River architecture

Rivers vary in their structure, their architecture (Fig. 7.1). This structure is, firstly, that which surrounds the water: the bank, bed, rocks, shoals, vegetation and its shade, together with artefacts such as bridge piers, weirs, mill-wheels and altered banks. The natural structures are due to the factors already discussed: land form, geomorphology, discharge, erosion, sedimentation, rock type (Chapters 3 and 5). These are then modified by engineering, trampling livestock, burrowing animals, boats, anglers, constructions and other activities.

Plants are influenced, often controlled by this inorganic structure. They grow because of, by, in and over it … or avoid it, as the case may be. The presence of some plants may also influence the structure of others (Chapter 5).

For fish, the vegetation is as much part of the river architecture as is the gravel in the bed. Some invertebrates live just on plants: plants are their river structure. The same applies to some diatoms and other periphyton.

The identity of ‘structure’ therefore varies with the organisms using it. Substrate structure is the most important building block in mountain and rocky streams, indeed may be the only sizeable building block in really swift water. But in lowland streams the gentle, earthy substrate is also a vital building block.