WHAT IS A VERNAL POOL ?

The term 'Vernal Pool' primarily comes from California where ephemeral ponds fill in the spring ('vernal' means 'spring').  Defined as seasonally-flooded depressions found on soils with an impermeable layer such as a hardpan, claypan, or volcanic basalt, California's vernal pools occur on a variety of landscape formations, but most often on alluvial formations deposited by ancient waterways and seas. The impermeable layer allows the pools to retain water much longer then the surrounding uplands; nonetheless, the pools are shallow enough to dry up each season.

Seasonally-flooded depressions also occur in Rhode Island and throughout the Northeast. Though the name vernal pool/ pond has long been adopted, the terminology may not be totally appropriate for southern New England, as some ponds fill up with water in the late fall (actually 'autumnal' ponds). Furthermore, in the northeast, the geologic factors that determine where on the landscape seasonally-flooded pools occur is poorly understood.

Nevertheless, the term 'vernal pool' has stuck and the definition remains inconsistent across the country, as well as, from state to state throughout the region. This webpage addresses this issue by listing some of the various definitions.

Two vernal pool studies currently underway at the university - one led by Dr. Peter Paton and another by Dr. Frank Golet (URI Dept. of Natural Resources Science) - are looking at vernal pools in Rhode Island and their relationship to geology, hydrology, vegetation, and other landscape aspects. The focus of Paton's research however is amphibians.

Vernal pools can be rich amphibian breeding areas.  In Rhode Island wood frogs, spotted salamanders, marbled salamanders, and, to a lesser degree, gray treefrogs, spring peepers, American toads, and red-spotted newts rely heavily on vernal pools to complete their breeding cycle. What appears to be one of the most important factors determining which species are found in a given pond is the length of time that surface water inundates the pond (termed "hydroperiod"). Fewer species are able to exist in ponds that dry up relatively rapidly and many species of amphibians avoid ponds that are flooded throughout the year. Permanent ponds tend to have fish populations, which are major predators of young amphibians.  When you look at the accompanying life histories, one of the most important figures to study is the relationship between hydroperiod and the probability of detecting a species. Species such as wood frog prefer pools that dry up each year, whereas bullfrog can only breed successfully in ponds with water year-round
DEFINITIONS VARY FROM STATE TO STATE

MASSACHUSETTS
Wicked Big Puddles. A Guide to the Study and Certification of Vernal Pools.
A vernal pool is a contained basin depression, holding water for two to three months or more, which lacks breeding populations of fish and which supports the breeding of wood frogs or mole salamanders or contains fairy shrimp.  
MAINE
Citizen's Guide to Documenting Vernal Pools.  Maine Audubon Society.
Vernal pools or seasonal wetlands, are defined as naturally occurring, seasonal bodies of water, free of predatory fish populations, that provide breeding habitat for one or more of Maine's four vernal pool indicator species- spotted and blue-spotted salamanders, wood frogs, and fairy shrimp.
NEW HAMPSHIRE 
Identification and Documentation of Vernal Pools in New Hampshire.
A vernal pool is typically a temporary body of water providing essential breeding habitat for certain amphibians and invertebrates and does not support fish.
CONNECTICUT
Vernal Pool Wetlands.
Vernal Pools are small bodies of standing fresh water that are most obvious in the landscape during the spring of the year. They are usually temporary in nature. In order to meet the definition of a vernal pool, a wetland must have the following physical characteristics: (1) it contains water for approximately two months during the growing season (2) it occurs within a confined depression or basin that lacks a permanent outlet stream (3) it lacks any fish population (4) it dries out most years, usually by late summer.
USGS
Vernal pools are small bodies of standing water that form in the spring from meltwater and are often dry by mid-summer or may even be dry before the end of the spring growing season. May vernal ponds occur in depressions in agricultural areas, but may also be found in woodlots. Wetland vegetation may become established but are usually dominated by annuals.