ABSTRACT
Extraction of the reserves of kaolin in the St. Austell granite outcrop of Cornwall, to supply the china and paper industries since the late eighteenth century, has imposed an abnormal load of up to 3 % by weight of fine suspended sediment on the streams draining the area. This pollution has caused adjustment of channel form, the width-depth ratio being substantially reduced, but channel capacity relatively unaltered. Analysis of the suspended sediment load of one stream by X-ray diffraction and Coulter Counter methods reveals downstream coarsening and mineralogical adjustment that jointly imply selective deposition of fines. These deposits are probably related to flocculation of kaolin in water of increasing downstream ionic concentration. Bank material of polluted and unpolluted streams differs in terms of plasticity, clay and silt content, and clay mineralogy. Bank clay content and mineralogical indices are used in multiple regressions to predict plasticity index and width-depth ratio, which themselves appear to be inversely related. The best description of width-depth ratio variation is provided by a regression model incorporating discharge or catchment area as a ‘scale’ effect, and indices of clay mineralogy. This satisfactorily predicts the different channel shapes of polluted and unpolluted streams as a result of the altered mineralogical balance of the fine fraction of the bank material of the former.
