The science of stream sorting

ELISABETH M. BROWN

For 25 years we lived next to an intermittent stream in Laguna

Canyon, taking the pulse of storms through their runoff.

About 15 minutes after measurable rain began, the creek would

begin to flow, turbulent and brown. In a significant storm, we would

hear deep bumping sounds -- boulders being dragged along. As the

storm eased and the torrent slowed, it all quieted down, and the

water cleared. When the stream stopped flowing and dried up, it left

a new layer of silt, clearly marked with the water currents.

After a particularly violent storm one wet year, I returned home

from flooded streets in North Laguna to find that the water had

deposited a foot-high bank of silt and rocks in the street. I still

regret not taking a picture: far from being a jumbled mess, the bank

was a record of the stream flows, indicated through the size of the

particles and pebbles deposited by the water.

The faster the water flow, the larger the particle size it can

carry. Water is a dense and heavy medium; in motion, it packs a real

wallop. Six inches of flowing water can knock a person off her feet.

At its height, even a modest creek can carry large boulders, but as

soon as the water slows down, the largest rocks drop out. As the

water slows further, it deposits smaller and smaller pebbles and rock

particles in the stream channel, leaving only fine silt in the flow.

Finally, even the silt drops out, and the water clears.

In the Canyon Acres tributary that March afternoon was a bank of

perfectly- layered and graded sediment: fist-sized rocks on the

bottom, then, in turn, smaller pebbles, coarse soil, and sand, topped

with clay and fine silt.

Visitors to the California desert notice the large fan-shaped

deposits that originate in the canyons between adjacent mountains.

These alluvial fans are mute evidence of the rainstorms that are

eroding the mountains. Water rushing out of the steep canyons slows

down on the broad desert floor, dropping its load of rocks, sand and

silt. The finer particles go farther before falling out, leaving

extensive beds of clay on the valley floor.

After a wildfire, burned hills erode especially quickly. A couple

of years after the 1993 firestorm in Laguna Canyon, a rainy winter

resulted in drifts of fine white sand in the flat grassland. Some

park visitors thought we had trucked the sand in for aesthetic

reasons!

Runoff rainwater also sorts the hillside rocks and soil as it

descends. As the water starts its journey at the top of a hill, it’s

little more than a trickle, and can move only small soil particles.

Over time, as smaller particles move downhill, a coarser,

faster-draining soil remains on ridges and hilltops.

Different soils add to the complexity of the coastal sage scrub

community.

Hilltop and ridgeline soils dry out faster after the rain, so

plants that tolerate or prefer this type of soil, like cactus, are

found there. Downslope, the soils become progressively finer-grained,

and hold on to the water longer.

This allows plants that need more moisture to grow there.

When the clouds open up, the rain is doing real work down here --

transforming the landscape by sorting the slopes.

Do you have a question about the natural world that might be the

subject of a column? E-mail me at [email protected].

* ELISABETH M. BROWN is a biologist and the president of Laguna

Greenbelt Inc.