Vanishing rivers

Overall vulnerability

Feature assessed:

• Vanishing rivers

Special qualities:

• Beautiful views created by contrasting landscapes and dramatic geology

Feature description:

Vanishing rivers are watercourses that disappear as they travel across the limestone bedrock, the main systems being the Hamps, Manifold, and Lathkill in the White Peak. For most of the year, these rivers can only be seen on the surface as dry riverbeds for much of their length. Disappearing under the ground, these rivers take a subterranean course through natural caves and mine workings before rising suddenly some miles further downstream. These rivers can only be seen as surface flow when in spate. Despite being dry, these rivers host a diverse freshwater community composed mostly of ephemeral and drought tolerant species. When the surface waters flow, opportunistic fish such as bullhead move in and become abundant, living elsewhere during the rest of the year. Some bullhead have been found without pigmentation at Ilam Risings. These fish presumably live at least partly in the underground section of the river. Vanishing rivers are a distinctive part of the White Peak landscape, forming some of the characteristic limestone dales and the obscure cave systems beneath.

How vulnerable are vanishing rivers?

Vanishing rivers in the PDNP have been rated ‘high’ on our vulnerability scale. This score is due to high sensitivity and exposure to climate change variables, with an unclear current condition, and a moderate adaptive capacity.

The greatest impact on vanishing river systems will be changes in rainfall across the year. Greater extremes in precipitation will leave rivers dry for longer and in higher flow when active on the surface. Vanishing rivers have some capacity for adaptation, as their characteristic community is relatively hardy and there are some financial resources available to increase their adaptive capacity.

Current condition:

As river systems, the current condition of vanishing rivers is largely the same as that for rivers and streams generally. When flowing, vanishing rivers are subject to the same pressures as other watercourses in the PDNP. Invasion by non-native species such as the American mink and signal crayfish will affect these systems, but these will be disadvantaged in the dry reaches. Similarly, invasive plant species will affect vanishing rivers, but riverbank erosion effects will be lessened in dry reaches due to the less defined riverbank. Runoff from agricultural land is likely to introduce nitrogen and phosphorous contamination and pesticide pollutants, as well as siltation from fine sediment.

An effect specific to vanishing rivers is present in the Lathkill. Due to the river’s emergence and flow through mines and mined sections of caves, heavy metal pollution affects some areas of the watercourse. The lower Lathkill has good or high status of biological and chemical elements apart from zinc. This is due to the blockage of Hillcar Sough, which has caused mine efflux to back up into the Lathkill rather than take its previous course into the Derwent.

What are the potential impacts of climate change?

Overall potential impact rating

Most climate change effects on vanishing rivers will be the same as for other watercourses assessed in the rivers and streams section. Only instances where the impacts are specific to vanishing rivers or differ from other PDNP watercourses are detailed here.

Direct impacts of climate change

Changes in annual precipitation cycles will prove very important for vanishing rivers. Greater rainfall in winter and lower levels in summer are likely to increase the variability of vanishing rivers, having negative effects on the river’s biological communities. Many aquatic invertebrate species persist as a ‘seedbank’ of drought resistant life stages in the sediment during dry periods. Seedbank diversity reduces after longer dry periods, and abundance decreases as fewer invertebrates survive more intense drying. An increase in the diversity of terrestrial species occupying the dry river bed may occur, but this has not been studied. Higher flow conditions when the river is flowing may also decrease abundance and diversity. Not all freshwater species are adapted to high flow conditions, meaning the species assemblage present may be reduced. Many colonists also swim upriver to colonise, especially in the Lathkill where the source is a cave system. Extra flow may reduce the upriver dispersal ability of colonists such as bullhead. Data Certainty: High

Increased annual average temperatures are likely to cause a change in the invertebrate community of vanishing rivers. Cool adapted species may be lost or replaced by southern species moving northwards with climate change. Many temporary river specialist species are found in the chalk rivers of the southeast that could replace PDNP species as climate space shifts, but dispersal across the gap between these two habitat areas may prove problematic. Data Certainty: High

Human behaviour change

Drier summers may lead to increased demand for water abstraction. This would exacerbate low water levels during increasingly frequent drought. As a result, vanishing rivers would be dry for longer periods, having negative effects on the aquatic communities that utilise the habitat. This effect may however be small as vanishing rivers are not generally targets for water abstraction. However, some rivers that were previously perennial may become temporary during extremely dry periods. This may allow the vanishing river assemblage to increase in range and abundance, but at the expense of the usual river assemblage. Data Certainty: High

Nutrient changes or environmental contamination

Longer dry periods may cause additional nutrient input to vanishing rivers during the wet period. During the time that no water is flowing on the riverbed, plants grow on the newly available space and plant debris such as leaves and branches collect. When flow resumes, this organic matter is picked up or decays in the water, increasing the nutrient content and organic matter in the watercourse. Longer dry periods may therefore allow more plant matter to accumulate, increasing the nutrient input to the temporary watercourse. Data Certainty: Low

What is the adaptive capacity of vanishing rivers?

Overall adaptive capacity rating

Vanishing rivers have some capacity to regenerate after extreme events. Species that inhabit the watercourse are already adapted to recolonise when the opportunity arises, through their drought tolerance or rapid dispersal ability. Therefore, changes that result in the watercourse being lost for longer periods may not result in complete loss of the species assemblage once suitable conditions return. Data Certainty: Very High

Vanishing rivers in the PDNP have reasonable connectivity. Some, such as the Hamps and Manifold, connect along their length. All are in the White Peak area and are not too far apart, meaning species with high dispersal such as those with aerial life stages have the potential for population movement between rivers. There are few artificial barriers along vanishing rivers as they are not ideal for retention of water. Data Certainty: Low

There is a reasonable diversity of vanishing rivers in the PDNP. While all are limestone rivers with some extent of underground flow, there are differences in their hydrogeology. Different river systems have different underground flows, with some travelling mostly through mine workings and some through natural caves. Vanishing rivers also flow above ground for different lengths and at different times of year. Data Certainty: Low

Some funding sources are available for work on rivers at a catchment level. However, these are unlikely to take into account the unique situation of temporary rivers, in which the river does not exist as overground flow for much of the year. Vanishing rivers may therefore remain less desirable for funders, as they do not fit the typical river structure. Data Certainty: Moderate

Vanishing rivers benefit from some of the same legislative and organisational support as other watercourses in the PDNP. Some rivers are covered by the Water Framework Directive and so are closely monitored. They will however be likely to receive less support from water companies, being on a porous limestone bedrock and so less useful for abstraction. Data Certainty: High

There is a general lack of understanding of temporary streams across the UK. PDNP vanishing rivers are one of the better studied systems, but still more research is required to effectively implement adaptive management. Some recent studies have aimed to quantify the invertebrate community of these systems, as well as the ecosystem services they provide. Such work will be needed to understand these systems and how to preserve them in the future. Data Certainty: Moderate

Key adaptation recommendations for vanishing rivers:

Improve current condition to increase resilience

The current condition of a feature is an important factor alongside its sensitivity and exposure, in determining its vulnerability to climate change. These recommendations are aimed at improving the condition of the feature at present, therefore making it better able to withstand future changes to climate.

• More research is required to better understand the hydrology of low flow river systems and their interaction with surface and underground workings associated with mining.
• Consider restoration measures to reduce the impact of human activity, such as features in mining (soughs) diverting water to different catchments. Especially where impacts are known.
• Restoration of natural processes across the catchment will improve watercourse health and therefore resilience to change.
• Consider removal of impoundments and weirs to restore natural river flow and improve species dispersal.
• Minimise agricultural inputs, especially fertilisers and pesticides. Give consideration to good management of waste to improve catchment quality, including effective slurry store management.
• Restoration of healthy soils in river catchments will increase infiltration of water and reduce runoff, thereby reducing sedimentation. Tree establishment is a known method to improve infiltration and reduce pollutant spread from runoff.
• Restoration of semi-natural vegetation on critical runoff pathways will slow the flow and reduce erosion.
• Use of low nutrient livestock feeds will reduce contamination of the watercourse.
• Manage invasive species, including botanical species and crayfish.
• Consider the impact on key views when planning adaptations.

Adaptations that could aid other features

These recommendations are changes that could be made to this feature, which will have a positive impact on the ability of other vulnerable features to withstand future climate change.

• Creation of riparian shade through tree and scrub establishment will be beneficial to freshwater habitats, and may offset some of the effects of temperature rises.