Geology & Hydrology of River Trent

Geology

Underlying the upper reaches of the Trent, are formations of Millstone Grit and Carboniferous Coal Measures which include layers of sandstones, marls and coal seams. The river crosses a band of Triassic Sherwood sandstone at Sandon, and it meets the same sandstone again as it flows beside Cannock Chase, between Great Haywood and Armitage, there is also another outcrop between Weston-on-Trent and King's Mill.

Mercia Mudstone formation atGunthorpe

Downstream of Armitage the solid geology is primarily Mercia Mudstones, the course of the river following the arc of these mudstones as they pass through the Midlands all the way to the Humber. The mudstones are not exposed by the bed of the river, as there is a layer of gravels and then alluvium above the bedrock. In places, however the mudstones do form river cliffs, most notably at Gunthorpe and Stoke Lock near Radcliffe on Trent, the village being named after the distinctive red coloured strata.

The low range of hills, which have been formed into a steep set of cliffs overlooking the Trent between Scunthorpe and Alkborough are also made up of mudstones, but are of the younger Rhaetic Penarth Group.

In the wider catchment the geology is more varied, ranging from the Precambrian rocks of the Charnwood Forest, through to the Jurassic limestone that forms the Lincolnshire Edge and the eastern watershed of the Trent. The most important in terms of the river are the extensive sandstone and limestone aquifers that underlie many of the tributary catchments. These include the Sherwood sandstones that occur beneath much of eastern Nottinghamshire, the Permian Lower Magnesian limestone and the carboniferous limestone in Derbyshire. Not only do these provide baseflows to the major tributaries, the groundwater is an important source for public water supply.

This ‘staircase’ of flat topped terraces was created as a result of successive periods of deposition and subsequent down cutting by the river, a product of the meltwater and glacially eroded material produced from ice sheets at the end of glacial periods through the Pleistocene epoch between 450,000 and 12,000 years BP. Contained within these terraces is evidence of the mega fauna that once lived along the river, the bones and teeth of animals such as the woolly mammoth, bison and wolves that existed during colder periods have all been identified. Another notable find in a related terrace system near Derby from a warmer interglacial period, was the Allenton hippopotamus.Sand, gravels and alluvium deposits that overlie the mudstone bedrock occur almost along the entire length of the river, and are an important feature of the middle and lower reaches, with the alluvial river silt producing fertile soils that are used for intensive agriculture in the Trent valley. Beneath the alluvium are widespread deposits of sand and gravel, which also occur as gravel terraces considerably above the height of the current river level. There is thought to be a complex succession of at least six separate gravel terrace systems along the river, deposited when a much larger Trent flowed through the existing valley, and along its ancestral routes through the water gaps at Lincoln and Ancaster.

Gravel extraction at Besthorpe

The lower sequences of these terraces have been widely quarried for sand and gravel, and the extraction of these minerals continues to be an important industry in the Trent Valley, with some three million tonnes of aggregates being produced each year. Once worked out, the remaining gravel pits which are usually flooded by the relatively high water table have been reused for a wide variety of purposes. These include recreational water activities, and once rehabilitated, as nature reserves and wetlands.

During the end of the last Devensian glacial period the formation of Lake Humber in the lowest reaches of the river, meant that substantial lake bed clays and silts were laid down to create the flat landscape of theHumberhead Levels. These levels extend across the Trent valley, and include the lower reaches of the Eau, Torne and Idle. In some areas, successive layers of peat were built up above the lacustrine deposits during the Holocene period, which created lowland mires such as the Thorne and Hatfield Moors.

Hydrology

The topography, geology and land use of the Trent catchment, all have a direct influence on the hydrology of the river. The variation in these factors is also reflected in the contrasting runoff characteristics and subsequent inflows of the principal tributaries. The largest of these is the River Tame, which contributes nearly a quarter of the total flow for the Trent, with the other significant tributaries being the Derwent at 18%, Soar 17%, the Dove 13%, and the Sow 8%. Four of these main tributaries, including the Dove and Derwent which drain the upland Peak District, all join within the middle reaches, giving rise to a comparatively energetic river system for the UK.

Rainfall in the Trent valley

Rainfall

Rainfall in the catchment generally follows topography with the highest annual rainfall of 1,450 millimetres and above occurring over the high moorland uplands of the Derwent headwaters to the north and west, with the lowest of 580 millimetres, in the lowland areas to the north and east. Rainfall totals in the Tame are not as high as would be expected from the moderate relief, due to the rain shadow effect of the Welsh mountains to the west, reducing amounts to an average of 691 millimetres for the tributary basin.^[ The average for the whole Trent catchment is 720 millimetres which is significantly lower than the average for United Kingdom at 1,101 millimetres and lower than that for England at 828 millimetres.

Like other large lowland British rivers, the Trent is vulnerable to long periods of rainfall caused by sluggish low pressure weather systems repeatedly crossing the basin from the Atlantic, especially during the autumn and winter when evaporation is at its lowest. This combination can produce a water-logged catchment that can respond rapidly in terms of runoff, to any additional rainfall. Such conditions occurred in February 1977, with widespread flooding in the lower reaches of the Trent when heavy rain produced a peak flow of nearly 1,000 cubic metres per second (35,000 cu ft/s) at Nottingham. In 2000 similar conditions occurred again, with above average rainfall in the autumn being followed by further rainfall, producing flood conditions in November of that year.

Another meteorological risk, although one that occurs less often, is that related to the rapid melting of snow lying in the catchment. This can be a result of a sudden rise in temperature after a prolonged cold period, or when combined with extensive rainfall. Many of the largest historical floods were caused by snowmelt, but the last such episode occurred when the bitter winter of 1946-7 was followed by a rapid thaw due to rain in March 1947 and caused severe flooding all along the Trent valley.

At the other extreme, extended periods of low rainfall can also cause problems. The lowest flows for the river were recorded during the drought of 1976, following the dry winter of 1975/6. Flows measured at Nottingham were exceptionally low by the end of August, and were given a drought return period of greater than one hundred years.

Discharge

The Trent has marked variations in discharge, with long term average monthly flows at Colwick fluctuating from 45 cubic metres per second (1,600 cu ft/s) in July during the summer, and increasing to 151 cubic metres per second (5,300 cu ft/s) in January. During lower flows the Trent and its tributaries are heavily influenced by effluent returns from sewage works, especially the Tame where summer flows can be made up of 90% effluent. For the Trent this proportion is lower, but with nearly half of low flows being made up of these effluent inflows, it is still significant. There are also baseflow contributions from the major aquifers in the catchment.The river's flow is measured at several points along its course, at a number of gauging stations. At Stoke-on-Trent in the upper reaches, the average flow is only 0.6 cubic metres per second (21 cu ft/s), which increases considerably to 4.4 cubic metres per second (160 cu ft/s), at Great Haywood, as it includes the flow of the upper tributaries draining the Potteries conurbation. At Yoxall, the flow increases to 12.8 cubic metres per second (450 cu ft/s) due to the input of larger tributaries including the Sow and Blithe. At Drakelow upstream of Burton the flow increases nearly three-fold to 36.1 cubic metres per second (1,270 cu ft/s), due to the additional inflow from the largest tributary the Tame. At Colwick near Nottingham, the average flow rises to 83.8 cubic metres per second (2,960 cu ft/s), due to the combined inputs of the other major tributaries namely the Dove, Derwent and Soar. The last point of measurement is North Muskham here the average flow is 88.4 cubic metres per second (3,120 cu ft/s), a relatively small increase due to the input of the Devon, and other smaller Nottinghamshire tributaries.

Average monthly flows of Trent in cubic metres per second measured at Colwick (Nottingham).

Sediment

In the lower tidal reaches the Trent has a high sediment load, this fine silt which is also known as ‘warp', was used to improve the soil by a process known as warping, whereby river water was allowed to flood into adjacent fields through a series of warping drains, enabling the silt to settle out across the land. Up to 0.3 metres (0.98 ft) of deposition could occur in a single season, and depths of 1.5 metres (4.9 ft) have been accumulated over time at some locations. A number of the smaller Trent tributaries are still named as warping drains, such as Morton warping drain, near Gainsborough.

Warp was also used as a commercial product, after being collected from the river banks at low tide, it was transported along the Chesterfield Canal to Walkeringham where it was dried out and refined to be eventually sold as a silver polish for cutlery manufacturers.

References

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