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　TPO1：Groundwater

　　1. The same thing happens to this day, though on a smaller scale, wherever asediment-laden river or stream emerges from a mountain valley onto relatively flat land,dropping its load as the current slows: the water usually spreads out fanwise, depositingthe sediment in the form of a smooth, fan-shaped slope.

　　2. Sediments are also dropped where a river slows on entering a lake or the sea, thedeposited sediments are on a lake floor or the seafloor at first, but will be located inlandat some future date, when the sea level falls or the land rises; such beds are sometimesthousands of meters thick.

　　3. In lowland country almost any spot on the ground may overlie what was once the bedof a river that has since become buried by soil; if they are now below the water’s uppersurface (the water table), the gravels and sands of the former riverbed, and its sandbars,will be saturated with groundwater.

　　4. This is because the gaps among the original grains are often not totally plugged withcementing chemicals; also, parts of the original grains may become dissolved bypercolating groundwater, either while consolidation is taking place or at any timeafterwards.

　　5. But note that porosity is not the same as permeability, which measures the ease withwhich water can flow through a material; this depends on the sizes of the individualcavities and the crevices linking them.

　　6. If the pores are large, the water in them will exist as drops too heavy for surface tensionto hold, and it will drain away; but if the pores are small enough, the water in them willexist as thin films, too light to overcome the force of surface tension holding them inplace; then the water will be firmly held.