The annual juvenile flounder migration has started, with these 1-2cm, translucent flatfish moving upstream from Gravesend and into the freshwater of the Beverley Brook. They will stay here for some time before returning to estuaries and saline water to spawn. As well as this, during a recent education session with Lowther School, we found minnow larvae so young that they still had their yolk sacs attached. For the first time on record outside of Richmond Park, we found juvenile stone loach. Stone loach prefer clean, healthy streams, so this is a good sign that the oil spill earlier in the year didn’t have too large an affect. In the same stretch of the brook, whilst carrying out surveys again, for the first time on record outside of Richmond Park, we found perch, roach and chub. Very good signs.
All these precious fish were found amongst the hemlock water dropwort, brambles and nettles that hang into the brook from the bank. This highlights the importance of woody debris in rivers. For too long we have managed our blue corridors poorly, thinking that straightening, widening, deepening, and clearing watercourses that rush water downstream is the best thing to do. As such, wood and other natural debris have historically been removed. Not only is this bad news for wildlife, but for flooding, too. With very little in-channel vegetation and features, it is suspected that most fry will be washed out to the Thames at the first high flow or heavy rain. To lend weight to this theory, after recent rainfall, we witnessed a shoal of small chub stuck down by Westfields sluice, and now even lower down the catchment. If this rushing torrent of water meets a culvert or road bridge it may burst its banks or wipe out whatever is in its path. We do not have to look far afield or even into the recent past for examples of this.
Fortunately, this is again where woody debris, like wind blown trees, or dare I say, even beaver dams, come into play. These trees and wood give fish and invertebrates a place to shelter from predation and reproduce. They also create a medium for algae and other vegetation to grow, trap silt and initiate scour of the riverbed, exposing the natural gravels which are so important for spawning. Trees (and beaver dams) in rivers hold back water, letting it flow through gradually. So, during periods of high flow (e.g. after a big storm), the downstream movement of water will be controlled; in places like Barnes and the Beverley Brook catchment, this is very important!
Some of the Beverley Brook is tidal and when the tide is high and there is a heavy flow, the top end of the tide acts as a wall, stopping water flow downstream. This is called ‘tide lock’ and is what resulted in the flooding last July. If we were to place or leave windblown trees and woody debris in the river, perhaps replicating a series of beaver dams, the water would take much longer getting downstream, decreasing the chances and risk of flood events in the future. This is something that we will be investigating as part of our new innovative, flood resilience project which we are partnering with the London Borough of Richmond-upon-Thames and the Wildfowl and Wetlands Trust to deliver. If you want to find out more about it or learn more about water systems, the Beverley Brook or fish, come along to Barnes Fair this Saturday the 9th of July and have a chat with us.