Geology and geomorphological background
The distinctive scenery of the Yorkshire Dales in upper Swaledale and Wensleydale is produced by the Yoredale rocks and carboniferous limestone. The broad terraces in the landscape are partly a result of geology and partly arise from the work of glaciers during the ice age, when large blocks of limestone and sandstone, which were loosely laid on the shales, were removed down to the next strong bed. Postglacial water washed away weathered shales and undercut the terraces. These Yoredale terraces now give many Dales hillsides a staircase appearance, with many waterfalls, like Hardraw Force. The famous Aysgarth Falls are different, having formed where a step has retreated in the limestone valley floor, where Wensleydale hangs above the glacially over-deepened Bishopdale.
The rocks become progressively younger downriver from the Dales. Sandstones and shales of the carboniferous millstone grit form an area of grit moorland in the catchments of the Burn and Laver, tributaries of the Ure, and in the catchment of some of the right bank tributaries of the River Swale.
The Permian sequence of magnesian limestone and marl units form a north-south ridge of higher land on the western side of the Vale of York. The rocks dip gently eastwards and are overlain by sandstone which forms the Vale of York. On the eastern side of the Vale the sandstone is overlain by areas of mudstone and marls. The Jurassic sandstone and limestones which form the North York Moors provide a westwards-facing escarpment along much of the eastern side of the Vale of York.
The limestones are permeable and much precipitation infiltrates in the summer, affecting the discharge of the upper sections of many tributaries, although this influence is less obvious on the Ouse. In the winter, when water tables are higher, the effect of the permeable rocks is diminished. Much of the remaining geology is relatively impermeable.
Vegetation
The upland parts of the basin are largely moorland with heather, bracken, grasses and scattered trees with extensive wooded areas on lower slopes. Interception is therefore relatively low, although this has to be balanced by considering the nature of moorland soils. Below the upper moorland, extensive areas are forested, and as much of the woodland is coniferous, interception is high throughout the year. Areas of deciduous woodland will have higher levels of interception during the summer when there is a cover of leaves.
Much of the middle and particularly the lower stages of the basin are areas of farmland. The landscape is consequently more open and interception levels are lower. The degree of interception depends on the nature of farming. Pastoral activities are associated with areas of grassed fields where interception is low; while interceptions is areas of arable farming depends on the type of crop and time of the year.
Soils
The upland areas of the Pennines and Yorkshire Moors are dominated by peat soils characteristic of moorlands. These have an important role, acting like a sponge in summer, absorbing a great deal of precipitation and affecting discharge. However, the effect is reduced from late autumn to early spring when wetter weather means that the ground is largely saturated and so infiltration levels are reduced. The upper valleys are typified by surface water gleys; these are seasonally waterlogged and are slowly permeable. Downstream, the river valleys quickly become dominated by brown soils; suitable for agriculture- this also applies to the Vale of York, where there are also areas of drier sandier soils. It is important to consider the impact of farming on the soils and the consequent implications for water movement in the basin. Areas of groundwater gley extend southeasterly from Thirsk, around York to Selby. The maintenance of well-drained soils, by such factors as healthy vegetation cover, extensive root systems, an open soil structure and plentiful soil fauna, helps to reduce run-off in the basin.
Climate
The topography of the Ouse sections of the basin, as we have seen, is almost uniformly flat. The annuals precipitation varies very little as a result, ranging between 540 and 640mm. Annual potential evaporation is approximately 520 to 540mm. However the upland areas receiver much higher amounts of precipitation. High rainfall over the tributaries makes the River Ouse very prone to flooding. The annual pattern of precipitation is reflected in the regime of the River Ouse and its tributaries.
Human influence on the River Ouse basin
Farming
Much of the catchment of the River Ouse and its tributaries is used for farming. The upland areas like the Pennines and Yorkshire Moors are dominated by pastoral activities, while the Vale of York has a mixture of arable and dairy farming.
There are large areas of moorland in the uplands, particularly to the west of the Ouse. Moorland ‘gripping’ (grips are drains) was carried out extensively in the Swale, Ure and Ouse catchments in the 1960s and 1970s, encouraged by grand aid from the Ministry of Agriculture, Fisheries and Food (MAFF). The consequence these works has been to lower the water table and increase stream base flows near the drained areas. At the catchment scale, drainage leads to larger peak flows downstream and a reduction in lag times. Dry weather base flows tend to be higher, due to water reaching the channels more quickly than would otherwise be the case.
Over-deepening of the grips has led to considerable erosion on some moors, an increased silting of watercourses downstream. MAFF and English Nature have since recognised these effects and have provided grant aid to block grips and thus reduce the rate of surface run-off.
Although the management of the soils in the lower parts of the basin helped to increase infiltration and reduce run-off, over many years’ agricultural drainage schemes and the removal of wooded areas to create space for farming have contributed to increases in the discharge of the Ouse and its tributaries.
Forestry
Afforestation varies within the entire basin. The Ure catchment has had more plantations than the Swale, but there are no significant forest plantations in the Ouse catchment. Forests increase interception and have the effect of reducing discharges. Of the tributaries and the Ouse itself. This effect applies throughout the year as the species planted are conifers, which are evergreen. It must be recognized that the effects of forestry are temporarily variable, as the plantations change, particularly towards maturity as felling and restocking takes place.
The current set-aside policy and farming recession has meant that the Environment Agency has had the opportunity to influence land use to benefit and improve the environment. Forestry schemes are grant-aided by the Forestry Commission, with the Woodland Grant schemes encouraging the development of new woodlands. As these woodland areas become more extensive, this will also have an impact on the basin, affecting interception levels and reducing discharges.
There is little significant deforestation in the basin today, although once the Vale of York was extensively forested in medieval times, with smaller areas of poorly drained land and lowland heath. Agricultural intensification over the centuries has been largely responsible for the loss of forest cover and the consequent reduction in interception with the basin.
Urban developments and communications
New housing areas, out of town shopping centres, and industrial and infrastructural developments all create impermeable surface within the basin. This turn leads to an increase in surface run-off as infiltration is reduced. Drains carry much of this water into local streams and rivers, which increased discharge and can contribute to a greater frequency and magnitude of flooding. In recent years urban development has included growth on the fringes of York, Ripon and other settlements in the basin, and motorway-standard widening of the A1 road (which runs north-south in the west of the basin)
The Environment Agency can impose conditions on new developments and can raise objections to a planning authority if it believes that a new development would have an adverse effect on both the quantity and quality of water reaching rivers. New urban development often incorporates balancing ponds or tanks that can be used to filter out pollutants. They also store drainage water at times of high discharge in nearby rivers, this water being released at a later dated when river levels are lower.
During the 1980s and 1990s there was much growth on the northern fringe of York, in Clifton Moor. This has included a new business park with light industry and offices and retail developments, and the creation of extensive areas of impermeable surfaces, such as car parks. Balancing pounds have been incorporated into the business park to handle excess surface water and any contaminants. The combined control of water quantity and quality is called ‘sustainable urban drainage’.
