Introduction

The variability between estuaries, and within a single system over time, is staggering. In North America alone, there are thousands, with the number depending on how one counts. The classification of these many estuaries is discussed in Chapter 1. Here we provide some examples of time and space variability and describe it more formally for typical shallow estuaries in a simple analytical framework. The discussion considers only positive estuaries, where river flow plus precipitation exceeds evaporation, so that salinity decreases toward the head of the estuary. Estuaries in arid climates are considered in Chapter 9.

Examples of estuarine variability

Estuarine variability can be described as being either:

• Intratidal variability, that occurs at tidal frequencies of 12–25 hours or on even shorter time scales. The diurnal (daily) and semidiurnal (twice-daily) astronomical tides and their “overtides” (circulation driven by non-linear processes and occurring at sums or multiples of the basic astronomical frequencies) are the most obvious examples. Also in this category are variations in scalar properties (e.g., salinity, temperature and density) driven directly by tidal currents, the effects of a daily sea breeze, harbor seiches with periods of minutes to hours, internal waves, inertial motion at the local pendulum frequency (periods of 12–20 hours at mid-latitudes) caused by impulsive wind forcing (large estuaries only), and variations in currents and scalar properties driven by tidal variations in vertical mixing and the along-channel density gradient. This last category includes the related topics of tidal straining, internal asymmetry, and strain-induced periodic stratification.

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