Why is sediment sorting important?

WTT Conservation Officer Andy Thomas writes:

I took this photo recently of a very small patch of river bed on a headwater stream of the River Test and it encapsulates in one shot why the sorting of river bed material is so important.

Habitat quality here is regulated by a combination of channel width, flow power, available sunlight and a lump of tree that’s fallen in the river (out of shot). This tiny strip of river margin supports four distinct habitats which have been sorted by a gradient of flow velocities, downstream of the tree limb, producing a sequence of habitat types favoured by a range of different fish and invertebrate species, even over such a tiny range.

• The gravel itself could potentially be a decent trout spawning site in winter but will also support cased caddis on the underside of the larger stones. I know – I looked!
• The weedy strip is brilliant trout fry habitat and will be favoured by certain olive species (particularly nymphs of the blue winged olive Serratella ignita) and the larvae of black gnats Simulium,
• The layer of woody detritus will be favoured by shredding invertebrates such as shrimp Gammarus,
• The inside line of finest settled material (silt) will be favoured by burrowing mayfly nymphs and young brook lampreys.

Four critically important habitats in a few square foot of river bed, all supported by a single, innocuous lump of fallen wood.