The hydrochemistry of backwaters and dead zones

This thesis investigates the inter-relationship between hydrological and water quality variations with reference to backwaters and dead zones in rivers. The examination of a series of controlled reservoir releases to an upland channel in Wales showed "in-channel" dead zones were important solute source and storage areas. Physico-chemical data have been presented to show the influence of main channel flow and quality variations on the hydrochemistry of a range of seven backwaters within the Trent basin. The hydrological regime of the mainstream seasonally modified backwater quality. The degree of main-flow influence declined with decreasing hydrological connectivity of the backwater environments with the mainstream. Furthermore other hydrological, biological and biogeochemical factors influenced backwater physico-chemistry. A generalised typology of permanent floodplain water bodies in the R. Trent catchment based upon water chemistry was established. An applied study examined the effect of reservoir cleaning/emptying operations and flood events on backwater hydrochemistry on the French Upper River Rhone. The simultaneous occurrence of decreasing mainstream discharge with the passage of the sediment laden release wave reduced its impact on backwater environments. Anthropogenic influences on riverine systems as a result of intensification of agriculture and urbanisation (e.g. nitrate pollution and river regulation) have influenced the hydrological and physico-chemical functioning of fluvial hydrosystems. In view of rising nitrate levels in many British rivers, trends within the Trent basin have been investigated. Rates of increase in concentration of 0.06 mg.l-1 N yr-1 were typical of rural catchments, whilst rates of up to 0.20 mg.l-1 N yr-1 were recorded in urbanised tributaries. Furthermore the impact of rising nitrate levels in the Trent catchment and river regulation on backwater hydrochemistry and ecology have been examined.