Vernal Pools
Description
Vernal pools are seasonal depressional wetlands that occur in glaciated areas of northeastern and midwestern states. They are covered by shallow water for variable periods from winter to spring, but may be completely dry for most of the summer and fall. These wetlands range in size from small puddles to shallow lakes and are usually found in a gently sloping plain of grassland. Western vernal pools are sometimes connected to each other by small drainages known as vernal swales, forming complexes. Beneath vernal pools lies either bedrock or a hard clay layer in the soil that helps keep water in the pool.
Climatic changes associated with each season cause dramatic changes in the appearance of vernal pools. The pools collect water during winter and spring rains, changing in volume in response to varying weather patterns. During a single season, pools may fill and dry several times. In years of drought, some pools may not fill at all.
The term vernal pool originally referred only to small, intermittently filled wetlands found in the Mediterranean-type climate of the western United States. Today it is used more broadly to include other small ephemeral wetlands found country-wide.
In the spring, wildflowers often bloom in brilliant circles of color that follow the receding shoreline of the pools. By early summer, the water has evaporated, and the clay pools appear brown, barren, and cracked.
Functions & Values
However, appearances may be deceiving. The unique environment of vernal pools provides habitat for numerous rare plants and animals that are able to survive and thrive in these harsh conditions. Many of these plants and animals spend the dry season as seeds, eggs, or cysts, and then grow and reproduce when the ponds are again filled with water. In addition, birds such as egrets, ducks, and hawks use vernal pools as a seasonal source of food and water.
Vernal Pool Geology
Vernal pools form in a variety of ways. The position of a vernal pool on the landscape can suggest its origin. A few common origins of vernal pools in New York are discussed below.
Floodplains
Vernal pools are often formed in the floodplains of streams and rivers. During floods, a stream will overflow its banks and enter the floodplain where the rushing high waters scour pockets in the floodplain. Some of these pockets retain water well and become vernal pools. The natural meandering of a stream channel across a floodplain over the years can also create vernal pool habitats in the form of oxbow wetlands.
Glaciers
Glaciers covered the area during the last ice age. Glaciers created many isolated depressions, through the actions of ice melt, scour, and erosion that fill up with precipitation and snowmelt.
Sag Ponds
Shallow depressions in limestone regions are called sag ponds. Sag ponds form in landscapes where easily dissolved bedrock such as limestone is found underneath an impermeable soil layer derived from sandstone. Over time, groundwater dissolves pockets and holes in the limestone. Eventually the surface soils 'sag' into the depression underneath. Surface water gathers in the depression but the impermeable layer in the soil restricts surface water infiltration and creates a perched water table.
Please message or email me your answers to the following questions:
1) Take a close look at the vernal pool in front of you. Use the Cache description to determine the possible geological origin of the vernal pool in front of you.
2) Based upon the cache page, and the information found at GZ, what unique qualities of this vernal pool make it an ideal habitat as a nursery for amphibians
3) In your estimation, what kind of soil forms the hardpan for the vernal pools that you can observe just a few feet of where you stand? a) clay or; b) hard bedrock layers. Look at the soil around you for a clue.
4) What is the approximate width and length of the pool?
5) Include the time of year that you visited the pool area, and how that may have impacted the width/length.
6) If you arrived during the pool’s dry season, what evidence did you observe that verified the existence of a vernal pool?
7) Post a photo of yourself (face not required) or a personal item at GZ.
REFERENCES:
United States Environmental Protection Agency
Wikipedia
New York Natural Heritage Program