Ignorance about river flow

Recent comments in the Press and on television suggest that well-meaning councillors and our MP are still betraying their poor understanding of the natural environment when discussing the River Severn.

Decisions made out of ignorance can cause problems. Rivers create their own channels to accommodate their most frequent flood discharge, most of the time they flow well within their banks doing little morphological work between the shorter bursts of more vigorous activity.

Channel capacity at such times is determined by its constant "cross-sectional area", which can be expressed as "deep and narrow" or "wide and shallow".

Interference with one will lead to change in the other, as the river seeks the status quo. For instance dredging "the flash" below Coton Hill in the 1920s certainly allowed pleasure craft to get through at low water, but caused the river to quickly abandon and silt up the medieval barge gutter. This should be a warning to all those in favour of dredging.

At the Welsh Bridge, removal of the medieval structure whose piers had taken up channel space naturally allowed the river to become narrower. Artificial infill of Frankwell Quay encouraged this too, as revealed by the theatre excavations.

The documentary evidence for many changes is readily available in Shropshire Archives. I hope this interpretation will contribute to a more sensible discussion of the river.

David Pannett, (Shropshire Geological Society) Shrewsbury