CHAPTER II
The Tidal Compartment of a River
   The tidal compartment of a river may be defined as that portion of the stream which intervenes between the area of unimpeded tidal action and that in which there is a complete cessation or absence of tidal action. On the seaward side of the tidal compartment the accession and recession of the tides follow conditions similar to those of the contiguous open sea, unless these are modified by what may be termed accidental, or by local causes, such as shoals, races, or the gorging action of wind.
   Rain and Rivers.—There is infinite variety in the régime of rivers, due to the contour and gradient of their bed and other physical conditions. The variations of rainfall on the earth’s surface lie between nil and about 300 inches per year. On portions of the Essex coast the average is as low as 13 or 14 inches, and the Highlander’s remark that the rainfall in his district was “aboot twelve fut” was under the mark in respect of the actual fall in some mountain districts of England. The extent to which percolation takes place varies relatively with the degree of impermeability of the subsoil, stiff clay or rock being water-arresting subsoils, whereas sand is penetrative in a high degree, until its pores become water-logged or choked with argillaceous matter. There is a wide range in the percentages of rainfall which reach their respective river systems. The average annual percentage of infiltration in England is about 42 per cent; the mean daily evaporation is
0.08 inch. These figures, however, have little value in relation to the problems of specific localities.
   River Contours.—The force of the ebb current in a river
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