You can always have too much or too little of a good thing. When it comes to revetments (re-inforcements) of river-banks; there are a whole host of pitfalls.
On the one hand, many sections of river suffer from excessive grazing of the land surrounding them that leads to a dramatic reduction in the variety of bugs, plants as well as fish populations that can be supported. In addition, many rivers that run through towns and cities often pass through quite intensively used land upstream of urban reaches. The excessive inputs of fine silt and sand where bank-erosion is rampant and extensive often end up being accumulated in the engineered sections of channels in towns and cities. As well as causing maintenance problems,this can smother what may, otherwise, be perfectly good spawning gravel.
Conversely, where efforts to "lock" a river channel in one place are over-zealous; the result will be the strangulation of supply of spawning gravels and a variety of cobbles and other river-bed material. Each of the different diameters of gravels/cobbles/boulders that come from eroded banks form a unique and vital habitat either for particular species or particular stages within the lifecycle of a species.
So the optimum for biodiversity (and, consequently...
Bank erosion - a matter of balance
Posted on October 23, 2013