Vernal Pools: The Habitat of Spring

Spring is the time to look for fleeting vernal pools. These habitats are teaming with aquatic and terrestrial plants and animals—especially with the amphibians that rely on these pools for their life cycles.

Vernal pools are unique and temporary habitats that form in the spring when shallow depressions fill with snowmelt and rainwater. As the temperatures rise and the sun shines, the surface water evaporates but the soil will hold moisture longer than the surrounding landscape.

These ephemeral wetlands are the perfect habitat for bugs and amphibians that need water for part of their life cycle and prefer to do their reproductive work without the pressure of predatory fish. The lack of fish in vernal pools is a key characteristic of these habitats.

Vernal pools are relatively small—100 to 1,000 square feet— but serve several key ecological functions. Not only are they important habitats for numerous species, they also help to reduce flooding during spring snow melt and rain storms. Being low on the landscape, they can collect runoff pollution and help clean the water before it seeps into our ground water.

Close up photo of flower.
Photo: Shallan Knowles

Eastern Washington’s climate and geography is ripe for the formation of these temporary wetlands. The majority of our precipitation falls in the winter months and then evaporates during the hot, dry summer. The channeled scablands and the underlying layer of basalt in our area allow for numerous shallow depressions where water can collect, creating vital habitat for aquatic species in an otherwise dry landscape.

There are many clues to the location of vernal pools. In the drier sage steppe, landscape vegetation will be denser and greener later in the season. Sometimes you will find dried films of algae or salt rings as the water evaporates. In early spring, one of the best clues to the location of vernal pools is the sound of frogs, especially the calls from the Pacific tree frog (Pseudacris regilla).

Both the presence of frogs and salamanders are a sign of a healthy environment. Many species of amphibians spend their youth in and around wetlands like vernal pools, hatching from eggs, swimming around as tadpoles, and finally growing legs to venture out onto dry land. During these formative times, amphibians breath through their skin and are particularly susceptible to pollutants that can be washed into vernal pools from upland environments.

Hiking along the Palisades Park trail in early spring, you will hear the chorus of the small, grey-green Pacific tree frogs. The males of this species use their call to stake out territory in vernal pools while they wait for a female to choose a mate. The females will then attach eggs to shallow underwater sticks or rocks. The proposal to turn 100 acres between Palisades City Park and Riverside State Park into the Rimrock to Riverside corridor will surely protect several vernal pools.

Turnbull National Wildlife Refuge has copious vernal pools in the early spring. The aptly named biscuit and swale prairie alternates between raised mounds of deep soil and shallow hardpan depressions. Keep a look out for long-toed salamanders, clams, fairy shrimp, cattails, and sedges.

The presence of vernal pools is a sure sign that spring has arrived and the region is teaming with life again. Sometimes, the ephemeral, the temporary, and the vanishing are the most worthy outdoor objectives.

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