Point Reyes Peninsula, west of the bay, consists of Mesozoic granites. The rift valley southeast of the bay contains Quaternary alluvium. Much of the grassland consists of introduced annual grasses used for pastures which replaced indigenous perennial grasses. Much of the woodland is forested. This area was harvested for both timber and firewood during the latter part of the 19th century, with some activity continuing into the 1940's.
Stream flow provides the major material linkage between the watershed and bay.
Two major stream systems drain into the bay. These streams drain about half the watershed and enter at the southeastern end of the bay. The remainder of the watershed is drained by local streams along the southwest and northeast shores of the bay. Small shoreline watersheds are unlabelled. USGS Stream gauging stations at locations of black circles.