Downtown Los Angeles, the center of the second largest city in the United States, population
in 1980, 11 million, the fastest growing city in the United States.
Extending for nearly 100 kilometers along the Pacific coast, this conurbation today
occupies most of the Los Angeles basin, a lowland, 3,000 square kilometers in extent,
a conurbation of low-rise, low-density housing, stretching as far as the eye can see, from
the Pacific Ocean to the San Gabriel Mountains, which almost encircle the lowlands.
These are the San Gabriel Mountains, young mountains, 1,000 meters high, still rising,
still very unstable, especially along the lines of the great faults which crisscross
the region, like the San Andreas Fault, seen here 150 kilometers north of Los Angeles.
This is where two of the great plates of the Earth's crust meet, and here's clear evidence
of recent Earth movement.
For example, this small river system has been displaced by the northward movement of one
of the plates.
The rocks which make up these mountains are soft and easily eroded.
This can be seen in the gullying which is taking place on the hillsides throughout the
region.
On a larger and more impressive scale, it can be seen in the dissection of the mountains
by rivers, a process aided by a semi-arid climate characterized by long droughts and
occasional rainstorms of great intensity, and rock surfaces protected only by sparse
open forest or scrub vegetation known as chaparral, surfaces which are in places completely exposed
by great fires which periodically destroy this thin protective cover.
Together, these factors have conspired to produce this landscape of sharp ridges, deeply
cut river valleys, and steeply plunging side canyons.
During the summer, the valleys and canyons are virtually dry.
The riverbeds string with boulders.
During the winter, they can be very different.
This is one small stream in flood.
Imagine the effects hundreds of streams like this entering a lowland basin like the Los
Angeles basin.
Rainstorms, floods, and mudslides hit southern California.
The inundation of this canyon near Burbank is worsened because the road follows the course
of an old streambed which the water seeks out as if by habit.
The water is
very shallow.
These floods in March 1938 were caused when several storms struck southern California.
Soils and surface rocks were already saturated and runoff was rapid.
The river channels in the Los Angeles basin couldn't cope with this increased flow from
the hills and a large part of lowland Los Angeles disappeared underwater.
Floods such as these have occurred at intervals throughout the history of Los Angeles.
The enormous growth of the city during the 20th century has made them more noticeable.
In 1900, the city of the angels, Los Angeles, was small, still tightly clustered around
the original center where an overland trail reached the Los Angeles River.
Twenty years later, and the city spread northwards towards Pasadena, westwards into the Hollywood
hills, and had reached the sea at Santa Monica and Long Beach.
By 1950, the city had grown enormously thanks to the motor car and the new freeways and
building had spread into the San Fernando Valley to the northwest.
Today the entire lowland is built up and low housing is being built far to the south in
Orange County and eastwards towards San Bernardino.
And Los Angeles has rightly earned the title of 60-mile city.
During this expansion, canyons have been occupied regardless of the risk of floods.
Entire mountains have been cropped, leveled off, to provide houses for the very rich,
and where slopes have been too steep for normal building, houses have been perched on stilts.
Such houses are the first casualties of the floods when they occur.
The weather is not entirely to blame for the runoff of water, mud, and rocks that forced
37 families to evacuate their homes.
The plight gloomily carries out a prediction that recent forest fires that stripped the
hills of rain-retaining Russian trees would lead to exactly this condition.
It was to protect housing developments like this that the Los Angeles Flood Control Authority
was set up in 1915, and a program of major engineering works began two years later which
has continued almost without pause ever since.
One of the most obvious problems was that in lowland areas like the Los Angeles Basin,
the rivers are shallow and flow in wide, ill-defined, braided channels.
For most of the year, there is little water in these channels, and most of the riverbed
is exposed.
After heavy rains, however, water levels rise rapidly.
The river not only covers its bed, it spreads onto the surrounding floodplain.
It was to prevent this that in the built-up area of Los Angeles, vast, raw concrete-lined
channels were built, a distinctive feature of the city landscape.
At the same time, a series of massive dams was built to control the flow of water from
the upland tributaries into the main channels.
Each of these dams is designed to hold back the flood waters entering the river systems,
to store this water, sometimes for considerable periods, and to release the water in a controlled
manner over a long period of time.
In short, these are dams designed to lower the flood hydrograph, and in doing so to reduce
the risk of severe flooding in the Los Angeles Basin below.
Of the dams, in the valleys and canyons, other large engineering works were needed.
Here the rivers are cutting back into the soft rock, and vast amounts of debris are
produced.
Checked dams have been built to trap this material, and prevent it moving downstream
into built-up areas.
This is a small dam in the tributary canyon, but the accumulation of debris during the
single winter storm is impressive.
After the rain, these dams have to be emptied, and the debris is transported either to dumping
grounds, or for use as aggregate in other engineering projects.
This then is the basis of the flood control system for central Los Angeles.
Debris dams form a great arc, marking the edge of the encircling hills.
Control dams, some in high mountains, others protecting the lowland basins.
Spreading grounds below the dams improve channels over 640 kilometers of them on virtually every
major river in the region.
We're going to look at one river, the San Gabriel, and the controls imposed upon it.
The river rises in the San Gabriel Mountains, 80 kilometers from the sea.
Here the river has cut a deep, steep-sided valley.
Gradients are generally steep.
The channel is strewn with boulders, many of them large, and in places a small floodplain
has been formed.
It is in the mountains that the first control dams have been built.
This is the San Gabriel Dam.
Just less than 25 kilometers from the source of the river, it controls discharge from an
area of 500 square kilometers.
Water levels are continuously monitored, and the great sluices can be opened or closed
to cope with changing weather conditions.
In this way, flood waters can be stored and released when the risk of flooding in the
lower river is reduced.
The dam has other important functions.
For example, water can be diverted along channels and conduits like these, and onto vast spreading
grounds on the floodplain below the dam.
Here the water is released into large basins to form shallow pools, and because the underlying
rocks are permeable, this water rapidly percolates into the water-bearing strata below.
The scale of the operation can be seen from this single large spreading ground serving
the San Gabriel Dam.
The outlines of the older, small rectangular pools still visible in the foreground even
during the long summer drought when little water is released.
This is just one of the spreading ground complexes serving Los Angeles, and they play an important
part in maintaining water supplies in a region which is forced to import over 60 percent
of its water, often over long distances.
For example, from Owens Lake in the Sierra Nevada, 400 kilometers to the northeast.
From the Parker Dam on the Colorado River, 300 kilometers to the east, and from as far
away as the Sacramento River, 600 kilometers to the north.
Together, these sources supply the Los Angeles region with about 310 million cubic feet of
water each day.
This prodigious amount of water is necessary to maintain Los Angeles industries, and even
more significant, the lifestyle of a population which is among the most affluent in the world.
Below the San Gabriel and Morris Dams, the river continues in its mountain course.
Its valley is steep-sided, its floodplain narrow, its channel shallow and braided.
Then as the river approaches the lowland basin of Los Angeles, its features change.
It's at this point that the next major control system has been built, its prime purpose,
to protect the city.
This is the Santa Fe Dam, a large earth-filled dam over seven kilometers long, enclosing
a lowland basin over 445 hectares in extent.
Unlike the San Gabriel and Morris Dam, this basin is occupied only during times of flood.
For the rest of the year, the dam is dry.
The great sluice gates stand open.
Further protection is needed at this point in the river's course.
This is the Maddock Debris Basin, still part full after the previous winter's floods.
It's one of a series of such dams built above the Santa Fe Dam at the mouth of the network
of canyons which dissect the hills, and its dams restrict the movement of debris whilst
allowing the free flow of water down the concrete slipways.
The need for such protection at this point is all too obvious.
Still in its mountain course, the San Gabriel is already a rigidly controlled river.
The San Gabriel and Morris Dam's high in the hills, the San Gabriel's spreading grounds
below the dams, the Santa Fe Dam and the Maddock Debris Basin where the river enters the lowlands.
And below this, an improved channel down to Whittier Narrows.
This is the channel below the Santa Fe Dam, straight, wide, flat bottomed, with high concrete
embankments to cope with the largest flood.
Even so, further safeguards are necessary, and at Whittier Narrows, the largest and most
impressive engineering works on the entire river have been undertaken.
Here two rivers have cut through the low ridge of the La Merced and San Jose Hills.
The Rio Hondo, a tributary of the neighbouring Los Angeles River, and the San Gabriel.
A large dam has been built across this gap, which is called the Whittier Narrows.
During floods, the sluice gates can be closed, and as at Santa Fe, a large lowland basin
behind the dam can be flooded.
Flood water can then be released in controlled amounts to the Rio Hondo and thence to the
Los Angeles River, to the San Gabriel River, or to both.
This is the Whittier Narrows flood control basin, nearly 976 hectares in area, enclosed
by a vast earth-filled dam, five kilometres long, and nearly 20 metres high.
For most of the year, the entire area is virtually dry.
The giant sluice gates stand unused.
These are sluices which control flow into the San Gabriel River, nine massive gates
raised and lowered electrically during times of flood.
Pressure on land is becoming significant even in low-rise, low-density Los Angeles, and
when the basin is dry, it's used as a public park and recreation centre for a large area
of the city.
More than two million people visit the basin each year to make use of the golf courses
and fishing lakes, or simply to walk or ride along the leisure trails.
Through the dam, the channel becomes completely artificial, virtually straight, fully lined,
a vast concrete drain, designed to carry water to the sea as quickly and efficiently as possible,
virtually unrecognisable as a river.
And this is the mouth of the San Gabriel, as artificial as the channel which leads into
it, a fitting end to a river apparently completely tamed by a man, so tamed that little more
than two percent of the water falling on the catchment is allowed to reach the sea.
Though in spite of this, the San Gabriel still has to be treated with respect.
Things on this scale may be a thing of the past, but no-one can predict when the river
will next rise in front, and no flood control system on earth can guarantee the safety of
all parts of the city the size of Los Angeles.
And there's a final twist to this complicated story.
Various pictures of damage on the Californian coast near Los Angeles during recent storms
show a coastline under threat, and the success of the flood control program may have contributed
to this threat.
As stream flowing rivers has decreased, so the amount of sediment reaching the sea has
been drastically reduced.
This in turn has meant that there is now less sediment available for beach building.
And in places particularly where harbor works have restricted long shore drift, erosion
has taken place and buildings are already under threat.
It is possible therefore that these magnificent beaches may, like the great Los Angeles floods,
also become a thing of the past.
And there's a final twist to this complicated story.
Various pictures of damage on the Californian coast near Los Angeles during recent storms