The Crane Valley Partnership

Heavily Modified Channels

River Channelisation

River channelisation involves a series of engineering practices most commonly used to control flooding, to drain wetlands and to improve navigation. Methods of channelisation found on the River Crane include: widening, straightening and bank enforcement.

Alterations to a river’s morphology, such as those found on many parts of the Crane, are known to negatively impact the way a river functions. By removing meanders, river channels are shortened and gradients are increased. This has been shown to alter flows, depth and affect the deposition of sediment along many reaches of river.

River channelisation has been shown to have a negative effect on wildlife, especially fish. This can be attributed to the loss of the natural pool-riffle sequence associated with the variety of low flows that fish require for shelter. Meanders create the areas of slack water fish that require to hold position when currents are fast. Straightened channels also lack the habitat complexity that diverse populations of macro-invertebrates need to thrive. Enforcing the banks of rivers also reduces biodiversity in terms of marginal and riparian plant life, avian species including kingfishers, and mammals such as water voles.

Barriers to Fish Migration

The Crane hosts a number of man-made barriers that prevent fish stocks utilising the river’s entire reach. These barriers also prohibit the ingress of fish species from the River Thames. This is hindering the rehabilitation of the river’s fish stocks and poses a serious threat to migratory species such as the European eel.

The Zoological Society of London have identified 19 barriers on the lower river and their eel monitoring surveys show that elvers (juvenile eels migrating to the river from the sea via the Thames) are unable to pass these obstructions. The EA have begun identifying barriers for removal or for mitigation via the installation of fish passes.

Kidds Mill Sluice

Restoring Channalised Stretches of River

In order to restore these heavily modified stretches of river we must redefine the river’s shape by: creating meanders and using flow deflectors to naturalise straightened channels; removing wooden toe-boarding where appropriate to improve the riparian zone; narrowing overly widened sections, in order to improve flow, by redefining banks through techniques such as faggoting and spiling; removing culverts to open up lost sections of river; improving substrate via the introduction of new gravels or the creation of structures that cause scouring.