April showers bring May flowers – and vernal pools, if you live in California or Southern Oregon.

Vernal pools are seasonal depressional wetlands that fill up with rainwater during winter and spring, but may be dry for part of the year. They are typically found on the West Coast, especially in California and Southern Oregon, but can also be found in parts of the Northeast and Midwest. Western vernal pools often occur within “vernal pool landscapes”  where swales connect vernal pools to each other and to seasonal streams. Vernal pools vary in size from 1 square meter to more than 2 acres.

Why Care About Western Vernal Pools?

Western vernal pools can connect to other pools and streams that flow seasonally or only after rain. Multiple studies [PDF] show that California vernal pools fill with water and flow into these channels [PDF], sending water downstream during many days of the year. These connections can impact the base flow of downstream waters, altering their physical characteristics. Western vernal pools are also hot spots of biodiversity, with native plant and animal species some of which can only be found in vernal pool habitats. In a study of vernal pools, 17 out of 67 species [PDF] were only found in one of the surveyed ponds. In turn, these plants and animals provide food and habitat for shorebirds and waterfowl. Vernal pools, like other depressional wetlands, can also help to store and slow floodwaters.

Are They Protected Under The Clean Water Act?

Right now, it’s unlikely that vernal pools would currently be protected under the Clean Water Act.

The proposed Clean Water Rule acknowledges these connections and sets up a process where similar “other waters” that lie outside of the floodplain can be protected under the Clean Water Act. These waters collectively with other similar waters must demonstrate a significant connection to downstream waters, meaning that those waters have a more than speculative impact on the chemical, physical, and biological integrity of downstream protected waters. Although playa lakes aren’t categorically protected right now under the draft rule, the EPA and the Army Corps are looking for input about whether they should be.