Biology Reference
In-Depth Information
Valley rivers and the mining debris they carried downstream. As early as
1914, mining had increased the amount of sediment coming into the bay
by tenfold over natural baseline levels. But in the late 1990s, U.S. Geologi-
cal Survey (USGS) scientists discovered that instead of absorbing a steady
supply of new sediment, the bay floor had actually started eroding. More
recently, they've noticed the water growing clearer. “Of course there is still
erodible material in the bay, but the huge excess from mining may now
have worked its way out of the system, or is no longer available for resus-
pension by winds and tides,” says the USGS's David Schoellhamer, the
bay's chief sediment accountant. As a result, erosion of existing mudflats
may be the most significant source of sediments to build marshes with in
the future (see also Figure 3, pp. 42-43).
To help restoration planners accommodate this change, Schoellhamer
calculated a series of sediment budgets for the entire bay system. He tracks
what comes in, what gets resuspended, and what escapes out the Golden
Gate via tides or dredge barges. His calculations suggest that sediment in-
flows from the delta have decreased over the second half of the twentieth
century, and sediment inflows from local tributaries are now greater than
what comes in from the delta. Despite these inputs, the annual net loss of
sediment from the bay to the ocean has almost doubled, from 1.4 million
metric tons (mmt) between 1955 and 1990 to 2.4 mmt, during normal
water years, since 1995.
Though the Bay as a whole is now eroding, some areas continue to have
strong deposition, says Schoellhamer, making them prime spots for wet-
land restoration. The Petaluma River, for example, exhibits some of the
largest sediment concentrations measured around the bay: the river chan-
nel traps a lot of sediment because flows and tides aren't strong enough to
carry it out into San Pablo Bay. This explains why just four years after a
Petaluma River levee near the Highway 37 bridge was breached, Carl's
Marsh had gained six vertical feet of sediment.
SUBSIDENCE: HOW IT HAPPENS
When landscapes are separated from the tides that once flooded them by
building dikes, their elevation subsides. The spongy soils, once expanded with
water and vegetation, dry out and contract. Initially, decomposing organic mat-
ter in once water-logged peat soils cause subsidence. Subsidence can be fur-
ther accentuated by agricultural tilling and groundwater withdrawal, depending
on land uses in the immediate area. As a result, many delta islands now lie
10-25 feet below sea level (see Map 12, p. 220). Former bay tidal marshes
have also dropped below tidal levels due to diking and loss of local aquifers,
but not to the same extent as their delta counterparts.
