Habitat Restoration at Little Ann

Brown trout (Salmo trutta) © Linda Pitkin/2020VISION

Chalk streams are fantastic places for wild fish, but only if they reach their full potential. The Watercress and Winterbournes partners have recently given the Pillhill Brook a helping hand.

In the hamlet of Little Ann around 700m of the Pillhill Brook runs through an area called The Water Meadows. Despite many years of careful management, the chalk stream has recently seen a decline in its once-healthy brown trout population. Keen to support this iconic local species, the landowner decided to call in some expert assistance.

The Wild Trout Trust led on delivering a solution through Watercress and Winterbournes, in which they and the Wildlife Trust are both partners. Site visits revealed that the loss of brown trout was caused by excess sediment on the stream bed - this is a problem for fish as it means shallow water, fewer invertebrates to eat, and insufficient oxygen for egg development.

In a healthy stream, the water's movement removes material from some areas (a process called 'scour') and deposits it in others. This forms a varied, meandering channel in which the water moves fast enough to carry sediment away. However, if environmental pressures introduce sediment or slow the flow then the stream can become overly shallow and sluggish.

The partners therefore aimed to not only restore the stream's fish habitat, but also maintain it by tackling the root causes of its reduction. We checked the area for water voles, and scheduled the works for October to avoid fish and bird breeding seasons. Then it was time to get cracking!

Fish habitat

In order to call a chalk stream home, fish like brown trout need places where they can stop to eat, rest, and spawn - this is called 'holding'. Scour pools are often ideal for this purpose: carved from the stream bed by flowing water, they offer cool, deep water and gravelly areas for spawning.

Scour pools form naturally under the right conditions, but the stream at Little Ann only had one suitable for adult fish. It lies just beyond a historic sluice gate, the remains of which narrow the channel and quicken the flow enough for a pool to form. We mimicked this natural process at five points along the stream, digging pools with tapered gravel 'tails' for a gentle, sloping exit.

To keep these new pools clear from sediment, we flanked them with logs and brushwood from nearby forestry work. These were angled into the stream to narrow and diversify the channel, much as a fallen tree would; the resulting faster flow will help to stop the pools from filling with sediment over time.

Tree cover

They may stay on the sidelines, but trees are actually integral to the health of adjacent chalk streams. Their branches give shelter to birds and insects, while their leaves keep the water cool in the shade. Their roots block sediment washed off the land and secure the banks against erosion. Even fallen trees are valuable, as their presence in the channel encourages the formation of scour pools.

It's possible to have too much of a good thing, however, as excessive shade can limit the growth of aquatic plants - this makes the channel wider and less welcoming for wildlife. The stream at Little Ann had spots that were over-shaded and others which needed more cover; we rectified this by thinning some trees and planting cuttings of fast-growing species like willow.

Softened banks

Artificial or 'hard' banks, such as brick walls and metal fences, can stop plants from growing in certain parts of the stream. This not only reduces wildlife habitat, but also leaves those banks exposed to erosion. This is especially true for the 'toe', the lowest part of the bank which is submerged most often, but all parts can be vulnerable when the water is high.

The stream at Little Ann had 250m of hard banks, which we 'softened' with a revetment - a parallel structure that acts as a buffer. Wood bundles were combined with biodegradable mesh, filled in with gravel and topsoil, then planted with aquatic species like iris, sedge, and reed canary grass. As these plants grow they will give the banks even more protection against erosion.

Drinking stations

The clean drinking water offered by the chalk streams is hugely important to many livestock owners. Unfortunately cattle can wear away a stream's banks by walking along the edge and grazing the vegetation; these actions loosen the soil, releasing sediment and widening the channel.

The stream at Little Ann had a partial solution in the form of fenced-in drinking bays, which had somewhat helped the issue. To offer a more comprehensive, long-term solution, however, we replaced these bays with two drinking stations in stream-adjacent fields where cattle graze.

These pools are connected to the stream by underground pipes, which top them up if their water levels get too low. This system will allow the cattle to drink from the stream without eroding its banks, and as a bonus will provide extra wetland habitat for the area's wildlife!

Cherishing our chalk streams

We're so pleased to have made these improvements at Little Ann - huge thanks to the landowner for approaching us and to the Wild Trout Trust for leading on the works. Watercress and Winterbournes is running a wide range of activities to protect, enhance, and celebrate seven local chalk streams - explore our plans.