kookamongus's Identifications

Total of 34 frogs were easy to count with binoculars. Photos blurry, video slightly better — but some frogs had leapt off main branch — and video doesn't show second branch nearer to bridge or righthand tiny stump with more frogs (upper right at :07 of video), plus heads or eyes popping up out of the water in the open and beneath log.

Described 20 minutes later to Marin Water District employee, who mentioned Calif. Red-Legged Frog.
UPDATE: After seeing these images, MWD has identified these as American Bullfrog.

PNG is a still from the video. View video: https://bit.ly/48lgwCc
(Not great video, but changed settings to allow download; let me know if that isn't working for you)

PS — We often visit the bridge from Lagunitas parking lot to Shadyside Trail to look for tadpoles and crayfish: 9/17, 3pm, on reservoir side of bridge spotted 1 large tadpole and a tadpole with well-developed legs.

At least six individuals directly observed flying through the forest and calling. I didn't have any means to get photos other than my phone but I was within a few meters of them at times as they landed near me for a moment before flying off.