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Earth Notes: An ancient surprise in Zion's waterways

cyanobacteria colony growing in shallow water
National Park Service
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The Southwest’s summer monsoon storms do more than provide much-needed water to parched landscapes. Gathered in arroyos, washes and canyons, their runoff scours out stagnant waterholes and provides new nutrient-rich sediment to wetland ecosystems.

Officials at Utah’s Zion National Park had hoped this summer’s monsoon runoff would help solve a simmering problem: namely, a toxic outbreak. In July of 2020, a puppy that belonged to a family visiting the park died not long after playing in the Virgin River. Testing soon revealed the culprit: cyanobacteria, microscopic organisms that can produce dangerous toxins when they grow in dense mats in shallow water.

Sometimes called blue-green algae, cyanobacteria have been around for a very long time. We probably wouldn’t be here without them, as they helped produce Earth’s oxygen-rich atmosphere billions of years ago. But for water managers, they can be a headache, especially when they congregate in amorphous mats in bodies of fresh water.

When disturbed, those colonies can disperse high levels of poisons called cyanotoxins. As a result, visitors to the Zion area have been encouraged to avoid direct contact with the water, or when dogs or children splash and play.

This past summer’s storms did reduce cyanobacteria levels, but ongoing monitoring has revealed they still occur in the park’s waterways. If you’re planning to visit, check the park website, or ask a ranger, before planning any activities involving contact with Zion’s beautiful waterways.