Purple toadflax (Linaria purpurea), a plant of garden origin that seems quite at home along the Brook.
Cherry Hinton Brook is a small chalk stream flowing from Giant's Grave near Cherry Hinton towards central Cambridge. A report by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire in 2012 identified a range of issues which were adversely affecting the ecology and flora and fauna of the Brook. It also made proposals for action to improve the Brook.
As chalk streams are a Biodiversity Action Plan habitat, potentially very species-rich and rare even at an international level, the Wildlife Trust and Friends of Cherry Hinton Brook were keen to improve the Brook for wildlife.
Since then, a variety of projects, in line with the report, and with the support, encouragement and assistance of Guy Belcher, the Cambridge City Council Biodiversity Officer, have been planned and implemented.
This has been done by contractors managed by the City Council. Cutting back or removing trees and scrub allows more light into the Brook, which encourages the growth of water plants, such as Water Starwort (Callitriche). It also reduces the amount of nutrients added to the Brook by fallen leaves.
Some plants, such as the big sedges (especially Carex riparia and Carex acutiformis), Bulrush or Reedmace (Typha latifolia) and Yellow Flag (Iris pseudacorus), grow very vigorously and have to be cut back periodically as otherwise they would completely block the flow of water. Selective cutting back can create meanders, allowing the water to flow freely while maintaining shelter for small animals, including water voles and moorhens.
Management of the vegetation is also a means of making the water flow faster, so it scours the stream bed, clearing existing silt and slowing the deposit of more silt.
Flow deflectors are one way of improving the flow of water in a stream. By narrowing the channel the flow of water is increased and becomes turbulent, which clears silt and exposes the gravel stream bed. They also trap silt, leading to the gradual narrowing of the channel and the development of channel-side vegetation.
The flow deflectors also create a greater variety of habitats: deep pools, fast-flowing water, exposed gravel and quiet backwaters which may silt up and provide habitat for bankside vegetation.
The gravel stream bed provides a spawning site for fish and home for insects such as the Banded Demoiselle damselfly.
The flow deflectors in Cherry Hinton Brook have been constructed using timber from the tree management work along the Brook. This has removed the need to buy in or transport timber.
It might be thought that the flow deflectors could increase the risk of flooding but if there is a lot of water in the Brook it just flows over the flow deflectors so they do not impede the flow of water.
They have been installed along Cherry Hinton Brook, from Sainsbury's on Coldham's Lane upstream to the grounds of Cherry Hinton Hall. Working parties have included volunteers from Friends of Cherry Hinton Brook, Friends of Cherry Hinton Hall, Cambridge Water, Natural England, Anglian Water and Community Volunteers.
Riffles, areas of shallow water, are created by dumping gravel into the stream bed. They provide new habitats for invertebrates and other water creatures. Some were installed at the same time as the flow deflectors were constructed, and by the same volunteers.
Some plants, particularly introduced species, can become invasive and over-dominant, taking over and adversely affecting stream habitats. One such is New Zealand Pigmyweed (Crassula helmsii). There is not a great deal of this along Cherry Hinton Brook, and it is proposed to remove it when found. Other plants which might need controlling are Japanese Knotweed (Fallopia japonica), Himalayan Balsam (Impatiens glandulifera) and Floating Pennywort (Hydrocotyle ranunculoides).
A wide range of people have helped with the improvement works, including volunteers from Friends of Cherry Hinton Brook, Friends of Cherry Hinton Hall, Cambridge Water, Natural England, Anglian Water and Community Volunteers. Guy Belcher has provided expertise, instruction, equipment and materials, and organised related work, such as cutting back or removing trees and scrub from along the bank. Sainsbury's provided refreshments for lunch and tea breaks. RiverCare have also provided much support and help generally. Funding has come from Cambridge City Council, the Cambridgeshire and Peterborough Biodiversity Partnership and Natural England.
April 2016