When Panther Hollow Lake overflowed on 20 Aug duckweed floated with it, 21 Aug 2026 (photo by Kate St. John)

23 August 2026

On Thursday 20 August, in 45 minutes from 8:10 and 8:55am, it rained 1.55″ at the Allegheny County Airport. For almost 15 minutes it came down at the rate of 3.4″ per hour. Schenley Park’s Panther Hollow Lake was one of many waterways that overflowed. By Friday morning the flood waters had receded, leaving behind a coating of duckweed right up to the high water mark. This was the first time I’ve ever seen a duckweed flood.

Ideally the flood would have been handled by the old concrete overflow channel but it’s blocked by a landslide so the flood ran over the channel edge and down the path to the lake.

An old landslide below the Panther Hollow Bridge blocks Panther Hollow Lake overflow channel, 22 Aug 2026 (photo by Kate St. John)

Water scoured new ditches and carried an old orange cone into the lake. It also beat down the cattails on its way to the duckweed zone.

Flood path to Panther Hollow Lake, 21 Aug 2026 (photo by Kate St. John)

Because Panther Hollow Lake is pond-sized and very shallow it is eutrophic in the summer, covered in duckweed and filamentous algae. On the lake, everything was a sea of green.

Panther Hollow Lake a sea of green, 21 Aug 2026 (photo by Kate St. John)

Duckweed heavily coated the concrete edge, …

Duckweed left behind by the 21 Aug flood, 22 Aug 2026 (photo by Kate St. John)

… and a lumpy log right next to the edge …

Duckweed coating everything from the 21 Aug flood, 22 Aug 2026 (photo by Kate St. John)

… and the grass.

Closeup of duckweed coating grass at the edge of Panther Hollow Lake, 22 Aug 2026 (photo by Kate St. John)

Usually duckweed (Lemnoideae) is a floating plant and each tiny leaf-like structure dangles a tiny root. But the water is gone.

Duckweed from Panther Hollow Lake, 22 June 2025 (photo by Kate St. John)

Will it take another rain storm to wash away the duckweed? Or will the sun desiccate it and make it brown? I’ll have to go back and see.