Mima Mounds - Vernal Pools

Vernal Pools

Vernal pools are shallow surficial depressions that seasonally fill with water during winter and spring rains and dry up during dry summer months. They get their name from the recognition of the seasonality of the habitat and the springtime flora associated with them. Vernal pools form where a impermeable or very slowly permeable layer underlies small and shallow depressions and creates a perched water table. The impermeable or very slowly permeable layer typically consists of either soil horizons such as duripans or claypans or bedrock in the form of volcanic mud or lava flows.

Within California, vernal pools are quite commonly associated with Mima Mounds. These Mima Mounds are typically located on stable landforms that are greater than 100,000 years old. These landforms are characterized by strongly developed soils that usually have a relatively impermeable layer (claypan or silcrete duripan) in the subsoil. This impermeable layer locally impedes drainage and creates perched water levels and causes the formation of vernal pools within the intermound depressions that are associated with Mima Mounds. Vernal pools are typically small, shallow, and complex ephemeral wetlands that only have internal drainage because they are hydrologically isolated from perennial inflow by a ring of Mima Mounds. Although the ponded water that fills vernal pools comes and goes throughout the year, it is present at least for a short time in most years. However, within California, the mound-depression microrelief associated with Mima Mounds is just one of a variety of geographic settings within which vernal pools occur. For example, in the Modoc Plateau region of California, numerous vernal pools are found on the surface of volcanic mudflows and basalt lava flows where Mima Mounds are completely absent.