%0 Journal Article %J Journal of Glaciology %D 1997 %T The relationship between subglacial water pressure and velocity of Findelengletscher, Switzerland, during its advance and retreat} volume = {43 %A Iken, A. %A Truffer, M. %X Findelengletscher, Switzerland, advanced about 250 m between 1979$\backslash$nand 1985, and retreated thereafter. Subglacial water pressure, surface$\backslash$nvelocity and surface strain rate were determined at several sites.$\backslash$nThe measurements were made early in the melt seasons of 1980, 1982,$\backslash$n1985 and 1994 and in the autumn of 1983 and the winter of 1984. Changes$\backslash$nof surface geometry were assessed from aerial photographs. The estimated$\backslash$nbasal shear stress changed little between 1982 and 1994. Nevertheless,$\backslash$nlarge changes in the relationship of subglacial water pressure and$\backslash$nsurface velocity were observed, which cannot be reconciled with the$\backslash$nmost commonly used sliding law unless it is modified substantially$\backslash$nConsideration of possible reasons indicates that a change in the$\backslash$nsubglacial drainage system occurred, probably involving a change$\backslash$nin the degree of cavity interconnection. Isolated cavities damp the$\backslash$nvariations in sliding velocity that normally result from changes$\backslash$nin water pressure, because the pressure in isolated cavities decreases$\backslash$nas the sliding speed increases. In contrast, by transmitting water-pressure$\backslash$nfluctuations to a larger area of the bed, interconnected cavities$\backslash$namplify the effect of water-pressure fluctuations on sliding speed.$\backslash$nThus, we suggest that an observed decrease in velocity (for a given$\backslash$nwater pressure) between 1982 and 1994 was a consequence of a decrease$\backslash$nin the interconnectedness of the subglacial cavity system. %B Journal of Glaciology %P 328–338 %@ 0022-1430 %G eng %R 10.1017/CBO9781107415324.004