therefore functionally extinct; this figure is likely an underestimate for British rivers, given only 3.3% of the total river network of Britain is fully connected (Jones et al. 2019). The post‐war land drainage grants ceased in the 1980s, but the loss of wetland has continued to the present‐day and current attempts to improve watercourse form and function through restoration is having little or no measurable impact on floodplains.
Figure 1.2 Cause of hydromorphic degradation in the UK.
Source: www.riverhabitatsurvey.org/wp-content/uploads/2012/07/RHS.pdf
It is interesting to review the figures above against the Water Framework Directive measure of river health currently being used across Europe. Entwistle et al. (2019a) used floodplain land‐use data for 2017 broken down according to current water body status generating 2975 auditable units. Water bodies presently at Good status were selected (n = 375), arable and horticulture covers in excess of 50% of the floodplain area on around 15% of Good status water bodies, this increases to around 50% for area under improved grassland and when the two are considered together between 70 and 75% of Good Status water bodies are covered by at least 50% farmland. Around half of these water bodies are utilised over 90% by farming.
1.4 River and Floodplain Recovery
The preceding section summarises several studies relating to river and floodplain degradation; however, it remains difficult for the reader to understand exactly what has happened to watercourses and valley bottoms in England. Rivers seen in the landscape today are often regarded as “natural” systems with little regard to the actual levels of degradation they have suffered. This is perfectly understandable as we have grown up with heavily modified and managed systems, and we have few natural analogues left against which we can judge the degree of alteration. “Stability” is seen as desirable with neatness and uniformity favoured above natural functionality. Fortunately, this attitude is changing as the economics of river and floodplain management has resulted in reduced river training and a consequent increase in the influence of natural fluvial processes. Many systems are showing small but significant signs of recovery towards a better functioning system aligned with current catchment controls. Recovery has not always been back to a previous river state before human intervention as conditions in the catchment have changed significantly enough to cause a state change in our rivers, but their diversity is improving.
The situation above is best illustrated through examples. Figure 1.3 illustrates СКАЧАТЬ