Vernal Pools

Overview

A vernal pool in Sligo Creek Park. [2]

A vernal pool is a seasonal pool of water that is typically characterized by being relatively small, shallow, and ephemeral. A vernal pool is filled in the spring by rain and snow melt, and then dry up when summer starts to come around. These pools can last longer if the soil beneath them is made up of fine silt; Making the percolation rate slower. These pools can form not only near other wetlands, but in any low lying area with a soil structure that is capable of holding water for a short period of time. Vernal pools have been found on the tops of upland areas, woodlands, and urban areas. The key characteristic that contributes to the importance of these pools is that they are separated from other water bodies. When considering the characteristics of vernal pools, it is evident that vernal pools are unique and temporary wetland ecosystems. [1]

Formation of Vernal Pools

In order for vernal pools to form, many factors have to align. The Topography, water table (sometimes), and soil history all have to be just right before a vernal pool will typically form. Most vernal pools only occur in the Western Region and the Northeastern Region of the United States. They will form however in many parts of Canada, and many other Mediterranean or Subtropical regions on earth. [3]

The topography of a region is very important in an area that has vernal features. The area typically will be a flat lying area that displays divots in the ground along the flat area. The type of rock in the area will also assist in the formation of vernal pools. If there is igneous rock or bedrock underneath the soil, the formation of vernal pools will be promoted greatly. Even if the rock is suspended on a hill or a mountain, it creates a situation where the infiltration of water into the soil is limited because the rock is essentially impermeable shortly under the ground. [3]

Most believe that the water table in a region is the sole reason behind vernal pool formation, but this is not the case. Although the water table in an area can be extremely important in vernal pool formation. If the area has a higher water table, vernal pool formation will be promoted because water is more likely to pool up on the surface in the Spring months and create vernal pools; This is common in wetland areas and near stream beds. However, the water table does not have to be high in order for a vernal pool to form. Vernal pools can form due to the rock below and holding runoff in an area, creating a suspended water table or a lower infiltration rate in that area. [3]

The soil in an area is often the fundamental reason that vernal pools are able to form in an area. Mainly because of what geological feature existed there once before. Areas that promote vernal pool formation are areas effected by glacial action, floodplains, sag ponds, pingos, and even areas with human activity.

References

[1]

[2] Murtagh, Ed. “Vernal Pools.” Friends of Sligo Creek, Takoma Park Newsletter, Aug. 2004, www.fosc.org/VernalPool.htm.

[3] “Vernal Pool EarthCache.” GC2G67F Diamond Head Crater (Earthcache) in Hawaii, United States Created by Martin 5, www.geocaching.com/geocache/GC6ZRQV_vernal-pool-earthcache?guid=451fa0e4-d882-4d81-936c-9e56bfb317ff.