Click here for a fascinating explanation and video by Paul Gaskell and the Wild Trout Trust on the real impact of dredging.
Paul used an Emriver kit (fluvial geomorphology in a box!) to replicate the impact of dredging a stable stream. As Paul’s film so clearly shows, dredging drastically de-stabilises a river not only at the dredged site, but a long way up and downstream of the site too.
I’ve been on an anti-dredging rant lately, ever since the government announced a plan to relax by-laws that protect small rivers from zealous farmers with diggers. A river is a dynamic entity, not a long bucket. It is dynamic because it flows, taking with that flow silt and sand and gravel which forms the shape of the channel. But it is also stable because there is a balance between the volume and speed of the water and the natural size of the channel.
So, as Paul’s film shows, when material is removed to make a section of river deeper and wider, that transformation creates a demand for material (to re-set the balance) which is pulled from upstream, and continues to be pulled until the river re-stabilises itself. At the same time, while the dredged section is re-forming, re-balancing its deficit if you like, the natural flow of material downstream is cut off: forcing the river to pull material from the river banks. The up and downstream erosion continues until the river has repaired the damage caused by the dredging.
High energy rivers can repair themselves quite quickly, but will cause unpredictable damage in the process.
Chalk-streams cannot. They are such low energy rivers, formed originally by forces that will never re-occur this side of an ice age, that the damage caused by dredging can be fatal to the habitat of the river. The more drastically dredged chalk streams in East Anglia for example, which we are trying to restore through the Norfolk Rivers Trust, are permanently unstable. The rivers have only sand and silt to work with nowadays as they try, year after year, to recreate the stable state they once had. And then, on a routine basis, this material is removed (sometimes in the name of weed control, although the weeds only need controlling because the river is filling with silt) and the process begins again.