Introduction

Worldwide there are many estuaries that experience low inflow at times, resulting in the absence of classical low-salinity, density-driven estuarine circulation. However, these systems function like classical hypopycnal basins during periods following rain and enhanced river flow (even if brief and infrequent). Further, the morphology and ecology of these low-inflow estuaries (LIEs) are clearly similar to those estuaries with more persistent freshwater inflow. Through an increase in the diversity of estuaries studied in recent decades, extending beyond cool, wet regions (e.g., western Europe, northeastern United States), it has become evident that estuaries with low-inflow periods are as common as estuaries with persistent inflow of significant volumes of freshwater (often called “classical” estuaries, as they are the focus of the older/classical estuarine literature). Wherever evaporation exceeds precipitation for a time longer than the basin residence time (e.g., during the dry season), it is possible to find LIEs associated with watersheds of smaller size and storage volume (where runoff approaches zero during the dry season). Recognizing that evaporation is on the order of 1–10 mm per day, a brief review of seasonal precipitation maps (e.g., http://geography.uoregon.edu/envchange/clim_animations/gifs/pminuse_web.gif) reveals that LIEs may be found along many coastlines of the world. Further, in regions where coastal lands slope steeply to the sea (e.g., California, Chile, South Africa), watersheds are smaller and faster, hydrographs are short-tailed and LIEs are a common occurrence (e.g., Largier et al., 1997).

The approach in this chapter is at the scale of the basin.