The construction of three dams induced large changes in the tide range in the Eastern Scheldt, a tidal inlet in the southwestern part of the Netherlands. In 1986 the mean high tide level was reduced by almost one metre. This excluded the greater part of the medium-high marshes almost completely from tidal flooding. The absence of tidal flooding in the summer of 1986, combined with a net precipitation deficit, increased the bulk density of the sediment irreversibly from 400 to 530 kg m–3 in the top 5 cm. The subsidence of the backmarshes varied from 1 to 8 cm. The loss of moisture allowed oxygen to diffuse into the initial reducing sediment. This altered the geochemistry of the sediment significantly. The redoxcline was lowered from a mean depth of about 15 cm in 1985 to 20–30 cm in the summer of 1986. Evidence of pyrite oxidation within a narrow depth interval of 15–30 cm was obtained from the change in the composition of pore waters. The rapid increase in redox potentials (up to 600 mV) and total dissolved iron (up to 5 mM 1–1) and SO inf4 sup2– (up to 65 mM 1–1) and the decrease in pH (up to 4.5–2.5) all suggest a rapid oxidation of pyrite. Acidic conditions were found only in sediments with low contents of calcium carbonate and high contents of pyrite. The importance of seasonal changes in redox processes on the partial decalcification of the salt marsh sediments is discussed. The established of a new tide range was reflected in the pH and Eh of the sediment. In December 1987 the depth profiles of pH and Eh were again close to those observed in 1985.