Fresh water, in comparison with the gigantic marine waters, occurs as small and very
small inland waters. Rivers and lakes are the most striking collections of water, and to
these must be added underground water.
The lakes are of very different sizes and depths. The Feldsee in the Black Forest is only
380 m long and 32 m deep; the Bodensee is about 63 km long with a maximum depth of
252 m; Lake Tanganyika is 650 km long and 1450 m deep. Shallow lakes, the bottom of
which may be everywhere colonized by plants, are called pools or—if they have an artificial
outlet—ponds. Funnel-shaped cavities full of water, marshes, bogs on moors, pools in woods
and meadows, and also drains (moats) are examples of small collections of water. Plant waters are also very interesting, being cavities in plants filled with water as, for example,
the cavities in beech-tree trunks. Collections of water which persist for only a short time,
but recur more or less regularly, are periodic or temporary waters.