TEMPORAL AND SPATIAL DEPOSITIONAL PATTERNS IN THE SEVERN ESTUARY, SOUTHWESTERN BRITAIN - INTERTIDAL STUDIES AT SPRING-REAP AND SEASONAL SCALES, 1991-1993

The estuary is a high-energy and very variable system, with a hypertid al, semidiurnal tidal regime. The amount and coarseness of the sedimen t trapped on filter papers, and the suspended silt concentration in th e flood tidal front, were measured daily from mudflat and marsh statio ns at each of six sites during eight spring-neap cycles spread over a two-year period. Four of the cycles were associated with vernal equino xes and four with autumnal equinoxes. River discharge, wind speed, tid al height and the suspended sediment concentration all affect the amou nt and texture of the trapped mud. Complex tidal patterns are unlikely to be registered other than very imperfectly in these tidal deposits. At any site, the silts deposited on the mudflats are slightly coarser than those accreted on the marshes. Both marsh and mudflat deposits t end to become less sandy but more clayey toward the mouth of the estua ry. The sediments trapped on the mudflats and marshes during vernal eq uinoxes tend at a site to be sandier and less clayey than their autumn al counterparts. As the vernal and autumnal, equinoctial tidal regimes are virtually identical, and river discharge has a negligible influen ce, winter storminess and the substantial annual change in water tempe rature, affecting viscosity, may largely be the explanation, with biol ogical factors possibly augmenting the effect. (C) 1998 Elsevier Scien ce B.V. All rights reserved.