The scientific literature often calls whole trees, logs, branches and twigs that lodge in a waterway as Course Woody Debris (CWD), Large Woody Debris (LWD) or Large Woody Habitat (LWH). Many of us in Australia refer to wood in streams as Snags, not to be confused with the good old Aussie sausage, also known as a snag!
Anyway, Snags (this is what we'll refer to as any wood in the stream) are a really important feature in our Australian Rivers.
Here are a few reasons why Wood is Good:
- ask a fisherman, they will tell you that is where fish are
- wood provides a hard surface for the colonisation of biofilm and macroinvertebrates, and a surface on which some fish lay their eggs
- wood forms compex 3-dimensional structures in the water column that provides different sized spaces and habitat zones. In a complex snag there are a range of nooks and crannies (love that word cranny) that also provide a diversity of hydraulic habitats (i.e. different flow velocities and resting places)
- Birds, reptiles, amphibians and mammals use logs and branches for resting, foraging and lookouts
- Logs spanning waterways can provide crossing points for a range of animals
- Wood provides 'hydraulic roughness' that slows flows, can moderate rates of erosion, slows overall sediment transport and yet provide areas of bed diversity (e.g. localised scour holes and downstream depositional bars). If wood is removed from a stream, you can be left with a featureless, flat stream bed without habitats and more susceptible to erosion processes.
The benefits of snags are now well known and wood is now incorporated into river stabilisation and habitat enhancement works. Stay turned for the upcoming blogs on timber revetment and fish hotels.