As you walk up to Chabot Dam the valley carved by San Leandro Creek opens up below, and it’s quite impressive (click photo for bigger version):
At the top, the dam is buttressed against a ridge of the volcanic rock of Leona Quarry. Before the dam was built, there must have been quite a defile here:
The rock is nicely exposed just next to the dam, worth taking a close look at:
The gorge is more than 100 meters deep and only a couple hundred meters wide. Like other Oakland stream valleys, it seems large for the stream running through it today. I think much of the hardest work carving the canyons was done during periods of much wetter climate than today. Or maybe it was drier, with the land being more prone to desert-style flash floods.