Changing sea level related to long term climate change has resulted in significant changes in the environments around the Fal Estuary, and indeed with rising sea level at the end of the last glaciation, the tidal limits within the estuary moved inland. However, over the last 2000 years there is evidence that the rivers and creeks in the northern part of the estuary have been the site of increased sediment deposition. A useful concept in understanding how the estuary is responding to changes in sea level and sediment supply is the idea of ‘accommodation space’. The ‘accommodation space’ is the available volume in which sediment can accumulate; thus if sea level is not changing (or even actually slowly rising) but the rate of sediment supply is high, then all of the accommodation space will be infilled and the estuary will apparently show a relative fall in sea level with the maximum tidal limit stepping seaward with time. This is what has happened in the Fal Estuary; initially during sea level rise at the end of the last glaciation the tidal limits stepped landward. However, with the expansion of the mining industry and rapid increase in the rate of sediment deposition, the narrow creeks and rivers in the northern part of the estuary were rapidly infilled with sediment, pushing the tidal limit seaward. Today, there is some evidence to suggest that with the current limited natural supply of sediment to the estuary, some of the intertidal areas are being eroded; the estuary is actually ‘under-supplied’ with respect to sediment input.
Aerial photograph of Ruan Lanihorne which was an important port in the 14th century. As can be seen, the valley floor is now extensively silted. Photograph courtesy of Steve Hartgroves at the Cornwall Archaeological Unit.
The recognition that the mining activity was having a significant environmental impact by causing extensive deposition of sediment was recognised very early on, initially in the Fowey Estuary and then in the Fal Estuary. In 1356, Abraham le Tynnere was charged with having caused ‘damage to the prince and haven of Fowey’ due to siltation in the estuary arising from tin streamworks he owned along the River Fowey (Gerrard, 2000).
Extensive areas of saltmarsh form an important habitat at Lostwithiel on the Fowey Estuary today but in the medieval period this was an important port.
Early environmental legislation tried to reduce the impact of tin streaming as it was realised that siltation was leading to the loss of the then important port of Lostwithiel. With respect to the Fal Estuary Queen Elizabeth I is quoted to have stated on the 20th June 1580 that:
‘the said Port of Falmouth at present is in decay and requireth speedy preservation for the protection and increase of shipping there, because that from the constant working of the tinners there, great quantities of stone have come into the said port to the great damage of the said port and same borough of Truro is likewise much the worse from the same cause’
Areas around the Fal which have seen the most extensive effects of increased siltation are on the River Fal, Restronguet Creek, Tresillian River and Calenick Creek. Of these sites the River Fal has probably experienced the most siltation. The river may have been navigable in Roman times up to Golden Mill, some 2 km north of the Medieval town of Tregony. Increased siltation and the construction of Tregony Bridge in 1382 meant that this was the limit to shipping at that time; however, by 1600 the port of Tregony was no longer accessible and the river was only tidal up to Lanihorne Woods. Today the tidal limit is just above Sett Bridge; effectively the tidal limit has moved some 6.5 km down the Fal River as a direct result of the release of sediment into the river due to mining.
Sett Bridge (built across the Fal River in 1885) marks the approximate present day tidal limit. Photograph courtesy of Steve Hartgroves at the Cornwall Archaeological Unit.
Restronguet Creek has been subjected to considerable sediment deposition from the rivers Carnon and Kennal primarily as a result of the tin and copper mining activity in the Gwennap mining district. The Carnon River, which had been tidal as far inland as Bissoe in the 16th Century (Ratcliffe, 1997), rises in the mining district of Gwennap and drains an area of 3,500 ha as well as receiving mine water from the County Adit which was constructed in 1748 above Nangiles. Whitley in 1881 stated that:
‘Carnon Creek.........shows the most serious accumulation of silt in Falmouth Haven’.
Whitley (1881) calculated that 456,000 m3 of sediment had been deposited in the lower Carnon Valley, with an accumulation rate of 6 cm per year in the period 1821-1842.
The River Kennal also experienced rapid siltation and the sediment supplied via both the Kennal and Carnon rivers inevitably reached Restronguet Creek where locally extreme sediment accretion rates of up to 8 cm per year occurred (Whitley, 1881). Stapleton and Pethick (1996) have estimated an average siltation rate of 5 cm year for the period 1500-1995 for Restronguet Creek which is reflected by the rapid expansion of the area of saltmarsh particularly during the late 19th and early 20th centuries.
The northern tributaries have also been subjected to considerable siltation although not as rapid as in Restronguet Creek or the River Fal. Within the Tresillian River Whitley (1881) calculated a sedimentation rate of 0.43 cm per year at St. Clements on the Tresillian River for the period 1597-1880. Calenick Creek also shows marked siltation with the loss of Calenick Quay which had served the smelting house and crucible works in the 18th Century.
Diagram illustrating the degree of siltation of the Fal River (not to scale)