floods. As water flows through a dam it
often runs through generators that create
electricity.
People also put waste into rivers. This
practice has made river pollution a serious
problem.
#More to explore
Canyon • Dam • Delta • Flood
• Irrigation
Rivera, Diego
The Mexican artist Diego Rivera is
known for creating large murals, or wall
paintings. In his murals he represented
Diego Rivera painted a mural in San Francisco, California, that combines images of traditional
Mexican art with images of modern North American technology.
68 Rivera, Diego BRITANNICA STUDENT ENCYCLOPEDIA
scenes from Mexico’s history, telling of
its cultures, farming and industry, wars,
and politics. His paintings feature bright
colors and are crowded with people and
symbols.
Rivera was born on December 8, 1886,
in Guanajuato, Mexico. At age 10 he
earned a scholarship to study art in
Mexico City. In 1907 he won a scholarship
that allowed him to travel to
Europe. He studied art in Spain and
then settled in Paris, France.
In Paris Rivera began painting in a style
called cubism. In this style the subject is
broken apart into basic shapes, such as
squares, triangles, and circles. In about
1917 his style changed. He began to use
simple forms and bold areas of color.
In 1921 Rivera returned to Mexico. He
and the Mexican painter David Alfaro
Siqueiros hoped to create a new kind of
art especially for Mexico. They wanted
to paint murals on buildings where
everyone could see them. The murals
would tell the stories of the Mexican
people.
Rivera completed his first important
mural in 1923. He was soon hired to
paint murals on several public buildings
in Mexico. Between 1930 and 1934
Rivera painted murals in the United
States. He died on November 25, 1957,
in Mexico City. Frida Kahlo, his wife,
also was a famous painter.
..More to explore
Kahlo, Frida • Mexico • Painting
Riyadh
Population
(2007
estimate), urban
area,
4,465,000
Riyadh is the capital of Saudi Arabia, a
country in the Middle East. It is the
largest city in Saudi Arabia. Huge
amounts of oil were discovered in the
country in the middle of the 1900s. The
Saudi government used money from the
country’s oil industry to turn Riyadh
into a very modern city.
Factories in Riyadh process oil, cement,
metals, and foods. The city is also a
major center of business and banking.
Riyadh became the capital of the Sa!ud
family lands in the early 1800s. In 1881
the Rashid family took control of the
A robed man walks in a garden in Riyadh,
Saudi Arabia. The water tower behind him
is a city landmark.
BRITANNICA STUDENT ENCYCLOPEDIA Riyadh 69
city. The Sa!ud family recaptured Riyadh
in the early 1900s. They established
Saudi Arabia as a unified country in
1932. Riyadh was the capital.
During the 1900s Riyadh grew greatly.
Money from the oil industry was used to
build modern hospitals, roads, and
gleaming new office buildings. In the
early 21st century the city was the site of
several violent terrorist attacks.
#More to explore
Saudi Arabia
Road
Cars, trucks, buses, motorcycles, and
bicycles travel on strips of land called
roads. Some roads are made of dirt or
gravel. A firm, strong material called
pavement covers most modern roads.
Types of roads include city streets, country
roads, and long-distance highways.
How Roads Are Built
There are many steps in road building.
First, road builders choose a route, or
path, for the road. They must carefully
study how the road will affect the businesses,
homes, traffic, plants, and animals
of the area. Next, road builders buy
the land on which they will build the
road.
Construction of unpaved roads (roads
without pavement) is simple. Builders
clear away trees, bushes, and plants from
the roadway—the strip of land over
which the road will pass. Then they
smooth out the roadway and press the
surface down with a machine called a
roller. This type of road is suitable for
areas where few cars travel.
Roads that many cars will use must be
stronger than roads that have less traffic.
These roads are paved. Paving materials
include asphalt and concrete. Asphalt is
a thick, dark, flexible material made
from petroleum, or oil. Concrete is a
mixture of cement, water, sand, and
gravel. When it dries it becomes as hard
as stone.
A strong road may start with a layer of
asphalt. On top of the asphalt is a layer
of sand, gravel, or crushed stone. A final
layer of asphalt or concrete finishes the
road. The concrete may have steel rods
or mesh inside it for extra strength.
Most major modern roads are built with
a gentle slope downward from the center
of the road outward. This slope is called
a crown. The crown allows water to run
off the road. These roads also have a
system for draining water away from the
sides.
An unpaved country road makes a pretty
picture. But such a road turns to mud in wet
weather, and it cannot handle heavy traffic.
The first major
road was the
Persian Royal
Road in western
Asia. It
was more than
1,700 miles
(2,700 kilometers)
long and
was used from
about 3500 to
300 BC.
70 Road BRITANNICA STUDENT ENCYCLOPEDIA
History
Peoples in ancient Egypt and western
Asia built the earliest roads more than
