Vernal pools generally have no currents because they have no regular sources or spills of their waters.
The two main abiotic factors — things other than plants and animals — related to vernal pools are how the water gets into pools in the spring of the year and the geological nature of the receptacle in which the pool forms. Vernal pools form in the spring and often vanish before the end of summer. These abiotic factors define pools found in two quite distinct main combinations in two regions of North America.
Water from Spring Rains and Snow Melt
Vernal pools form with water that simply accumulates in a basin or depression in any kind of soil. The Massachusetts Division of Fisheries and Wildlife, Natural Heritage and Endangered Species Program, identifies a vernal pool as one “where water is contained for more than two months in the spring and summer of most years and where no reproducing fish populations are present.” These pools vary a great deal in size, but the prototypical pool is only 3 feet deep at its peak season.
Water Rising through the Ground
Especially in the forested eastern parts of the continent, the soil is often saturated with water to a much higher level in the spring than at other times of the year, again due to melting of the winter’s snow and spring rains. Some depressions in the soil may be deep enough to meet this rising groundwater table and form vernal pools. As the soil dries out around the pool and the water table falls again, the pool will shrink and possibly disappear altogether, but it may also come back in a rainy autumn to persist and even freeze over through the winter.
Vernal pools that reflect groundwater levels are generally depressions in porous soils. These pools can be distinguished from spring-fed ponds in that the source of the water appears generalized rather than focused on an identifiable eruption, and in their diminishment or vanishing with the water table. The soil banks of such a pool may rise quite high above the highest water level, or the basin may be nearly indistinguishable from surrounding terrain when the pool is dry.
Hardpan and Rock
Vernal pools may also form without access to groundwater, in depressions in soil or rock where accumulating water seeps very little if at all. One of the places such pools have been studied is in California’s Central Valley, where identified deposits of soil and clay have formed mounds and basins. The vernal pools collect the water that falls on the mounded features of the same soil and hold it until it evaporates.
Vulnerability to Development
Despite attention to wetlands conservation, if an environmental impact survey is conducted at the wrong time of year, it may completely miss the existence of vernal pools, especially in rocky formations. To prevent such errors, conservation groups need to document the existence of pools in land that may be vulnerable to development.