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Vernal PoolA vernal pool is usually a shallow depression in level ground with no permanent above-ground outlet. In the northeast United States vernal pools fill with the rising water table or with the meltwater and rain of spring. Many vernal pools in the northeast are covered with ice in the winter. Vernal pools are best known in California, Oregon and Washington. In southern California they form on the coastal plateaus where the clays below the surface soil prevent water from percolating into the soil. Further north they form on ancient lava flows where lavas perform the same function as the clays further south. They also proliferate in the California Central Valley. They can form anywhere winters are wet and summers are dry and conditions hold water in the pools. The pools are dry most of the year and fill with the winter rains. They are called vernal pools because they are at their peak in the spring, ("vernal" meaning of, relating to, or occurring in the spring). Despite being dry most of the year, once filled they are teeming with life. The most obvious inhabitants are various species of frogs and toads. Living in the water the most notable inhabitants are daphnia and fairy shrimp. Further north tadpole shrimp are also common. Vernal pools tend to be on land that is ideal for development. As a result the pools have all but disappeared except on protected land. The San Diego Mesa Mint, a highly endangered plant, is found exclusively in vernal pools in the San Diego area. See also *Pot hole
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