Biology Reference
In-Depth Information
MIGRATIONS: PARTICULATE TRAVELS FROM MOUNTAIN TO SEA
Imagine three rocks sitting on the sunny slope of a Sierra foothill in the
1850s: one a speck of quartz, one a particle of clay, and one a skipping
stone-sized pebble. A miner aims a hose at the hill and blasts all three rocks
off the slope and down into a sluice box, where they are all separated from
their gold-bearing peers and dropped on the bank of a creek. The pebble is
too large and heavy to move far, and it ends up stuck on the bank for centu-
ries, until a major storm carries it down to a sandbar a few miles downriver,
where it spends more centuries. The speck, however, soon gets carried away
and joins the swell of spring rainwater rushing downstream. Being very small
and fine, the speck moves as fast as the water, surfing the surface flows right
through the bay and out the Golden Gate within a matter of days. The heavier
clay particle, meanwhile, follows the speck but makes a few stops along the
way downstream until it settles on a channel bank at the bottom of San Pablo
Bay. Here it soon gets a lot of company, as a blanket of other particles of clay,
mud, and silt slowly builds over it until it's buried under six feet of other solids.
Several decades later the clay particle might be scooped up and deposited on
a dike by a dredger or scoured out by a 100-year flood, resuspended, and
grabbed by a scientist in a bay-floor sample.
shoreline jaunts and barbecue spots, and for egrets, rails, and sandpipers
probing the mud for goodies.
The bay may not be murky forever. Between 1957 and 2001, the amount
of sediment coming down the Sacramento River decreased by 50 percent.
But the supply will never dwindle away to nothing. Sediments will always
be eroded off banks and shores by water and will move downstream.
Climate often dictates the state of bay waters. It drives winds, currents, and
waves, and it alters the temperature of air and water. Central California
experiences a Mediterranean climate pattern with two distinct seasons: a
warm, dry period from April to October, followed by a cool, rainy period
from November to March. As the rainy season ends, snowmelt fortified by
the season's final storms swells the banks of streams and lowers the salinity
of estuary waters. This runoff wanes with summer, reducing many creeks
to a trickle.
Westerly winds that began in spring intensify in summer, forcing ocean
waters shoreward. Near land, the rotation of the earth deflects currents to
