The river

Ecosystem

Chalk rivers like the Darent arise from underground chalk aquifers and flow through the landscape on flinty gravel beds. They are characterised by crystal-clear water which is rich in minerals and remains at a relatively constant temperature throughout the year.

Typically, chalk rivers have stable flow levels year-round too. Healthy chalk rivers support a rich diversity of species which enjoy a mosaic of different habitats.

The more time you spend enjoying and exploring the Darent, the more you discover the interconnections and interdependencies of the entire river ecosystem. All parts need to be kept in healthy balance for the whole system to thrive.

Walking the length of the river, it is clear we have much to be grateful for. Many stretches buzz with insects, there is abundant bird life, and the water is crystal-clear.

However, there are areas of concern.

The Middle and Lower Darent water body has fluctuated between poor and moderate ecological status over the past decade. In 2022, it was assessed as having good ecological status, but fish and aquatic plants only achieve moderate health and it is not clear what has improved to raise the status (unless the goal posts have been moved). The Upper Darent – from its source above Westerham to the outskirts of Otford – has been assessed as having poor to moderate ecological status throughout this period.

The river as a whole failed chemical classification in 2019 due to the presence of Polybrominated Diphenyl Ethers (PBDE). These were used as a flame retardant for consumer products in the past and, although they are now restricted, releases still occur during use or disposal of the items.  

Fish

Fish found in the Darent have traditionally included species associated with chalk streams such as native brown trout, European bullhead, chub, dace, roach and others.

Due to the fact that it flows through and alongside several gravel pits, species including carp, tench and bream have also been reported in the river.

Eels once thrived – swimming up from the Thames -but the number of young joining adult populations has dropped dramatically since the 1980s.

Eels have a fascinating lifecycle. They live for 5-20 years in fresh and brackish European waters before returning to the Sargasso Sea where they spawn and die. The young elvers then make their way back to rivers across Europe and the cycle starts again. Eels are critically endangered and appear on the Red List of Threatened Species – as do European Bullhead.

The most recent Environment Agency fish survey on the Darent and Cray dates from 2019. The number, range of species and sizes of fish caught are indicators of a fishery’s health – above 24 grammes of fish per square metre indicates good health.

The results appear in the table:

Plants

Plants – within and alongside the river – are a vital part of the river ecosystem. They provide food and shelter for fish and invertebrates and help to improve water quality.

Plant habitats along the river margins – known as riparian vegetation and buffer zones – help to stabilise the riverbank, control sediment and erosion, filter nutrients and other pollutants, reduce flood impact and provide shade for streams – quite apart from providing food and shelter for wildlife and pollinators!

Water plants growing in most chalk rivers are characterised by water crowfoot, water starwort and lesser water parsnip. These dominate in spring and early summer and, as summer progresses, watercress and water forget-me-nots encroach from the margins. Look out too for brooklime (a succulent herb belonging to the speedwell family) and water mint (a member of the dead nettle family which attracts small tortoiseshell, peacock and comma butterflies, as well as the green tortoise beetle).

Alongside the river, you find plants such as: meadowsweet – a food plant for 74 species of invertebrates including the extremely rare mullein wave moth; hemp-agrimony which has frothy pinkish flower clusters from July to September and is very attractive to all kinds of insects including butterflies; hemlock water dropwort, a food plant for the rare hypera pollux weevil; marsh marigold – a perennial from the buttercup family; and yellow iris, a source of food for many beetles, moths and other bugs.

Did you know?

Stream water-crowfoot (of the genus Ranunculus) is extremely important in the ecology of the river system. It supports the larvae of Simulium black fly or reed smuts, which feed by filtering bacterial and algal matter from the water. A healthy stand of Ranunculus will house up to 200,000 reed smuts per square metre which helps maintain the gin clear water associated with healthy chalk streams. Stream water-crowfoot can also help to diversify the flow, oxygenate the water and provide a refuge for fish fry and other creatures.

Mammals

Population numbers of many of the mammals found along the river have declined significantly over recent years. Action on habitat restoration and pollution is helping to reverse trends for some species, notably otters. Predation by non-native species such as mink continues to be a problem for smaller mammals.

Water shrew

Water shrews are the largest of Britain’s shrews. They have low population densities compared with most small mammals, travelling up to 160m along the water’s edge to find food and shelter. Mostly nocturnal, they are particularly active just before dawn. These shrews have short velvety fur, a long, pointed snout, small ears and tiny eyes. Unusually among mammals, they possess venomous saliva capable of paralysing prey such as small fish and frogs.

Water voles

Water voles – immortalised as Ratty in The Wind in the Willows – are the largest species of vole in Britain and sometimes mistaken for brown rats. They have glossy brown or black fur and a blunt nuzzle. Mostly active during the day, their head and back are visible when swimming. Water voles underwent one of the most serious declines of any wild mammal in Britain during the 20th century with populations falling by almost 90% due to the spread of American mink. Eradication of this invasive voracious predator is necessary if native wildlife is to recover.

Otters

These secretive semi-aquatic species occupy large territories which can extend beyond 20km, depending on the availability of food. Their sudden decline across the UK last century was connected to the use of organochlorine pesticides as well as the destruction of habitat. Since these pesticides were withdrawn from use and focus has been placed on the management of land and riverbanks – otters have been spreading back into many areas, including the Darent Valley.

Birds

Trees and bushes along the riverbank are a haven for birdlife – particularly in the more secluded sections of the Darent. Look out for grey wagtail, great tits, great white and little egrets, grey herons and more.

If you’re lucky, you may catch sight of the iridescent turquoise flash of a kingfisher – or even see one surveying the water from a nearby branch. These stunning birds mainly eat fish such as minnows and sticklebacks, but top up their diet with aquatic insects, freshwater shrimps and tadpoles. Territory is extremely important for their survival and typically covers at least a kilometre of river – but can extend as far as 5 km.

Insects

Insects are key to healthy, thriving and sustainable ecosystems and can tell us much about the health of a river system. An abundance of insects enables fish and amphibians to feed and thrive. In turn these attract mammals such as otters and magnificent birds.  Insects act as an indicator species, with short lifespans, small body sizes and specific food requirements. They are very sensitive to changes in oxygen and chemical levels in the water and many species are intolerant to pollution. 

Look out for the full range of insect life – colourful beetles, striking damselflies and dragonflies, beautiful butterflies and moths. Dragonflies and damselflies spend their entire nymphal stage under the water feeding and growing, which can last for a year or more. The male banded demoiselle is metallic blue with a distinctive dark back across its wings. Females are a shiny green and lay their eggs by injecting them into plant stems under the surface of the water. The eggs take two weeks to hatch and the larvae about two years to develop.

If you’re lucky, you might see a beautiful elephant hawk moth which feeds on great willowherb, in the riparian zone adjacent to the river.

Understanding aquifers and flow

The River Darent is a product of the local geology – notably the chalk landscapes of the North Downs and sandstone of the Lower Greensand Ridge. These substrates are highly permeable and hold water like a sponge, creating vast underground aquifers.

The amount of water in the aquifer determines the amount of water discharging to form the river. Flow is critical to the health of the river which is why over-abstraction of aquifers is so damaging. Reduced flow results in fewer, lower quality aquatic habitats. It disrupts the natural processes of erosion and deposition needed to shape and refresh niche habitats needed by fish and invertebrates to complete their life cycle. Low summer flows resulting from the 2022 drought reduced the quality and quantity of clean gravel habitats that are so important for trout spawning.

To understand how the river is impacted by abstraction and changing weather patterns, we need to know more about the aquifers from which the river is fed. Boreholes can measure the height of the water table at various locations. This information, used with knowledge of the geometry of the rock formation, provides indications of the aquifer storage volume at any point in time.

 

Groundwater models covering the main aquifers in England and Wales are owned by the Environment Agency which regulates abstraction based on an assessment of the needs of the environment.

For abstraction to be sustainable, licences should then be allocated on the basis of what remains. However, as we have seen, too often abstraction from chalk catchments is not leaving enough water to sustain the river and its environment.

Groundwater levels fluctuate annually and are normally at their lowest 3-4 months after the end of summer. This is because soil is usually very dry at the end of summer and so will happily soak up any rainfall. It is not until the soil moisture deficit has been addressed that rainfall can start to recharge the aquifer.

Keep track of the local water situation through monthly reports on rainfall, soil moisture deficit, river flows and groundwater levels.

 

Photos: With thanks to Jaanei Walshe for many of the stunning wildlife photographs used on this page.