Introduction

Macrotidal estuaries can be viewed within a continuum of deltaic-estuarine coastal depositional settings, influenced by riverine processes, wave regime and tidal energy (Wright & Coleman, 1973; Wright, 1985; Boyd, Dalrymple & Zaitlin, 1992; see Fig. 1.6). The morphodynamics of individual macrotidal estuaries are a function of sea-level changes and prior or inherited topography. The response of estuaries to sea-level changes, past and future, is affected by tidal range, nearshore wave climate, river inflow and the nature and supply of sediment. All estuaries assumed their present form during the rise of sea level that followed the last glacial maximum, about 18000 years ago. In areas that are relatively stable, such as northern Australia, estuaries have had similar sealevel histories, whereas in areas of very rapid crustal uplift or glacioisostatic response, estuaries are more likely to have experienced highly individual relative sea-level histories. The variety of estuaries reflects the range of submerged prior landforms, from relatively straight, steep coastlines through valleys in differing stages of infill to rock-barred basins. These different types of prior topography, which are manifest in coasts such as Has and dendritic drowned valley harbours, influence the interplay of processes which redistribute sediment to produce estuarine channels, tidal basins, backwaters and floodplains. Vegetation can modify estuarine morphodynamics through its effects upon sediment trapping and through its influence on the shear strength of channel banks.

This chapter is concerned with macrotidal estuaries, mostly in northern Australia which tectonically is a very stable region, with low sediment yield (about 5–15tkm−2a−1) and broad continental shelves.