A filtration spring is a spring whose water percolates from numerous small openings in permeable material. Discharge may be large or small. It is often used as a synonym to seepage spring, although a seepage spring is generally regarded as having small discharge. Pocket spring is another, albeit rarely used, synonym (it is not included in the American Geological Institute's Glossary of Geology, for example). See
http://de.wikipedia.org/wiki/Schuttquelle for a definition of Schuttquelle. It can be seen that no definition is entirely congruent, although pocket spring is further from the mark IMO, because neither filtration spring nor seepage spring rely on depressions, irregular or otherwise.
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