The present work describes spatial and seasonal variability in grain-size, pH and elemental characteristics (TOC, TIC, TN and C/N) in superficial sediments, as well as salinity, temperature and oxygen in the water, within the intertidal range of the Guadiana Estuary, SW Iberian Peninsula, during the year 2010. The results indicate that environmental parameters in the estuary are strongly dependent on spatial patterns, which reflect seasonal oscillations in freshwater discharge. The sediment is generally poorly sorted, with a symmetrical to very finely skewed distribution, in accordance with the low-energy conditions typical of the deposition areas. During winter, heavy rains forced the continuous discharge of Europe's largest reservoir, the Alqueva dam, creating exceptional hydrodynamic conditions and causing coarser sediment deposition in the estuary. High marsh areas are controlled by flood tides, promoting vertical accretion of muddy sediments, especially silts. The lower areas of the saltmarsh and-mud flat areas are controlled by the ebb, with more efficient export of fines toward the platform and retention of sands, sometimes with significant amounts of bioclasts. In winter (average Q = 654 m 3 s-1), salinity remained low throughout the estuary with the highest values (< 3) recorded up to 3 km inland from the river mouth, except in the most confined environments subject to greater evaporation. In summer (average Q = 52 m 3 s-1), the estuarine waters are better mixed and significant saltwater intrusion extends approximately 8 km inland. The seasonal variation in surface water temperature was very high, with a difference of ca 14 º C in the maximum temperature recorded in winter and summer. A clear relationship between elevation (in relation to mean sea level) and organic matter, pH and particle size was observed. In the higher areas of the marsh, where the hydrodynamics is attenuated, differential deposition of fine sediments promotes organic matter entrapment and low pH. Sedimentary organic matter derives from a mixture of native aquatic and terrestrial sources. The mid-upper estuary areas and higher zones of the saltmarsh in the lower estuary incorporate a greater terrestrial component, whereas the low-middle marsh areas of the lower estuary experience a substantial contribution from indigenous aquatic sources. The present data help to understand the present environmental condition of the Guadiana Estuary and contribute baseline data for future climatic and environmental management studies based on sediment-dependent proxies.
