The River Thame Conservation Trust installed a fish easement in May 2018 to help fish navigate over a stone sluice structure that they would otherwise be unable to swim over.
History – In the past we humans have changed once natural river systems for our own uses, for agriculture, building developments, navigation or water supply. Fish evolved in rivers long before humans had this influence and fish species are often not adapted to the changes we’ve made over time, such as dredged river beds, straightened channels and weirs which hold back water flow and block movement.
The River Thame and its tributaries has many blockages up its course, most of which are easily explained by the large number of watermills – signs of a time when the river was a major source of power in the rural economy and weirs were important to help provide that power.
Impact on River Species – When habitats which were once continuous are now divided by weirs and sluices, it restricts the movement of freshwater species, separating them from their food sources and the variety of habitats they need to complete their life cycles. This can badly affect their survival and breeding success, sometimes leading to species disappearing altogether in some stretches of river.
To add to the challenges for river species, the River Thame has been a victim of several pollution incidents over the last few decades and thousands of fish have been killed. Fragmented habitats are less resilient to these pollution incidents as the blockages prevent fish and other mobile species from moving away from the pollution and being able to re- colonise after the events.
What has happened at Waterstock? The sluice on the River Thame at Waterstock Mill was a complete barrier to fish passage (except when the river was in very high flow). On each flank, the concrete and stone walls were also structurally failing. The removal of the sluice structure was not an option so instead, to provide fish easement a pool was created on the structure itself by constructing a wall at the upstream and downstream ends of the structure to back up the flow of water.
A channel which is a secondary channel to the main River Thame was also restored as part of the project. It had been highly shaded by trees, with steep banks, it was over wide and VERY silty. Brushwood berms were created by hand with the help of a team of Fishery Officers from the Environment Agency and gravel was added. These works narrow the channel and speed up the flow of water which helps to keep the new gravels clean of silt. It creates a variety of habitat for fish and invertebrate species. The clean gravel is also important for gravel spawning fish such as chub and dace. Trees were managed along this stretch of channel to create a mixture of shaded and open channel. It’s all about variety when it comes to river habitats!