Answer:
   
   A mikva or mikveh is a place used for ritual immersions. A proper
   mikva contains a minimum of 40 SE'AH--about 191 U.S. gallons (Sorry,
   but you'll have to do your own metric conversion.) of undrawn water.
   In general, if there are more than 40 SE'AH, then the remainder of the
   water may come from any source.
   
   "Undrawn" means not filled by bucket or by metal pipes.
   
   Natural lakes, whether or not fed by streams or rivers, fall into the
   category of "undrawn waters." Many synagogues also provide indoor
   mikvas.
   
   Additional information may be found in [5]N. Lamm, A Hedge of Roses;
   and I. Klein, A Guide to Jewish Religious Practice (Chapter 37).

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