Did you know there are only 260 chalk streams in the world, and Hertfordshire and Middlesex is home to 10% of this global resource? These unique river systems are globally rare and support some of our most vulnerable species, including the critically endangered Water Vole, wild Brown Trout, European Eel and Kingfisher. That’s why we’re coordinating a week of highlighting and celebrating these fantastic ecosystems – the jewel in our county’s crown! What better way than to kick this off than sharing eight indicator species for you to look out for.

Individually the presence of these species shows that some elements of the chalk stream are healthy but collectively, if we have them all present and thriving, we should expect a wholly healthy chalk river. We have chosen them as indicators of the trinity of what is needed for chalk stream health – good water quality, sufficient flow and good physical habitat.

Water-crowfoot – Indicator of good flow and good water quality

Water-crowfoot is a member of the buttercup family, although that might not be immediately obvious as it has delicate white flowers. These can be seen bobbing amidst strands or mats of foliage partly-submerged or on the surface of our chalk rivers, from May to September. Take a closer look and you’ll notice the similarity of its five-petalled flowers to its better-known cousin and its leaves bear an uncanny resemblance too. Water-crowfoot doesn’t just look pretty, it provides valuable micro-habitats, with its floating canopy and underwater structure, providing shelter for fish, offering security from predators and, also, shade during hot weather.

European Eel – Indicator of barriers being overcome

The European Eel is a long, smooth, snake-like fish, which can grow to over a metre in length. They vary in colouring, depending on their age, ranging from silver to olive-green but, in common, they all have yellow-bellies. Eels are predators and scavengers – they feed on dead animals, fish eggs, invertebrates and other fish. They are also known to eat the American Signal Crayfish, an invasive non-native species. The unique eco-system of a chalk river provides a suitable habitat for young eels (known as elvers), facilitates their migration to their breeding ground in the west Atlantic and acts as a refuge for declining populations. With their water quality being high, diverse plant life and constant water temperature, chalk rivers are a prime habitat for eels to thrive within.  Currently though, European Eels are not reaching the classic chalk streams, mostly the tributaries. There are currently 316 barriers to eels in the Lea catchment, including dams and weirs. Our work, which we have started, aims to make these barriers passable to eels, therefore their presence as an indicator will highlight that the barriers are being overcome.

Water Vole - Indicator of well vegetated and diverse banks of chalk streams where American Mink have been eradicated

The Water Vole has chestnut-brown fur, a blunt, rounded nose, small ears and a furry tail. Water Voles may be seen at the water’s edge or, look out for signs of their occupation, which include roundish burrows in the river bank, small piles of nibbled reeds, or latrines – green-brown, tic-tac shaped droppings. The species is often referred to as a ‘mini-ecosystem engineer’ and its presence is invaluable to creating the balance of a healthy chalk river; it maintains and shapes bankside vegetation, moves nutrients, and is food source for other species, including predatory fish and birds-of-prey.

Did you know? Water Voles used to be commonly found on rivers throughout Hertfordshire and the rest of the country, but over the last 50 years they have become the UK’s fastest-declining mammal, with populations decreasing by 95%. This has been due to two main factors - loss of quality wetland habitats and predation by American Mink, an invasive non-native species. There is hope though; The Trust has managed a number of successful Water Vole reintroductions and is working to get them back on all of the county’s rivers by 2030.

Yellow Sally Stonefly - Indicator of good water quality and in stream habitat

The Yellow Sally Stonefly is one of 34 species of stonefly in the UK but they can be tricky to tell apart – this one though can be identified as a sulphur yellow blur when hit by sunlight. In general stoneflies have narrow, dark wings, long antennae and two stout tail bristles. When at rest, they fold their wings flat over their bodies and when in fast-flowing water they can be witnessed clinging to the under-sides of rocks. Stoneflies are sensitive to environmental changes so give us early warning of pollution or habitat degradation and, as such, their presence gives us an indication of water quality and a healthy chalk river. They are also a vital food source for fish, birds and other wildlife.

Yellow May Dun mayfly - Indicator of good water quality and in stream habitat

The Yellow May Dun is one of 51 species of mayfly recorded in the UK. The main hatch for this species starts in May and continues through to July, with their nymphs present all year round. A distinctive-looking invertebrate, Yellow May Dun, are, as their name suggests vibrant yellow in colour, and they have broad, lace-like, clear wings, very short antennae and up to three, very long, fine tail bristles.

Did you know? Fishing-flies are made to look like mayflies as they are the favourite food for another of our chalk river dwellers, Brown Trout!

Beautiful Demoiselle damselfly - Indicator of good riparian habitat – gravelly beds, diverse emergent vegetation with a diverse bankside including trees and open grassland

This stunning damselfly lives up to its name – the Beautiful Demoiselle is certainly a looker with its metallic blue and green colours making it hard to miss. There are distinctive differences between the male and the female of the species; males have dark-coloured wings and metallic blue-green bodies, whilst females have brown wings and green bodies. Both are around 4.5cm in length and can be seen from May to August on our fast-flowing rivers. Males will often take rest on bankside vegetation until a female comes along and then, they will try to impress with a fluttering, courtship dance! This species is currently rare but as we improve the riparian habitat, we can expect the Beautiful Demoiselle to colonise, hence a good indicator of a quality chalk river.

Kingfisher – Indicator of all-round river health

The striking Kingfisher needs little introduction and is often seen in a flash of bright blue and metallic copper, diving into our chalk rivers, or sitting patiently on a low-hanging branch waiting for its next meal to swim by! An apex predator, the Kingfisher’s presence indicates that suitable food is present, and that nesting banks are appropriate for breeding. With chalk rivers being fed by underground chalk aquifers, the nutrient-rich, gin-clear, clean water offers aquatic life, including the fish that Kingfisher prey upon, ideal conditions. The abundance of food sources these rivers offer has a direct impact on the populations of these attractive and iconic birds.

Brown Trout - Indicator of good water quality and good flow

The clear, cool and consistently flowing water afforded by our chalk rivers provide ideal conditions for wild Brown Trout to spawn and thrive in – a habitat which is crucial for populations of these fish. Females lay their eggs on the gravel beds after cutting a redd (using her tail to dig a shallow pot in the river bed) and they are fertilised by males externally before being buried in the gravel. Good flow is needed to keep bed gravels clean for this breeding process. Brown Trout are golden-brown in colour with a dark back, creamy-yellow belly, and plain, dark tail fin. They are predatory fish, feeding on mayflies, damselflies and other small insects, as well as insect larvae and small fish.

We hope you have discovered more about why chalk rivers are so special and that this ‘What to See’ demonstrates the importance of this fragile ecosystem and the wildlife it supports. You can discover more about our chalk rivers this month with our news and blogs being focused on Herts Rivers Week and do check out our programme of events too.