The Orinoco begins at an elevation of 1,074 m
(3,523 ft) in the Parima Mountains close to the border
between Venezuela and Brazil. It soon bifurcates into
the southern and northern streams, the former of which
flows southward, eventually joining the Río Negro and
flowing into the Amazon, the latter flowing north and
east, joining major tributaries such as the Río Meta, Río
Arauca, and Río Apure. The major city located along the
Orinoco is Ciudad Bolívar, in Venezuela, where the river
is typically about 244 m (800 ft) wide. Ships can navigate
the Orinoco for about 1,120 km (700 mi), from its mouth
to the Cariben rapids, 9.6 km (6 mi) from the Río Meta.
Like the Amazon and its tributaries, the Orinoco is
strongly seasonal. At Ciudad Bolívar the annual variation
between high and low water level is between 15 and
18 m (approx. 50– 60 ft.). Rainy season in the Orinoco
Basin is from April to October; dry season extends from
November through March. Discharge rate varies with
seasonality; the lowest flow during dry season is only 1/25
to 1/30 of the highest flow during wet season.
The Orinoco River bisects two distinct geological
areas. The right (southern) bank of the Orinoco borders
Precambrian bedrock from the Guiana Shield, at 1.7
billion years old one of the oldest geological formations
on Earth. In contrast, the land bordering the left
(northern) bank of the main river is geologically recent,
formed only a few centuries ago from sediments washed
from the Andes and transported across the flattened
Llanos. The effect of these differing geological histories is
reflected in the differing characteristics of the tributary
rivers that drain into the main Orinoco. The right- side
tributaries typically are stable, constrained by crystalline
bedrock and, especially within the Guiana Shield,
abundantly supplied with rapids and waterfalls. The
left side tributaries are unstable, with shifting channels
formed by alluvial deposits from the river.
The Orinoco flows west, then north, before beginning
its major eastward flow. The river itself has had a strong
influence on the geology of the region, having helped
cut channels through parts of the Guiana Shield, thus
contributing to the isolation of a unique series of flat-
topped table mountains called tepuis, some over a mile
high (see “The Unique Te p u i s” sidebar). The highest
waterfall on the planet, Angel Falls (Kerepakupai
Merú), drops from the top of one of these tepuis. Much
of the Orinoco flows quietly and slowly through the
vast marshy Llanos region of Venezuela (chapter 14), a
region of relatively flat plains and marshes that supports
abundant wildlife.The Orinoco meets the Atlantic Ocean at the
Amacuro Delta and Gulf of Paria, an area of extensive
mangrove forests that Columbus explored in 1498,
calling it a “gateway of the Celestial Paradise.”The Amazon River
The Amazon River, or Río Amazonas, is a vast river that
forms at the confluence of the Marañón and Ucayali
rivers just west of Iquitos, Peru, flowing eastward
6,437 km (4,000 mi) to the sea (fig. 12- 2). In Brazil the
name Amazon is formally used from Manaus, Brazil,
eastward, but west of Manaus the river is called the Río
Solimões. As mentioned, the main river first takes shape
as a confluence of several major Andean tributaries,
principally the Ucayali, the Marañón, and, to a lesser
degree, the Tigre, all just west of Iquitos, Peru.
The headwaters of the Amazon were difficult to
discover because of the challenging climatic conditions
that prevail at high elevations in the Andes. But the
river was eventually traced to a small, unremarkable
tributary, the Carruhasanta, at an elevation of 5,598 m
(18,363 ft) in the cold, windswept Peruvian Andes, only
about 192 km (120 mi) from the Pacific Ocean. The
Carruhasanta flows into the Hornillos, which in turn
joins the Apurímac, a major tributary that eventually
joins the Ene, the Tambo, and finally the Ucayali. The
Amazon system plunges in elevation initially but drops
only about 5 cm per 1.6 km (2 in/mi) once outside the
Andes, eventually flowing to the Atlantic Ocean.
The Amazon is the world’s longest river, measuring
6,993 km (4,345 mi), just surpassing the Nile River in
length. However, the Amazon carries the world’s largest
volume of water by a wide margin. About 16% of all river
water in the world passes through the 320 km (200 mi)
wide delta of the Amazon, which daily discharges about
17 trillion liters (4.5 trillion gal), or about 200,000 cubic
meters of water per second (7.4 million ft^3 /sec). This
represents a discharge of about 4.4 times that of the Congo
(Zaire) River, the next most voluminous river. The plume
of sediment- laden water from the Amazon can be seen
as far as 100 km (about 60 mi) out to sea and has been
traced by NASA’s Coastal Zone Color Scanner (CZCS)
as it moves toward Africa between June and January and
toward the Caribbean from February through May. The
river itself is over 10 km (about 6 mi) wide as far as 1,600
km (1,000 mi) upriver, and large ships can navigate for
over 3,700 km (2,300 mi), eventually docking at Iquitos,chapter 12 cruising the rivers to the sea 207