The Mawddach Estuary and Barmouth Bay now occupy what was a deeply incised glaciated valley at the last ice maximum. This valley is now filled by a complex suite of sediments, up to 75m thick at the present estuary mouth, which record increasing marine influence. A regressive coastline is also present today. In the Bay, Welsh glacial drifts were eroded to form deep depressions, infilled by complex cross- stratified Late- or Postglacial sediments, overlain by a parallel-stratified sequence, perhaps formed behind a coast-parallel morainal barrier. Net sediment transport of the fine sand in the Bay is shoreward, controlled by a strong mesotidal and wave regime. Calculated sediment transport rates infer long term sediment accumulation rates of 82mm per year for the estuary as a whole, but repeated surveys suggest much spatial variation within the estuary. Waves greatly enhance sediment transport in the bay, by approximately 30 times in storms, when coarse sand is mobilised. The estuarine sand fines landward, from medium- to very finegrained. Grain size and petrological evidence suggest sediment is derived from erosion of glacial deposits in Cardigan Bay. A highly skewed tidal wave creates strong flood currents and net flood sediment transport throughout the estuary and in the southwest portion of the bay. The estuary is generally well-mixed, and the tidal wave is a standing wave with a progressive component. The estuary comprises a two part hydrodynamic system, with decreased currents and sediment transport inland of a hard rock constriction. Intertidal megaripples dominate large areas of the seaward portion of the estuary. They show very complex morphological and dynamic behaviour over a lunar cycle. Detailed velocity profile measurements over these megaripples, show complex relationships of bedform to flow. Flow parameters show considerable scatter, and have only weak correlations with bedform morphology or migration. Within each flood-ebb cycle, the relationship of shear velocity to roughness length appears related to the underlying bedforms. Natural variation of megaripple behaviour is a major factor in limiting palaeoflow estimates from deposits of intertidal megaripples. A concluding chapter discusses the relationship between sea-level, sedimentation, and estuarine hydrodynamics, through the Holocene. Sea level rise brought drowning and erosion of coastal glacial complexes, leading to conditions allowing spit formation and growth, with, in the estuary, increased tidal asymmetry, salinity and low water elevation. Fine-grained estuarine sedimentation was initiated in the sheltered back-barrier region, intertidal flats and supratidal marshes prograded, and tidal channels were progressively restricted towards the northern side of the estuary mouth. Within the modern estuarine sand wedge, preserved estuarine sedimentary structures may exhibit an upwards-increasing expression of flood- directed sediment transport.