Add extremes of grazing, shading or even ploughing and the equilibrium described above is broken: no summer growth to restore original width.

The net effect is that streams become wider and shallower. Silt settles and the gravels become less suitable for fish spawning while also supporting fewer and less diverse invertebrates. Speed of flow and depth (optimum approx. 12cms per second and 30cms deep) is especially critical for juvenile salmon and so the area of fish production is significantly reduced.