Over the years human influences have affected the Ouse. The river Ouse is the principle drainage basin in Yorkshire and is naturally very large, covering 10,770 square km. It has four main tributaries; they are the Wharfe, Derwent, Aire and Don, these all contribute to the Ouse having a large base flow.
The relief of the Ouse section of the basin is almost uniformly flat. The annual precipitation varies very little as a result, ranging from between 540 and 640 mm. Annual potential evaporation is approximately 520 to 540mm. However the upland areas receive much higher amounts of precipitation. High rainfalls over the tributaries of the Ouse make it very prone to flooding, as the tributaries are unable to cope with amount of water.
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. Moor land ‘Gripping’ (grips are drains) was carried out extensively in the Swale, Ure and Ouse catchments in the 60’s and 70’s, the consequence of these works has been to lower the water table and increase stream base flows near the drained areas. This has lead to larger peak flows downstream and a reduction in lag times. Dry weather base flow tends to be higher, due to water reaching the channels quicker than would otherwise be the case.
Over many years agriculture 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.
Agricultural intensification over the centuries has been largely responsible for the loss of forest cover and the consequent reduction in interception within the basin.
New housing areas, out of town shopping centres, and infrastructural developments all create impermeable surfaces within the basin. This then leads to an increase in surface run-off as infiltration is reduced. Drains carry much of this water into local streams and rivers, which increases discharge and can contribute to a greater frequency and magnitude of flooding There have been numerous floods in the past in York and other parts of the basin. Recent floods include those on the Ure affecting the area around Boroughbridge in 1991, and major floods around Selby in 1995 and along the Derwent in 1999.