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Evolution of The Harnham Water Meadows

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Like everything else in the countryside, the area we know today as the Harnham Water Meadows, came into being over a long period of time involving both natural processes and human intervention. Locally, this may be regarded as starting with the formation of the chalk which occurred between about 100 and 66 million years ago. The chalk was later uplifted and folded around the time the Alps were formed. Later still, there was erosion of the valleys first by water, then by freeze-thaw processes

during the Ice Age and again by water again during the past 10,000 years that broke up the surface rocks, Later on, melt-waters from the ice sheets located further north than modern Salisbury during the later Ice Age (Pleistocene) cut the valleys deeper and deposited several metres of sands and gravels, although at no time during the Pleistocene did an ice sheet extend southwards into the area of south Wiltshire. The deepened valleys were filled with chalky and flinty rubble that can be seen in the river beds today around Harnham (Figure 1).

These rivers occupied different levels in the past. These are identified today as ‘river terraces’ – flattish areas in the valleys situated above the modern floodplain. One such example is located immediately to the north of the present watermeadows in what is today the area around Salisbury railway station. The late Pleistocene rivers that deposited the gravels below the alluvium are likely to be ‘braided’ in form. This meant there was a large proportion of gravel in a matrix of finer material that was moved by seasonally high river discharges. During this time, the rivers were most likely classic ‘braided’ rivers (Figure 2).

Resting on top of the gravels is an appreciable thickness of alluvium (sediment transported then deposited by a river). Because this reaches typically between 0.5m and 1.0m at Harnham, there is a lot of fine sediment (mostly silty clay loam) forming the floodplain – and hence the soils of the watermeadows – and from which they were constructed. This fine alluvium also includes a fair amount of peaty material belonging to the ‘Holocene’, the last 10,000 years or so, a time when human beings colonised the area following the global warming and ice melt at the end of the Pleistocene. The rivers that deposited the early Holocene alluvium were most likely meandering streams, but may have tended towards having more than one channel – due to the new imposed, finer sediment load (Figures 1 and 3)

The human technological period called the ‘Mesolithic’ lasted until about 6,500 years ago in southern England. During this time there was appreciable broad-leaved, temperate forest cover that may not have been continuous, but provided the resources for humans to gather food from plants, hunt and gather. It is thought that towards the end of this time, bands of hunter-gathers were making small temporary clearances within the forest, perhaps with a view to clearing areas for planting, clearance of non-hazel trees for nuts, felling for animal enclosures. It is further believed that soil degradation commenced during this time, although the main means of human livelihood was hunting, fishing and gathering.

Figure 1 - Cross-section of local river

Figure 1

Figure 2 - Classic braided rivers

Figure 2

Figure 3 - Early meandering/multiple channel

Figure 3

In the following Neolithic period, there was a dramatic revolution in which it is believed the majority of the population changed their mode of subsistence from hunter gathering to farming. It is estimated that the area covered by woodlands fell, perhaps to one-third across Britain, during the Neolithic. During this time there was serious deforestation, meaning that small areas cleared by Mesolithic people would become larger, join up units and hence present further threats to the soil. As farming expanded into the Bronze and Iron Ages, so the extent of soil erosion increased. The fine, silty soils that had been deposited by wind over the chalk downlands at the end of the Pleistocene and before there was any forest cover, became prone to being eroded, washed down the valley sides from plough-land laid bare by agriculture, and thus contributed in a significant way to the modern alluvium. In the sides of valleys of the Nadder and Wylye upstream of Harnham  is abundant evidence for arable farming, dating from periods most likely stretching from the Bronze Age to the medieval. These are often marked as ‘field systems’ (sometimes inappropriately named ‘celtic fields’). These ‘lynchets’ show extension of arable agriculture, often to steeper slopes, most vulnerable to erosion. It is estimated there was originally about one metre of löess (‘coverloams’) covering the chalk; today topsoil thickness is reduced to perhaps 300mm.

Changes in climate, hydrology and land cover inevitably influence the form of river channels. Removal of forest cover increases runoff to river channels, and most importantly transports sediment eroded from soils on the valley sides and above. Rivers would have responded to these changing conditions. First by increased flow and flood peaks (this river flow in the chalklands is largely regulated by passage through groundwater) and then by moving away from being conventional ‘meandering’ streams to developing a tendency towards multiple channels, responding to the increased supply of sediment. The new multiple-channels are best described as ‘anastomosing’ or branching rivers, but with the sense that the dividing channels will again re-join at some point downstream. There are important differences between classic braided rivers (of the kind assumed to have predominated during the later Pleistocene) and anastomosed systems: for example, the sediment load is no longer gravel-dominated, but instead is dominated by silt material (2μm to 60μm); a second feature is that the river system displays highly irregular channel patterns.

This irregular, multiple channel pattern is attributed to the periodic occurrence of  ‘avulsion’ events’; the abandonment of an existing channel whose bed has been built up by sediment during a flood event to create a new channel that heads across the floodplain, often causing an abandonment of the original channel. Eventually the new channel will re-enter another channel at some point downstream. This kind of multiple channel typically has fewer channels than typical braided streams – the average channel number (measured across the floodplain) is generally close to two. This explains the occurrence of islands within the river network, and especially prominent at confluences, such as that of the Nadder and Avon (Figures 4 and 5) that bound the watermeadows.

Stabilised by bank vegetation such as reeds or grass, or by wet woodland, this multiple channel network is naturally more stable than meandering streams and the channels display a lower sinuosity. Also, people will wish to protect their investments through maintaining banks and other structures in order to protect property, including mills, and properly manage watermeadows and canals. Anastomosed rivers present opportunities for economic development because the complexity of channels provides access across the floodplain, including opportunities for irrigation.

We can be confident that the modern situation (Figure 5) has been more or less stable for over 230 years, demonstrated by existing maps and aerial photographs. The inherent stability would be improved by the desire to protect the sites of the former mill on the north branch of the Nadder at Fisherton (today there is only the large mill house remains in Mill Road) and at the Old Mill (formerly Harnham Mill) on the south branch of the Nadder. Embankments are also present along the downstream end of the watermeadows.

The present watermeadows most likely date from the second half of the seventeenth century and these were constructed on a floodplain of silty and peaty soils. The pre-floating area was most likely a marsh including meadows arranged in strips in the medieval period. Evidence on the ground shows that watermeadow construction used  features of the medieval marsh, especially former natural drainage channels.

A simplified soil map of the Harnham Water Meadows is shown in Figure 6; these soils are typical of the chalk stream floodplains used in the construction of watermeadows. The best surviving watermeadow structures are on the mineral alluvium, the humose (‘peaty’) and true peat areas were evidently abandoned earlier, due perhaps to their liability to being trampled by animals, or high infiltration capacity making watering difficult. A schematic showing the relationship between the main watermeadow soils and structures is shown in Figure 7.

Figure 4 - Schematic showing avulsions across the floodplain

Figure 4

Figure 5 - The present arrangement of channels at Harnham

Figure 5

Figure 6 - Simplified soil map of the Harnham Water

Figure 6

Relationships between watermeadow features and soil series

Figure 7

Framework developmental sequence at the Harnham Water Meadows

Approx Time

Description

Diagram

Late Pleistocene
(Ice Age)

River gravels deposited in valleys and wind-blown loess deposited over the chalk land surface

Figures 1 and 2

Postglacial ‘wildwood’ c.4,500 BC

Natural woodland, stabilised banks, finer alluviation begins

Figure 3

Prehistoric to early historic period

Accelerated deposition in valley of silty alluvium from soil erosion with progressive deforestation, leads to multiple channels and avulsions

Figure 4

Later prehistory into historic times

Multiple thread channels achieve stabilisation with grazing marsh, wet woodland and fringing vegetation.

Figure 5

Watermeadows constructed 17th century

Pattern of carriers, bedwork ridges and drains leads to modern soil pattern

Figures 6 & 7

Further reading

  1. 1. British Geological Survey (2005). Sheet 298 Salisbury, scale 1:50,000, bedrock and superficial

  2. deposits.

  3. 2. Cook, H. F. (2008). Evolution of a floodplain landscape: A case study of the Harnham Water

  4. Meadows at Salisbury England. Landscapes (2008) 1 50-73.

  5. 3. Cook, H.F., Cowan M. and Tatton-Brown T (2008). Harnham Water Meadows: History and

  6. Description. Hobnob Press, Salisbury.

  7. 4. Cook, H.F and Williamson T. (eds.) (2007): Water Meadows: History, Ecology and Conservation

  8. Windgather Press, 152 pp.

  9. 5. Cutting R., Cook, H.F. and Cummings, I. (2003). Hydraulic conditions, oxygenation, temperature

  10. and sediment relationships of bedwork watermeadows. Hydrological Processes 17, 1823-1843.

  11. 6. Hopson, P.M. et al (2007). Geology of the Salisbury district, British Geological Survey, Keyworth
  12. 38pp.

Hadrian Cook, April 2009

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