The proper way to begin seems to be to make things as simple aspossible by considering different mechanisms in turn. Most of what isseen in an estuary can be related to one of three sources, the wind, thetide, and the river. Most of the analyses to be found in the engineeringtechnical literature discuss the effect of only one or at most two sources,for example the current driven by the wind in a tideless bay or thecirculation driven by the river inflow in a tideless estuary. Taking theliterature as a guide, the next three sections discuss in turn the isolatedeffects of wind, tide, and river. In each case we will discussqualitatively why and how the source causes mixing, and will quotewhat analytical results can be found. Later, in Section 7.4, we attemptan analytical synthesis based on a decomposition of the salinity andvelocity profiles.